diff --git a/Cargo.lock b/Cargo.lock
index 100e909570310..7c3879fdd98d4 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3631,6 +3631,7 @@ dependencies = [
  "rustc_macros",
  "rustc_middle",
  "rustc_serialize",
+ "rustc_session",
  "rustc_span",
  "rustc_target",
  "smallvec",
diff --git a/compiler/rustc_ast/src/visit.rs b/compiler/rustc_ast/src/visit.rs
index f687bfeced841..db4c5fe6b4426 100644
--- a/compiler/rustc_ast/src/visit.rs
+++ b/compiler/rustc_ast/src/visit.rs
@@ -244,14 +244,12 @@ pub trait Visitor<'ast>: Sized {
 
 #[macro_export]
 macro_rules! walk_list {
-    ($visitor: expr, $method: ident, $list: expr) => {
-        for elem in $list {
-            $visitor.$method(elem)
-        }
-    };
-    ($visitor: expr, $method: ident, $list: expr, $($extra_args: expr),*) => {
-        for elem in $list {
-            $visitor.$method(elem, $($extra_args,)*)
+    ($visitor: expr, $method: ident, $list: expr $(, $($extra_args: expr),* )?) => {
+        {
+            #[cfg_attr(not(bootstrap), allow(for_loop_over_fallibles))]
+            for elem in $list {
+                $visitor.$method(elem $(, $($extra_args,)* )?)
+            }
         }
     }
 }
diff --git a/compiler/rustc_borrowck/src/constraint_generation.rs b/compiler/rustc_borrowck/src/constraint_generation.rs
index 5e9cec5c350c9..144fd15fc2407 100644
--- a/compiler/rustc_borrowck/src/constraint_generation.rs
+++ b/compiler/rustc_borrowck/src/constraint_generation.rs
@@ -31,7 +31,7 @@ pub(super) fn generate_constraints<'cx, 'tcx>(
         body,
     };
 
-    for (bb, data) in body.basic_blocks().iter_enumerated() {
+    for (bb, data) in body.basic_blocks.iter_enumerated() {
         cg.visit_basic_block_data(bb, data);
     }
 }
diff --git a/compiler/rustc_borrowck/src/dataflow.rs b/compiler/rustc_borrowck/src/dataflow.rs
index 97d5a8d158e1e..816288eb50b29 100644
--- a/compiler/rustc_borrowck/src/dataflow.rs
+++ b/compiler/rustc_borrowck/src/dataflow.rs
@@ -143,7 +143,7 @@ struct OutOfScopePrecomputer<'a, 'tcx> {
 impl<'a, 'tcx> OutOfScopePrecomputer<'a, 'tcx> {
     fn new(body: &'a Body<'tcx>, regioncx: &'a RegionInferenceContext<'tcx>) -> Self {
         OutOfScopePrecomputer {
-            visited: BitSet::new_empty(body.basic_blocks().len()),
+            visited: BitSet::new_empty(body.basic_blocks.len()),
             visit_stack: vec![],
             body,
             regioncx,
diff --git a/compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs b/compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs
index 68f9a7c5007c8..a6b8c6057e05e 100644
--- a/compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs
@@ -459,7 +459,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 return outmost_back_edge;
             }
 
-            let block = &self.body.basic_blocks()[location.block];
+            let block = &self.body.basic_blocks[location.block];
 
             if location.statement_index < block.statements.len() {
                 let successor = location.successor_within_block();
@@ -518,7 +518,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         }
 
         if loop_head.dominates(from, &self.dominators) {
-            let block = &self.body.basic_blocks()[from.block];
+            let block = &self.body.basic_blocks[from.block];
 
             if from.statement_index < block.statements.len() {
                 let successor = from.successor_within_block();
@@ -568,7 +568,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             UseSpans::PatUse(span)
             | UseSpans::OtherUse(span)
             | UseSpans::FnSelfUse { var_span: span, .. } => {
-                let block = &self.body.basic_blocks()[location.block];
+                let block = &self.body.basic_blocks[location.block];
 
                 let kind = if let Some(&Statement {
                     kind: StatementKind::FakeRead(box (FakeReadCause::ForLet(_), _)),
diff --git a/compiler/rustc_borrowck/src/diagnostics/move_errors.rs b/compiler/rustc_borrowck/src/diagnostics/move_errors.rs
index cb3cd479ae2b6..16c2f9ccc6aa4 100644
--- a/compiler/rustc_borrowck/src/diagnostics/move_errors.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/move_errors.rs
@@ -88,7 +88,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                 if let Some(StatementKind::Assign(box (
                     place,
                     Rvalue::Use(Operand::Move(move_from)),
-                ))) = self.body.basic_blocks()[location.block]
+                ))) = self.body.basic_blocks[location.block]
                     .statements
                     .get(location.statement_index)
                     .map(|stmt| &stmt.kind)
diff --git a/compiler/rustc_borrowck/src/location.rs b/compiler/rustc_borrowck/src/location.rs
index 70a3116949894..5ca3f2f4d033c 100644
--- a/compiler/rustc_borrowck/src/location.rs
+++ b/compiler/rustc_borrowck/src/location.rs
@@ -33,7 +33,7 @@ impl LocationTable {
     pub(crate) fn new(body: &Body<'_>) -> Self {
         let mut num_points = 0;
         let statements_before_block = body
-            .basic_blocks()
+            .basic_blocks
             .iter()
             .map(|block_data| {
                 let v = num_points;
diff --git a/compiler/rustc_borrowck/src/region_infer/values.rs b/compiler/rustc_borrowck/src/region_infer/values.rs
index c81ef10f7c740..de20a4bb465c2 100644
--- a/compiler/rustc_borrowck/src/region_infer/values.rs
+++ b/compiler/rustc_borrowck/src/region_infer/values.rs
@@ -25,7 +25,7 @@ impl RegionValueElements {
     pub(crate) fn new(body: &Body<'_>) -> Self {
         let mut num_points = 0;
         let statements_before_block: IndexVec<BasicBlock, usize> = body
-            .basic_blocks()
+            .basic_blocks
             .iter()
             .map(|block_data| {
                 let v = num_points;
@@ -37,7 +37,7 @@ impl RegionValueElements {
         debug!("RegionValueElements: num_points={:#?}", num_points);
 
         let mut basic_blocks = IndexVec::with_capacity(num_points);
-        for (bb, bb_data) in body.basic_blocks().iter_enumerated() {
+        for (bb, bb_data) in body.basic_blocks.iter_enumerated() {
             basic_blocks.extend((0..=bb_data.statements.len()).map(|_| bb));
         }
 
diff --git a/compiler/rustc_borrowck/src/type_check/input_output.rs b/compiler/rustc_borrowck/src/type_check/input_output.rs
index 4431a2e8ec60d..84d2bfe04de05 100644
--- a/compiler/rustc_borrowck/src/type_check/input_output.rs
+++ b/compiler/rustc_borrowck/src/type_check/input_output.rs
@@ -225,7 +225,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
 
         debug!("{:?} normalized to {:?}", t, norm_ty);
 
-        for data in constraints {
+        if let Some(data) = constraints {
             ConstraintConversion::new(
                 self.infcx,
                 &self.borrowck_context.universal_regions,
diff --git a/compiler/rustc_borrowck/src/type_check/mod.rs b/compiler/rustc_borrowck/src/type_check/mod.rs
index 293d847ec9ab7..c1812aa4bbab6 100644
--- a/compiler/rustc_borrowck/src/type_check/mod.rs
+++ b/compiler/rustc_borrowck/src/type_check/mod.rs
@@ -2633,7 +2633,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             self.check_local(&body, local, local_decl);
         }
 
-        for (block, block_data) in body.basic_blocks().iter_enumerated() {
+        for (block, block_data) in body.basic_blocks.iter_enumerated() {
             let mut location = Location { block, statement_index: 0 };
             for stmt in &block_data.statements {
                 if !stmt.source_info.span.is_dummy() {
diff --git a/compiler/rustc_codegen_cranelift/src/analyze.rs b/compiler/rustc_codegen_cranelift/src/analyze.rs
index 35b89358b1984..0cbb9f3ec2d80 100644
--- a/compiler/rustc_codegen_cranelift/src/analyze.rs
+++ b/compiler/rustc_codegen_cranelift/src/analyze.rs
@@ -26,7 +26,7 @@ pub(crate) fn analyze(fx: &FunctionCx<'_, '_, '_>) -> IndexVec<Local, SsaKind> {
         })
         .collect::<IndexVec<Local, SsaKind>>();
 
-    for bb in fx.mir.basic_blocks().iter() {
+    for bb in fx.mir.basic_blocks.iter() {
         for stmt in bb.statements.iter() {
             match &stmt.kind {
                 Assign(place_and_rval) => match &place_and_rval.1 {
diff --git a/compiler/rustc_codegen_cranelift/src/base.rs b/compiler/rustc_codegen_cranelift/src/base.rs
index 44c34d6c8cb79..3011813c7035b 100644
--- a/compiler/rustc_codegen_cranelift/src/base.rs
+++ b/compiler/rustc_codegen_cranelift/src/base.rs
@@ -73,7 +73,7 @@ pub(crate) fn codegen_fn<'tcx>(
     // Predefine blocks
     let start_block = bcx.create_block();
     let block_map: IndexVec<BasicBlock, Block> =
-        (0..mir.basic_blocks().len()).map(|_| bcx.create_block()).collect();
+        (0..mir.basic_blocks.len()).map(|_| bcx.create_block()).collect();
 
     // Make FunctionCx
     let target_config = module.target_config();
@@ -271,7 +271,7 @@ fn codegen_fn_body(fx: &mut FunctionCx<'_, '_, '_>, start_block: Block) {
     }
     fx.tcx.sess.time("codegen prelude", || crate::abi::codegen_fn_prelude(fx, start_block));
 
-    for (bb, bb_data) in fx.mir.basic_blocks().iter_enumerated() {
+    for (bb, bb_data) in fx.mir.basic_blocks.iter_enumerated() {
         let block = fx.get_block(bb);
         fx.bcx.switch_to_block(block);
 
diff --git a/compiler/rustc_codegen_cranelift/src/constant.rs b/compiler/rustc_codegen_cranelift/src/constant.rs
index 7f7fd0e9c579d..e2b68f24a21dc 100644
--- a/compiler/rustc_codegen_cranelift/src/constant.rs
+++ b/compiler/rustc_codegen_cranelift/src/constant.rs
@@ -505,7 +505,7 @@ pub(crate) fn mir_operand_get_const_val<'tcx>(
                 return None;
             }
             let mut computed_const_val = None;
-            for bb_data in fx.mir.basic_blocks() {
+            for bb_data in fx.mir.basic_blocks.iter() {
                 for stmt in &bb_data.statements {
                     match &stmt.kind {
                         StatementKind::Assign(local_and_rvalue) if &local_and_rvalue.0 == place => {
diff --git a/compiler/rustc_codegen_ssa/src/mir/analyze.rs b/compiler/rustc_codegen_ssa/src/mir/analyze.rs
index 24da48ead63a2..c7617d2e464fa 100644
--- a/compiler/rustc_codegen_ssa/src/mir/analyze.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/analyze.rs
@@ -266,7 +266,7 @@ pub fn cleanup_kinds(mir: &mir::Body<'_>) -> IndexVec<mir::BasicBlock, CleanupKi
         result: &mut IndexVec<mir::BasicBlock, CleanupKind>,
         mir: &mir::Body<'tcx>,
     ) {
-        for (bb, data) in mir.basic_blocks().iter_enumerated() {
+        for (bb, data) in mir.basic_blocks.iter_enumerated() {
             match data.terminator().kind {
                 TerminatorKind::Goto { .. }
                 | TerminatorKind::Resume
@@ -296,7 +296,7 @@ pub fn cleanup_kinds(mir: &mir::Body<'_>) -> IndexVec<mir::BasicBlock, CleanupKi
     }
 
     fn propagate<'tcx>(result: &mut IndexVec<mir::BasicBlock, CleanupKind>, mir: &mir::Body<'tcx>) {
-        let mut funclet_succs = IndexVec::from_elem(None, mir.basic_blocks());
+        let mut funclet_succs = IndexVec::from_elem(None, &mir.basic_blocks);
 
         let mut set_successor = |funclet: mir::BasicBlock, succ| match funclet_succs[funclet] {
             ref mut s @ None => {
@@ -359,7 +359,7 @@ pub fn cleanup_kinds(mir: &mir::Body<'_>) -> IndexVec<mir::BasicBlock, CleanupKi
         }
     }
 
-    let mut result = IndexVec::from_elem(CleanupKind::NotCleanup, mir.basic_blocks());
+    let mut result = IndexVec::from_elem(CleanupKind::NotCleanup, &mir.basic_blocks);
 
     discover_masters(&mut result, mir);
     propagate(&mut result, mir);
diff --git a/compiler/rustc_codegen_ssa/src/mir/mod.rs b/compiler/rustc_codegen_ssa/src/mir/mod.rs
index c70c2bd9f741d..d6bbcd9923455 100644
--- a/compiler/rustc_codegen_ssa/src/mir/mod.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/mod.rs
@@ -150,13 +150,13 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     let start_llbb = Bx::append_block(cx, llfn, "start");
     let mut bx = Bx::build(cx, start_llbb);
 
-    if mir.basic_blocks().iter().any(|bb| bb.is_cleanup) {
+    if mir.basic_blocks.iter().any(|bb| bb.is_cleanup) {
         bx.set_personality_fn(cx.eh_personality());
     }
 
     let cleanup_kinds = analyze::cleanup_kinds(&mir);
     let cached_llbbs: IndexVec<mir::BasicBlock, Option<Bx::BasicBlock>> = mir
-        .basic_blocks()
+        .basic_blocks
         .indices()
         .map(|bb| if bb == mir::START_BLOCK { Some(start_llbb) } else { None })
         .collect();
@@ -172,8 +172,8 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         unreachable_block: None,
         double_unwind_guard: None,
         cleanup_kinds,
-        landing_pads: IndexVec::from_elem(None, mir.basic_blocks()),
-        funclets: IndexVec::from_fn_n(|_| None, mir.basic_blocks().len()),
+        landing_pads: IndexVec::from_elem(None, &mir.basic_blocks),
+        funclets: IndexVec::from_fn_n(|_| None, mir.basic_blocks.len()),
         locals: IndexVec::new(),
         debug_context,
         per_local_var_debug_info: None,
diff --git a/compiler/rustc_const_eval/src/interpret/eval_context.rs b/compiler/rustc_const_eval/src/interpret/eval_context.rs
index 92596d059cd49..2da9123b84610 100644
--- a/compiler/rustc_const_eval/src/interpret/eval_context.rs
+++ b/compiler/rustc_const_eval/src/interpret/eval_context.rs
@@ -782,7 +782,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         assert_eq!(
             unwinding,
             match self.frame().loc {
-                Ok(loc) => self.body().basic_blocks()[loc.block].is_cleanup,
+                Ok(loc) => self.body().basic_blocks[loc.block].is_cleanup,
                 Err(_) => true,
             }
         );
diff --git a/compiler/rustc_const_eval/src/interpret/intrinsics/caller_location.rs b/compiler/rustc_const_eval/src/interpret/intrinsics/caller_location.rs
index 5864b92155287..91f4f04251721 100644
--- a/compiler/rustc_const_eval/src/interpret/intrinsics/caller_location.rs
+++ b/compiler/rustc_const_eval/src/interpret/intrinsics/caller_location.rs
@@ -28,7 +28,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             let mut source_info = *frame.body.source_info(loc);
 
             // If this is a `Call` terminator, use the `fn_span` instead.
-            let block = &frame.body.basic_blocks()[loc.block];
+            let block = &frame.body.basic_blocks[loc.block];
             if loc.statement_index == block.statements.len() {
                 debug!(
                     "find_closest_untracked_caller_location: got terminator {:?} ({:?})",
diff --git a/compiler/rustc_const_eval/src/interpret/step.rs b/compiler/rustc_const_eval/src/interpret/step.rs
index fea158a9fe450..683e11ff7e0e5 100644
--- a/compiler/rustc_const_eval/src/interpret/step.rs
+++ b/compiler/rustc_const_eval/src/interpret/step.rs
@@ -53,7 +53,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             self.pop_stack_frame(/* unwinding */ true)?;
             return Ok(true);
         };
-        let basic_block = &self.body().basic_blocks()[loc.block];
+        let basic_block = &self.body().basic_blocks[loc.block];
 
         if let Some(stmt) = basic_block.statements.get(loc.statement_index) {
             let old_frames = self.frame_idx();
diff --git a/compiler/rustc_const_eval/src/transform/check_consts/check.rs b/compiler/rustc_const_eval/src/transform/check_consts/check.rs
index 0adb88a180f8b..cbfdb47dd1a40 100644
--- a/compiler/rustc_const_eval/src/transform/check_consts/check.rs
+++ b/compiler/rustc_const_eval/src/transform/check_consts/check.rs
@@ -135,7 +135,7 @@ impl<'mir, 'tcx> Qualifs<'mir, 'tcx> {
         // qualifs for the return type.
         let return_block = ccx
             .body
-            .basic_blocks()
+            .basic_blocks
             .iter_enumerated()
             .find(|(_, block)| matches!(block.terminator().kind, TerminatorKind::Return))
             .map(|(bb, _)| bb);
diff --git a/compiler/rustc_const_eval/src/transform/promote_consts.rs b/compiler/rustc_const_eval/src/transform/promote_consts.rs
index 161c89e3242b9..6301388d1e840 100644
--- a/compiler/rustc_const_eval/src/transform/promote_consts.rs
+++ b/compiler/rustc_const_eval/src/transform/promote_consts.rs
@@ -710,7 +710,7 @@ impl<'a, 'tcx> Promoter<'a, 'tcx> {
     }
 
     fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
-        let last = self.promoted.basic_blocks().last().unwrap();
+        let last = self.promoted.basic_blocks.last().unwrap();
         let data = &mut self.promoted[last];
         data.statements.push(Statement {
             source_info: SourceInfo::outermost(span),
@@ -803,7 +803,7 @@ impl<'a, 'tcx> Promoter<'a, 'tcx> {
                         self.visit_operand(arg, loc);
                     }
 
-                    let last = self.promoted.basic_blocks().last().unwrap();
+                    let last = self.promoted.basic_blocks.last().unwrap();
                     let new_target = self.new_block();
 
                     *self.promoted[last].terminator_mut() = Terminator {
@@ -1041,7 +1041,7 @@ pub fn is_const_fn_in_array_repeat_expression<'tcx>(
         _ => {}
     }
 
-    for block in body.basic_blocks() {
+    for block in body.basic_blocks.iter() {
         if let Some(Terminator { kind: TerminatorKind::Call { func, destination, .. }, .. }) =
             &block.terminator
         {
diff --git a/compiler/rustc_const_eval/src/transform/validate.rs b/compiler/rustc_const_eval/src/transform/validate.rs
index 1a14cd79fa069..45a94972c1134 100644
--- a/compiler/rustc_const_eval/src/transform/validate.rs
+++ b/compiler/rustc_const_eval/src/transform/validate.rs
@@ -140,8 +140,8 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         if bb == START_BLOCK {
             self.fail(location, "start block must not have predecessors")
         }
-        if let Some(bb) = self.body.basic_blocks().get(bb) {
-            let src = self.body.basic_blocks().get(location.block).unwrap();
+        if let Some(bb) = self.body.basic_blocks.get(bb) {
+            let src = self.body.basic_blocks.get(location.block).unwrap();
             match (src.is_cleanup, bb.is_cleanup, edge_kind) {
                 // Non-cleanup blocks can jump to non-cleanup blocks along non-unwind edges
                 (false, false, EdgeKind::Normal)
@@ -881,13 +881,13 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
             }
             TerminatorKind::Resume | TerminatorKind::Abort => {
                 let bb = location.block;
-                if !self.body.basic_blocks()[bb].is_cleanup {
+                if !self.body.basic_blocks[bb].is_cleanup {
                     self.fail(location, "Cannot `Resume` or `Abort` from non-cleanup basic block")
                 }
             }
             TerminatorKind::Return => {
                 let bb = location.block;
-                if self.body.basic_blocks()[bb].is_cleanup {
+                if self.body.basic_blocks[bb].is_cleanup {
                     self.fail(location, "Cannot `Return` from cleanup basic block")
                 }
             }
diff --git a/compiler/rustc_error_messages/locales/en-US/infer.ftl b/compiler/rustc_error_messages/locales/en-US/infer.ftl
new file mode 100644
index 0000000000000..60086cd6e477f
--- /dev/null
+++ b/compiler/rustc_error_messages/locales/en-US/infer.ftl
@@ -0,0 +1,112 @@
+infer_opaque_hidden_type =
+    opaque type's hidden type cannot be another opaque type from the same scope
+    .label = one of the two opaque types used here has to be outside its defining scope
+    .opaque_type = opaque type whose hidden type is being assigned
+    .hidden_type = opaque type being used as hidden type
+
+infer_type_annotations_needed = {$source_kind ->
+    [closure] type annotations needed for the closure `{$source_name}`
+    [normal] type annotations needed for `{$source_name}`
+    *[other] type annotations needed
+}
+    .label = type must be known at this point
+
+infer_label_bad = {$bad_kind ->
+    *[other] cannot infer type
+    [more_info] cannot infer {$prefix_kind ->
+        *[type] type for {$prefix}
+        [const_with_param] the value of const parameter
+        [const] the value of the constant
+    } `{$name}`{$has_parent ->
+        [true] {" "}declared on the {$parent_prefix} `{$parent_name}`
+        *[false] {""}
+    }
+}
+
+infer_source_kind_subdiag_let = {$kind ->
+    [with_pattern] consider giving `{$name}` an explicit type
+    [closure] consider giving this closure parameter an explicit type
+    *[other] consider giving this pattern a type
+}{$x_kind ->
+    [has_name] , where the {$prefix_kind ->
+        *[type] type for {$prefix}
+        [const_with_param] the value of const parameter
+        [const] the value of the constant
+    } `{$arg_name}` is specified
+    [underscore] , where the placeholders `_` are specified
+    *[empty] {""}
+}
+
+infer_source_kind_subdiag_generic_label =
+    cannot infer {$is_type ->
+    [true] type
+    *[false] the value
+    } of the {$is_type ->
+    [true] type
+    *[false] const
+    } {$parent_exists ->
+    [true] parameter `{$param_name}` declared on the {$parent_prefix} `{$parent_name}`
+    *[false] parameter {$param_name}
+    }
+
+infer_source_kind_subdiag_generic_suggestion =
+    consider specifying the generic {$arg_count ->
+    [one] argument
+    *[other] arguments
+    }
+
+infer_source_kind_fully_qualified =
+    try using a fully qualified path to specify the expected types
+
+infer_source_kind_closure_return =
+    try giving this closure an explicit return type
+
+# generator_kind  may need to be translated
+infer_need_type_info_in_generator =
+    type inside {$generator_kind ->
+    [async_block] `async` block
+    [async_closure] `async` closure
+    [async_fn] `async fn` body
+    *[generator] generator
+    } must be known in this context
+
+
+infer_subtype = ...so that the {$requirement ->
+    [method_compat] method type is compatible with trait
+    [type_compat] associated type is compatible with trait
+    [const_compat] const is compatible with trait
+    [expr_assignable] expression is assignable
+    [if_else_different] `if` and `else` have incompatible types
+    [no_else] `if` missing an `else` returns `()`
+    [fn_main_correct_type] `main` function has the correct type
+    [fn_start_correct_type] #[start]` function has the correct type
+    [intristic_correct_type] intrinsic has the correct type
+    [method_correct_type] method receiver has the correct type
+    *[other] types are compatible
+}
+infer_subtype_2 = ...so that {$requirement ->
+    [method_compat] method type is compatible with trait
+    [type_compat] associated type is compatible with trait
+    [const_compat] const is compatible with trait
+    [expr_assignable] expression is assignable
+    [if_else_different] `if` and `else` have incompatible types
+    [no_else] `if` missing an `else` returns `()`
+    [fn_main_correct_type] `main` function has the correct type
+    [fn_start_correct_type] #[start]` function has the correct type
+    [intristic_correct_type] intrinsic has the correct type
+    [method_correct_type] method receiver has the correct type
+    *[other] types are compatible
+}
+
+infer_reborrow = ...so that reference does not outlive borrowed content
+infer_reborrow_upvar = ...so that closure can access `{$name}`
+infer_relate_object_bound = ...so that it can be closed over into an object
+infer_data_borrowed = ...so that the type `{$name}` is not borrowed for too long
+infer_reference_outlives_referent = ...so that the reference type `{$name}` does not outlive the data it points at
+infer_relate_param_bound = ...so that the type `{$name}` will meet its required lifetime bounds{$continues ->
+[true] ...
+*[false] {""}
+}
+infer_relate_param_bound_2 = ...that is required by this bound
+infer_relate_region_param_bound = ...so that the declared lifetime parameter bounds are satisfied
+infer_compare_impl_item_obligation = ...so that the definition in impl matches the definition from the trait
diff --git a/compiler/rustc_error_messages/src/lib.rs b/compiler/rustc_error_messages/src/lib.rs
index b18d1f553e499..b17668dc0ae77 100644
--- a/compiler/rustc_error_messages/src/lib.rs
+++ b/compiler/rustc_error_messages/src/lib.rs
@@ -41,6 +41,7 @@ fluent_messages! {
     driver => "../locales/en-US/driver.ftl",
     expand => "../locales/en-US/expand.ftl",
     interface => "../locales/en-US/interface.ftl",
+    infer => "../locales/en-US/infer.ftl",
     lint => "../locales/en-US/lint.ftl",
     parser => "../locales/en-US/parser.ftl",
     passes => "../locales/en-US/passes.ftl",
diff --git a/compiler/rustc_infer/Cargo.toml b/compiler/rustc_infer/Cargo.toml
index 02ac83a5e8b25..aced787d67116 100644
--- a/compiler/rustc_infer/Cargo.toml
+++ b/compiler/rustc_infer/Cargo.toml
@@ -15,6 +15,7 @@ rustc_hir = { path = "../rustc_hir" }
 rustc_index = { path = "../rustc_index" }
 rustc_macros = { path = "../rustc_macros" }
 rustc_serialize = { path = "../rustc_serialize" }
+rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
 rustc_target = { path = "../rustc_target" }
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
diff --git a/compiler/rustc_infer/src/errors.rs b/compiler/rustc_infer/src/errors.rs
new file mode 100644
index 0000000000000..938f8aa77a5b4
--- /dev/null
+++ b/compiler/rustc_infer/src/errors.rs
@@ -0,0 +1,254 @@
+use rustc_errors::{fluent, AddSubdiagnostic, DiagnosticMessage, DiagnosticStyledString};
+use rustc_hir::FnRetTy;
+use rustc_macros::SessionDiagnostic;
+use rustc_span::{BytePos, Span};
+
+use crate::infer::error_reporting::{
+    need_type_info::{GeneratorKindAsDiagArg, UnderspecifiedArgKind},
+    ObligationCauseAsDiagArg,
+};
+
+#[derive(SessionDiagnostic)]
+#[diag(infer::opaque_hidden_type)]
+pub struct OpaqueHiddenTypeDiag {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    #[note(infer::opaque_type)]
+    pub opaque_type: Span,
+    #[note(infer::hidden_type)]
+    pub hidden_type: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(infer::type_annotations_needed, code = "E0282")]
+pub struct AnnotationRequired<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub source_kind: &'static str,
+    pub source_name: &'a str,
+    #[label]
+    pub failure_span: Option<Span>,
+    #[subdiagnostic]
+    pub bad_label: Option<InferenceBadError<'a>>,
+    #[subdiagnostic]
+    pub infer_subdiags: Vec<SourceKindSubdiag<'a>>,
+    #[subdiagnostic]
+    pub multi_suggestions: Vec<SourceKindMultiSuggestion<'a>>,
+}
+
+// Copy of `AnnotationRequired` for E0283
+#[derive(SessionDiagnostic)]
+#[diag(infer::type_annotations_needed, code = "E0283")]
+pub struct AmbigousImpl<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub source_kind: &'static str,
+    pub source_name: &'a str,
+    #[label]
+    pub failure_span: Option<Span>,
+    #[subdiagnostic]
+    pub bad_label: Option<InferenceBadError<'a>>,
+    #[subdiagnostic]
+    pub infer_subdiags: Vec<SourceKindSubdiag<'a>>,
+    #[subdiagnostic]
+    pub multi_suggestions: Vec<SourceKindMultiSuggestion<'a>>,
+}
+
+// Copy of `AnnotationRequired` for E0284
+#[derive(SessionDiagnostic)]
+#[diag(infer::type_annotations_needed, code = "E0284")]
+pub struct AmbigousReturn<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub source_kind: &'static str,
+    pub source_name: &'a str,
+    #[label]
+    pub failure_span: Option<Span>,
+    #[subdiagnostic]
+    pub bad_label: Option<InferenceBadError<'a>>,
+    #[subdiagnostic]
+    pub infer_subdiags: Vec<SourceKindSubdiag<'a>>,
+    #[subdiagnostic]
+    pub multi_suggestions: Vec<SourceKindMultiSuggestion<'a>>,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(infer::need_type_info_in_generator, code = "E0698")]
+pub struct NeedTypeInfoInGenerator<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub generator_kind: GeneratorKindAsDiagArg,
+    #[subdiagnostic]
+    pub bad_label: InferenceBadError<'a>,
+}
+
+// Used when a better one isn't available
+#[derive(SessionSubdiagnostic)]
+#[label(infer::label_bad)]
+pub struct InferenceBadError<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub bad_kind: &'static str,
+    pub prefix_kind: UnderspecifiedArgKind,
+    pub has_parent: bool,
+    pub prefix: &'a str,
+    pub parent_prefix: &'a str,
+    pub parent_name: String,
+    pub name: String,
+}
+
+#[derive(SessionSubdiagnostic)]
+pub enum SourceKindSubdiag<'a> {
+    #[suggestion_verbose(
+        infer::source_kind_subdiag_let,
+        code = ": {type_name}",
+        applicability = "has-placeholders"
+    )]
+    LetLike {
+        #[primary_span]
+        span: Span,
+        name: String,
+        type_name: String,
+        kind: &'static str,
+        x_kind: &'static str,
+        prefix_kind: UnderspecifiedArgKind,
+        prefix: &'a str,
+        arg_name: String,
+    },
+    #[label(infer::source_kind_subdiag_generic_label)]
+    GenericLabel {
+        #[primary_span]
+        span: Span,
+        is_type: bool,
+        param_name: String,
+        parent_exists: bool,
+        parent_prefix: String,
+        parent_name: String,
+    },
+    #[suggestion_verbose(
+        infer::source_kind_subdiag_generic_suggestion,
+        code = "::<{args}>",
+        applicability = "has-placeholders"
+    )]
+    GenericSuggestion {
+        #[primary_span]
+        span: Span,
+        arg_count: usize,
+        args: String,
+    },
+}
+
+// Has to be implemented manually because multipart suggestions are not supported by the derive macro.
+// Would be a part of `SourceKindSubdiag` otherwise.
+pub enum SourceKindMultiSuggestion<'a> {
+    FullyQualified {
+        span: Span,
+        def_path: String,
+        adjustment: &'a str,
+        successor: (&'a str, BytePos),
+    },
+    ClosureReturn {
+        ty_info: String,
+        data: &'a FnRetTy<'a>,
+        should_wrap_expr: Option<Span>,
+    },
+}
+
+impl AddSubdiagnostic for SourceKindMultiSuggestion<'_> {
+    fn add_to_diagnostic(self, diag: &mut rustc_errors::Diagnostic) {
+        match self {
+            Self::FullyQualified { span, def_path, adjustment, successor } => {
+                let suggestion = vec![
+                    (span.shrink_to_lo(), format!("{def_path}({adjustment}")),
+                    (span.shrink_to_hi().with_hi(successor.1), successor.0.to_string()),
+                ];
+                diag.multipart_suggestion_verbose(
+                    fluent::infer::source_kind_fully_qualified,
+                    suggestion,
+                    rustc_errors::Applicability::HasPlaceholders,
+                );
+            }
+            Self::ClosureReturn { ty_info, data, should_wrap_expr } => {
+                let (arrow, post) = match data {
+                    FnRetTy::DefaultReturn(_) => ("-> ", " "),
+                    _ => ("", ""),
+                };
+                let suggestion = match should_wrap_expr {
+                    Some(end_span) => vec![
+                        (data.span(), format!("{}{}{}{{ ", arrow, ty_info, post)),
+                        (end_span, " }".to_string()),
+                    ],
+                    None => vec![(data.span(), format!("{}{}{}", arrow, ty_info, post))],
+                };
+                diag.multipart_suggestion_verbose(
+                    fluent::infer::source_kind_closure_return,
+                    suggestion,
+                    rustc_errors::Applicability::HasPlaceholders,
+                );
+            }
+        }
+    }
+}
+
+pub enum RegionOriginNote<'a> {
+    Plain {
+        span: Span,
+        msg: DiagnosticMessage,
+    },
+    WithName {
+        span: Span,
+        msg: DiagnosticMessage,
+        name: &'a str,
+        continues: bool,
+    },
+    WithRequirement {
+        span: Span,
+        requirement: ObligationCauseAsDiagArg<'a>,
+        expected_found: Option<(DiagnosticStyledString, DiagnosticStyledString)>,
+    },
+}
+
+impl AddSubdiagnostic for RegionOriginNote<'_> {
+    fn add_to_diagnostic(self, diag: &mut rustc_errors::Diagnostic) {
+        let mut label_or_note = |span, msg: DiagnosticMessage| {
+            let sub_count = diag.children.iter().filter(|d| d.span.is_dummy()).count();
+            let expanded_sub_count = diag.children.iter().filter(|d| !d.span.is_dummy()).count();
+            let span_is_primary = diag.span.primary_spans().iter().all(|&sp| sp == span);
+            if span_is_primary && sub_count == 0 && expanded_sub_count == 0 {
+                diag.span_label(span, msg);
+            } else if span_is_primary && expanded_sub_count == 0 {
+                diag.note(msg);
+            } else {
+                diag.span_note(span, msg);
+            }
+        };
+        match self {
+            RegionOriginNote::Plain { span, msg } => {
+                label_or_note(span, msg);
+            }
+            RegionOriginNote::WithName { span, msg, name, continues } => {
+                label_or_note(span, msg);
+                diag.set_arg("name", name);
+                diag.set_arg("continues", continues);
+            }
+            RegionOriginNote::WithRequirement {
+                span,
+                requirement,
+                expected_found: Some((expected, found)),
+            } => {
+                label_or_note(span, fluent::infer::subtype);
+                diag.set_arg("requirement", requirement);
+
+                diag.note_expected_found(&"", expected, &"", found);
+            }
+            RegionOriginNote::WithRequirement { span, requirement, expected_found: None } => {
+                // FIXME: this really should be handled at some earlier stage. Our
+                // handling of region checking when type errors are present is
+                // *terrible*.
+                label_or_note(span, fluent::infer::subtype_2);
+                diag.set_arg("requirement", requirement);
+            }
+        };
+    }
+}
diff --git a/compiler/rustc_infer/src/infer/error_reporting/mod.rs b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
index 59ea1f3f9de45..465508e12058f 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/mod.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
@@ -58,7 +58,7 @@ use crate::traits::{
 };
 
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
-use rustc_errors::{pluralize, struct_span_err, Diagnostic, ErrorGuaranteed};
+use rustc_errors::{pluralize, struct_span_err, Diagnostic, ErrorGuaranteed, IntoDiagnosticArg};
 use rustc_errors::{Applicability, DiagnosticBuilder, DiagnosticStyledString, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
@@ -78,7 +78,7 @@ use std::{cmp, fmt, iter};
 
 mod note;
 
-mod need_type_info;
+pub(crate) mod need_type_info;
 pub use need_type_info::TypeAnnotationNeeded;
 
 pub mod nice_region_error;
@@ -1588,9 +1588,14 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                             Mismatch::Variable(infer::ExpectedFound { expected, found }),
                         )
                     }
+                    ValuePairs::Terms(infer::ExpectedFound {
+                        expected: ty::Term::Const(_),
+                        found: ty::Term::Const(_),
+                    }) => (false, Mismatch::Fixed("constant")),
                     ValuePairs::TraitRefs(_) | ValuePairs::PolyTraitRefs(_) => {
                         (false, Mismatch::Fixed("trait"))
                     }
+                    ValuePairs::Regions(_) => (false, Mismatch::Fixed("lifetime")),
                     _ => (false, Mismatch::Fixed("type")),
                 };
                 let vals = match self.values_str(values) {
@@ -2881,6 +2886,30 @@ impl<'tcx> ObligationCauseExt<'tcx> for ObligationCause<'tcx> {
     }
 }
 
+/// Newtype to allow implementing IntoDiagnosticArg
+pub struct ObligationCauseAsDiagArg<'tcx>(pub ObligationCause<'tcx>);
+
+impl IntoDiagnosticArg for ObligationCauseAsDiagArg<'_> {
+    fn into_diagnostic_arg(self) -> rustc_errors::DiagnosticArgValue<'static> {
+        use crate::traits::ObligationCauseCode::*;
+        let kind = match self.0.code() {
+            CompareImplItemObligation { kind: ty::AssocKind::Fn, .. } => "method_compat",
+            CompareImplItemObligation { kind: ty::AssocKind::Type, .. } => "type_compat",
+            CompareImplItemObligation { kind: ty::AssocKind::Const, .. } => "const_compat",
+            ExprAssignable => "expr_assignable",
+            IfExpression { .. } => "if_else_different",
+            IfExpressionWithNoElse => "no_else",
+            MainFunctionType => "fn_main_correct_type",
+            StartFunctionType => "fn_start_correct_type",
+            IntrinsicType => "intristic_correct_type",
+            MethodReceiver => "method_correct_type",
+            _ => "other",
+        }
+        .into();
+        rustc_errors::DiagnosticArgValue::Str(kind)
+    }
+}
+
 /// This is a bare signal of what kind of type we're dealing with. `ty::TyKind` tracks
 /// extra information about each type, but we only care about the category.
 #[derive(Clone, Copy, PartialEq, Eq, Hash)]
diff --git a/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs b/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs
index 561d1354edd21..e990fe7ecb504 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs
@@ -1,6 +1,10 @@
+use crate::errors::{
+    AmbigousImpl, AmbigousReturn, AnnotationRequired, InferenceBadError, NeedTypeInfoInGenerator,
+    SourceKindMultiSuggestion, SourceKindSubdiag,
+};
 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use crate::infer::InferCtxt;
-use rustc_errors::{pluralize, struct_span_err, Applicability, DiagnosticBuilder, ErrorGuaranteed};
+use rustc_errors::{DiagnosticBuilder, ErrorGuaranteed, IntoDiagnosticArg};
 use rustc_hir as hir;
 use rustc_hir::def::Res;
 use rustc_hir::def::{CtorOf, DefKind, Namespace};
@@ -14,6 +18,7 @@ use rustc_middle::ty::print::{FmtPrinter, PrettyPrinter, Print, Printer};
 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, Subst, SubstsRef};
 use rustc_middle::ty::{self, DefIdTree, InferConst};
 use rustc_middle::ty::{IsSuggestable, Ty, TyCtxt, TypeckResults};
+use rustc_session::SessionDiagnostic;
 use rustc_span::symbol::{kw, Ident};
 use rustc_span::{BytePos, Span};
 use std::borrow::Cow;
@@ -60,38 +65,49 @@ pub struct InferenceDiagnosticsParentData {
     name: String,
 }
 
+#[derive(Clone)]
 pub enum UnderspecifiedArgKind {
     Type { prefix: Cow<'static, str> },
     Const { is_parameter: bool },
 }
 
 impl InferenceDiagnosticsData {
-    /// Generate a label for a generic argument which can't be inferred. When not
-    /// much is known about the argument, `use_diag` may be used to describe the
-    /// labeled value.
-    fn cannot_infer_msg(&self) -> String {
-        if self.name == "_" && matches!(self.kind, UnderspecifiedArgKind::Type { .. }) {
-            return "cannot infer type".to_string();
-        }
-
-        let suffix = match &self.parent {
-            Some(parent) => parent.suffix_string(),
-            None => String::new(),
-        };
-
-        // For example: "cannot infer type for type parameter `T`"
-        format!("cannot infer {} `{}`{}", self.kind.prefix_string(), self.name, suffix)
+    fn can_add_more_info(&self) -> bool {
+        !(self.name == "_" && matches!(self.kind, UnderspecifiedArgKind::Type { .. }))
     }
 
-    fn where_x_is_specified(&self, in_type: Ty<'_>) -> String {
+    fn where_x_is_kind(&self, in_type: Ty<'_>) -> &'static str {
         if in_type.is_ty_infer() {
-            String::new()
+            "empty"
         } else if self.name == "_" {
             // FIXME: Consider specializing this message if there is a single `_`
             // in the type.
-            ", where the placeholders `_` are specified".to_string()
+            "underscore"
         } else {
-            format!(", where the {} `{}` is specified", self.kind.prefix_string(), self.name)
+            "has_name"
+        }
+    }
+
+    /// Generate a label for a generic argument which can't be inferred. When not
+    /// much is known about the argument, `use_diag` may be used to describe the
+    /// labeled value.
+    fn make_bad_error(&self, span: Span) -> InferenceBadError<'_> {
+        let has_parent = self.parent.is_some();
+        let bad_kind = if self.can_add_more_info() { "more_info" } else { "other" };
+        let (parent_prefix, parent_name) = self
+            .parent
+            .as_ref()
+            .map(|parent| (parent.prefix, parent.name.clone()))
+            .unwrap_or_default();
+        InferenceBadError {
+            span,
+            bad_kind,
+            prefix_kind: self.kind.clone(),
+            prefix: self.kind.try_get_prefix().unwrap_or_default(),
+            name: self.name.clone(),
+            has_parent,
+            parent_prefix,
+            parent_name,
         }
     }
 }
@@ -113,18 +129,24 @@ impl InferenceDiagnosticsParentData {
     fn for_def_id(tcx: TyCtxt<'_>, def_id: DefId) -> Option<InferenceDiagnosticsParentData> {
         Self::for_parent_def_id(tcx, tcx.parent(def_id))
     }
+}
 
-    fn suffix_string(&self) -> String {
-        format!(" declared on the {} `{}`", self.prefix, self.name)
+impl IntoDiagnosticArg for UnderspecifiedArgKind {
+    fn into_diagnostic_arg(self) -> rustc_errors::DiagnosticArgValue<'static> {
+        let kind = match self {
+            Self::Type { .. } => "type",
+            Self::Const { is_parameter: true } => "const_with_param",
+            Self::Const { is_parameter: false } => "const",
+        };
+        rustc_errors::DiagnosticArgValue::Str(kind.into())
     }
 }
 
 impl UnderspecifiedArgKind {
-    fn prefix_string(&self) -> Cow<'static, str> {
+    fn try_get_prefix(&self) -> Option<&str> {
         match self {
-            Self::Type { prefix } => format!("type for {}", prefix).into(),
-            Self::Const { is_parameter: true } => "the value of const parameter".into(),
-            Self::Const { is_parameter: false } => "the value of the constant".into(),
+            Self::Type { prefix } => Some(prefix.as_ref()),
+            Self::Const { .. } => None,
         }
     }
 }
@@ -303,11 +325,44 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         arg_data: InferenceDiagnosticsData,
         error_code: TypeAnnotationNeeded,
     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
-        let error_code = error_code.into();
-        let mut err =
-            self.tcx.sess.struct_span_err_with_code(span, "type annotations needed", error_code);
-        err.span_label(span, arg_data.cannot_infer_msg());
-        err
+        let source_kind = "other";
+        let source_name = "";
+        let failure_span = None;
+        let infer_subdiags = Vec::new();
+        let multi_suggestions = Vec::new();
+        let bad_label = Some(arg_data.make_bad_error(span));
+        match error_code {
+            TypeAnnotationNeeded::E0282 => AnnotationRequired {
+                span,
+                source_kind,
+                source_name,
+                failure_span,
+                infer_subdiags,
+                multi_suggestions,
+                bad_label,
+            }
+            .into_diagnostic(&self.tcx.sess.parse_sess),
+            TypeAnnotationNeeded::E0283 => AmbigousImpl {
+                span,
+                source_kind,
+                source_name,
+                failure_span,
+                infer_subdiags,
+                multi_suggestions,
+                bad_label,
+            }
+            .into_diagnostic(&self.tcx.sess.parse_sess),
+            TypeAnnotationNeeded::E0284 => AmbigousReturn {
+                span,
+                source_kind,
+                source_name,
+                failure_span,
+                infer_subdiags,
+                multi_suggestions,
+                bad_label,
+            }
+            .into_diagnostic(&self.tcx.sess.parse_sess),
+        }
     }
 
     pub fn emit_inference_failure_err(
@@ -340,48 +395,39 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             return self.bad_inference_failure_err(failure_span, arg_data, error_code)
         };
 
-        let error_code = error_code.into();
-        let mut err = self.tcx.sess.struct_span_err_with_code(
-            span,
-            &format!("type annotations needed{}", kind.ty_msg(self)),
-            error_code,
-        );
-
-        if should_label_span && !failure_span.overlaps(span) {
-            err.span_label(failure_span, "type must be known at this point");
-        }
+        let (source_kind, name) = kind.ty_localized_msg(self);
+        let failure_span = if should_label_span && !failure_span.overlaps(span) {
+            Some(failure_span)
+        } else {
+            None
+        };
 
+        let mut infer_subdiags = Vec::new();
+        let mut multi_suggestions = Vec::new();
         match kind {
             InferSourceKind::LetBinding { insert_span, pattern_name, ty } => {
-                let suggestion_msg = if let Some(name) = pattern_name {
-                    format!(
-                        "consider giving `{}` an explicit type{}",
-                        name,
-                        arg_data.where_x_is_specified(ty)
-                    )
-                } else {
-                    format!(
-                        "consider giving this pattern a type{}",
-                        arg_data.where_x_is_specified(ty)
-                    )
-                };
-                err.span_suggestion_verbose(
-                    insert_span,
-                    &suggestion_msg,
-                    format!(": {}", ty_to_string(self, ty)),
-                    Applicability::HasPlaceholders,
-                );
+                infer_subdiags.push(SourceKindSubdiag::LetLike {
+                    span: insert_span,
+                    name: pattern_name.map(|name| name.to_string()).unwrap_or_else(String::new),
+                    x_kind: arg_data.where_x_is_kind(ty),
+                    prefix_kind: arg_data.kind.clone(),
+                    prefix: arg_data.kind.try_get_prefix().unwrap_or_default(),
+                    arg_name: arg_data.name,
+                    kind: if pattern_name.is_some() { "with_pattern" } else { "other" },
+                    type_name: ty_to_string(self, ty),
+                });
             }
             InferSourceKind::ClosureArg { insert_span, ty } => {
-                err.span_suggestion_verbose(
-                    insert_span,
-                    &format!(
-                        "consider giving this closure parameter an explicit type{}",
-                        arg_data.where_x_is_specified(ty)
-                    ),
-                    format!(": {}", ty_to_string(self, ty)),
-                    Applicability::HasPlaceholders,
-                );
+                infer_subdiags.push(SourceKindSubdiag::LetLike {
+                    span: insert_span,
+                    name: String::new(),
+                    x_kind: arg_data.where_x_is_kind(ty),
+                    prefix_kind: arg_data.kind.clone(),
+                    prefix: arg_data.kind.try_get_prefix().unwrap_or_default(),
+                    arg_name: arg_data.name,
+                    kind: "closure",
+                    type_name: ty_to_string(self, ty),
+                });
             }
             InferSourceKind::GenericArg {
                 insert_span,
@@ -393,19 +439,20 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 let generics = self.tcx.generics_of(generics_def_id);
                 let is_type = matches!(arg.unpack(), GenericArgKind::Type(_));
 
-                let cannot_infer_msg = format!(
-                    "cannot infer {} of the {} parameter `{}`{}",
-                    if is_type { "type" } else { "the value" },
-                    if is_type { "type" } else { "const" },
-                    generics.params[argument_index].name,
-                    // We use the `generics_def_id` here, as even when suggesting `None::<T>`,
-                    // the type parameter `T` was still declared on the enum, not on the
-                    // variant.
+                let (parent_exists, parent_prefix, parent_name) =
                     InferenceDiagnosticsParentData::for_parent_def_id(self.tcx, generics_def_id)
-                        .map_or(String::new(), |parent| parent.suffix_string()),
-                );
+                        .map_or((false, String::new(), String::new()), |parent| {
+                            (true, parent.prefix.to_string(), parent.name)
+                        });
 
-                err.span_label(span, cannot_infer_msg);
+                infer_subdiags.push(SourceKindSubdiag::GenericLabel {
+                    span,
+                    is_type,
+                    param_name: generics.params[argument_index].name.to_string(),
+                    parent_exists,
+                    parent_prefix,
+                    parent_name,
+                });
 
                 let args = fmt_printer(self, Namespace::TypeNS)
                     .comma_sep(generic_args.iter().copied().map(|arg| {
@@ -435,15 +482,11 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     .unwrap()
                     .into_buffer();
 
-                err.span_suggestion_verbose(
-                    insert_span,
-                    &format!(
-                        "consider specifying the generic argument{}",
-                        pluralize!(generic_args.len()),
-                    ),
-                    format!("::<{}>", args),
-                    Applicability::HasPlaceholders,
-                );
+                infer_subdiags.push(SourceKindSubdiag::GenericSuggestion {
+                    span: insert_span,
+                    arg_count: generic_args.len(),
+                    args,
+                });
             }
             InferSourceKind::FullyQualifiedMethodCall { receiver, successor, substs, def_id } => {
                 let printer = fmt_printer(self, Namespace::ValueNS);
@@ -468,37 +511,54 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     _ => "",
                 };
 
-                let suggestion = vec![
-                    (receiver.span.shrink_to_lo(), format!("{def_path}({adjustment}")),
-                    (receiver.span.shrink_to_hi().with_hi(successor.1), successor.0.to_string()),
-                ];
-                err.multipart_suggestion_verbose(
-                    "try using a fully qualified path to specify the expected types",
-                    suggestion,
-                    Applicability::HasPlaceholders,
-                );
+                multi_suggestions.push(SourceKindMultiSuggestion::FullyQualified {
+                    span: receiver.span,
+                    def_path,
+                    adjustment,
+                    successor,
+                });
             }
             InferSourceKind::ClosureReturn { ty, data, should_wrap_expr } => {
-                let ret = ty_to_string(self, ty);
-                let (arrow, post) = match data {
-                    FnRetTy::DefaultReturn(_) => ("-> ", " "),
-                    _ => ("", ""),
-                };
-                let suggestion = match should_wrap_expr {
-                    Some(end_span) => vec![
-                        (data.span(), format!("{}{}{}{{ ", arrow, ret, post)),
-                        (end_span, " }".to_string()),
-                    ],
-                    None => vec![(data.span(), format!("{}{}{}", arrow, ret, post))],
-                };
-                err.multipart_suggestion_verbose(
-                    "try giving this closure an explicit return type",
-                    suggestion,
-                    Applicability::HasPlaceholders,
-                );
+                let ty_info = ty_to_string(self, ty);
+                multi_suggestions.push(SourceKindMultiSuggestion::ClosureReturn {
+                    ty_info,
+                    data,
+                    should_wrap_expr,
+                });
+            }
+        }
+        match error_code {
+            TypeAnnotationNeeded::E0282 => AnnotationRequired {
+                span,
+                source_kind,
+                source_name: &name,
+                failure_span,
+                infer_subdiags,
+                multi_suggestions,
+                bad_label: None,
+            }
+            .into_diagnostic(&self.tcx.sess.parse_sess),
+            TypeAnnotationNeeded::E0283 => AmbigousImpl {
+                span,
+                source_kind,
+                source_name: &name,
+                failure_span,
+                infer_subdiags,
+                multi_suggestions,
+                bad_label: None,
+            }
+            .into_diagnostic(&self.tcx.sess.parse_sess),
+            TypeAnnotationNeeded::E0284 => AmbigousReturn {
+                span,
+                source_kind,
+                source_name: &name,
+                failure_span,
+                infer_subdiags,
+                multi_suggestions,
+                bad_label: None,
             }
+            .into_diagnostic(&self.tcx.sess.parse_sess),
         }
-        err
     }
 
     pub fn need_type_info_err_in_generator(
@@ -510,15 +570,26 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         let ty = self.resolve_vars_if_possible(ty);
         let data = self.extract_inference_diagnostics_data(ty.into(), None);
 
-        let mut err = struct_span_err!(
-            self.tcx.sess,
+        NeedTypeInfoInGenerator {
+            bad_label: data.make_bad_error(span),
             span,
-            E0698,
-            "type inside {} must be known in this context",
-            kind,
-        );
-        err.span_label(span, data.cannot_infer_msg());
-        err
+            generator_kind: GeneratorKindAsDiagArg(kind),
+        }
+        .into_diagnostic(&self.tcx.sess.parse_sess)
+    }
+}
+
+pub struct GeneratorKindAsDiagArg(pub hir::GeneratorKind);
+
+impl IntoDiagnosticArg for GeneratorKindAsDiagArg {
+    fn into_diagnostic_arg(self) -> rustc_errors::DiagnosticArgValue<'static> {
+        let kind = match self.0 {
+            hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "async_block",
+            hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "async_closure",
+            hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "async_fn",
+            hir::GeneratorKind::Gen => "generator",
+        };
+        rustc_errors::DiagnosticArgValue::Str(kind.into())
     }
 }
 
@@ -579,22 +650,22 @@ impl<'tcx> InferSource<'tcx> {
 }
 
 impl<'tcx> InferSourceKind<'tcx> {
-    fn ty_msg(&self, infcx: &InferCtxt<'_, 'tcx>) -> String {
+    fn ty_localized_msg(&self, infcx: &InferCtxt<'_, 'tcx>) -> (&'static str, String) {
         match *self {
             InferSourceKind::LetBinding { ty, .. }
             | InferSourceKind::ClosureArg { ty, .. }
             | InferSourceKind::ClosureReturn { ty, .. } => {
                 if ty.is_closure() {
-                    format!(" for the closure `{}`", closure_as_fn_str(infcx, ty))
+                    ("closure", closure_as_fn_str(infcx, ty))
                 } else if !ty.is_ty_infer() {
-                    format!(" for `{}`", ty_to_string(infcx, ty))
+                    ("normal", ty_to_string(infcx, ty))
                 } else {
-                    String::new()
+                    ("other", String::new())
                 }
             }
             // FIXME: We should be able to add some additional info here.
             InferSourceKind::GenericArg { .. }
-            | InferSourceKind::FullyQualifiedMethodCall { .. } => String::new(),
+            | InferSourceKind::FullyQualifiedMethodCall { .. } => ("other", String::new()),
         }
     }
 }
diff --git a/compiler/rustc_infer/src/infer/error_reporting/note.rs b/compiler/rustc_infer/src/infer/error_reporting/note.rs
index 8c465b0876002..cffdf56bb6d48 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/note.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/note.rs
@@ -1,96 +1,78 @@
-use crate::infer::error_reporting::{note_and_explain_region, ObligationCauseExt};
+use crate::errors::RegionOriginNote;
+use crate::infer::error_reporting::note_and_explain_region;
 use crate::infer::{self, InferCtxt, SubregionOrigin};
-use rustc_errors::{struct_span_err, Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
+use rustc_errors::{
+    fluent, struct_span_err, AddSubdiagnostic, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
+};
 use rustc_middle::traits::ObligationCauseCode;
 use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::{self, Region};
 
+use super::ObligationCauseAsDiagArg;
+
 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     pub(super) fn note_region_origin(&self, err: &mut Diagnostic, origin: &SubregionOrigin<'tcx>) {
-        let mut label_or_note = |span, msg: &str| {
-            let sub_count = err.children.iter().filter(|d| d.span.is_dummy()).count();
-            let expanded_sub_count = err.children.iter().filter(|d| !d.span.is_dummy()).count();
-            let span_is_primary = err.span.primary_spans().iter().all(|&sp| sp == span);
-            if span_is_primary && sub_count == 0 && expanded_sub_count == 0 {
-                err.span_label(span, msg);
-            } else if span_is_primary && expanded_sub_count == 0 {
-                err.note(msg);
-            } else {
-                err.span_note(span, msg);
-            }
-        };
         match *origin {
-            infer::Subtype(ref trace) => {
-                if let Some((expected, found)) = self.values_str(trace.values) {
-                    label_or_note(
-                        trace.cause.span,
-                        &format!("...so that the {}", trace.cause.as_requirement_str()),
-                    );
-
-                    err.note_expected_found(&"", expected, &"", found);
-                } else {
-                    // FIXME: this really should be handled at some earlier stage. Our
-                    // handling of region checking when type errors are present is
-                    // *terrible*.
-
-                    label_or_note(
-                        trace.cause.span,
-                        &format!("...so that {}", trace.cause.as_requirement_str()),
-                    );
-                }
-            }
-            infer::Reborrow(span) => {
-                label_or_note(span, "...so that reference does not outlive borrowed content");
+            infer::Subtype(ref trace) => RegionOriginNote::WithRequirement {
+                span: trace.cause.span,
+                requirement: ObligationCauseAsDiagArg(trace.cause.clone()),
+                expected_found: self.values_str(trace.values),
             }
+            .add_to_diagnostic(err),
+            infer::Reborrow(span) => RegionOriginNote::Plain { span, msg: fluent::infer::reborrow }
+                .add_to_diagnostic(err),
             infer::ReborrowUpvar(span, ref upvar_id) => {
                 let var_name = self.tcx.hir().name(upvar_id.var_path.hir_id);
-                label_or_note(span, &format!("...so that closure can access `{}`", var_name));
+                RegionOriginNote::WithName {
+                    span,
+                    msg: fluent::infer::reborrow,
+                    name: &var_name.to_string(),
+                    continues: false,
+                }
+                .add_to_diagnostic(err);
             }
             infer::RelateObjectBound(span) => {
-                label_or_note(span, "...so that it can be closed over into an object");
+                RegionOriginNote::Plain { span, msg: fluent::infer::relate_object_bound }
+                    .add_to_diagnostic(err);
             }
             infer::DataBorrowed(ty, span) => {
-                label_or_note(
+                RegionOriginNote::WithName {
                     span,
-                    &format!(
-                        "...so that the type `{}` is not borrowed for too long",
-                        self.ty_to_string(ty)
-                    ),
-                );
+                    msg: fluent::infer::data_borrowed,
+                    name: &self.ty_to_string(ty),
+                    continues: false,
+                }
+                .add_to_diagnostic(err);
             }
             infer::ReferenceOutlivesReferent(ty, span) => {
-                label_or_note(
+                RegionOriginNote::WithName {
                     span,
-                    &format!(
-                        "...so that the reference type `{}` does not outlive the data it points at",
-                        self.ty_to_string(ty)
-                    ),
-                );
+                    msg: fluent::infer::reference_outlives_referent,
+                    name: &self.ty_to_string(ty),
+                    continues: false,
+                }
+                .add_to_diagnostic(err);
             }
-            infer::RelateParamBound(span, t, opt_span) => {
-                label_or_note(
+            infer::RelateParamBound(span, ty, opt_span) => {
+                RegionOriginNote::WithName {
                     span,
-                    &format!(
-                        "...so that the type `{}` will meet its required lifetime bounds{}",
-                        self.ty_to_string(t),
-                        if opt_span.is_some() { "..." } else { "" },
-                    ),
-                );
+                    msg: fluent::infer::relate_param_bound,
+                    name: &self.ty_to_string(ty),
+                    continues: opt_span.is_some(),
+                }
+                .add_to_diagnostic(err);
                 if let Some(span) = opt_span {
-                    err.span_note(span, "...that is required by this bound");
+                    RegionOriginNote::Plain { span, msg: fluent::infer::relate_param_bound_2 }
+                        .add_to_diagnostic(err);
                 }
             }
             infer::RelateRegionParamBound(span) => {
-                label_or_note(
-                    span,
-                    "...so that the declared lifetime parameter bounds are satisfied",
-                );
+                RegionOriginNote::Plain { span, msg: fluent::infer::relate_region_param_bound }
+                    .add_to_diagnostic(err);
             }
             infer::CompareImplItemObligation { span, .. } => {
-                label_or_note(
-                    span,
-                    "...so that the definition in impl matches the definition from the trait",
-                );
+                RegionOriginNote::Plain { span, msg: fluent::infer::compare_impl_item_obligation }
+                    .add_to_diagnostic(err);
             }
             infer::CheckAssociatedTypeBounds { ref parent, .. } => {
                 self.note_region_origin(err, &parent);
diff --git a/compiler/rustc_infer/src/infer/opaque_types.rs b/compiler/rustc_infer/src/infer/opaque_types.rs
index e579afbf38954..233a5004a3931 100644
--- a/compiler/rustc_infer/src/infer/opaque_types.rs
+++ b/compiler/rustc_infer/src/infer/opaque_types.rs
@@ -1,3 +1,4 @@
+use crate::errors::OpaqueHiddenTypeDiag;
 use crate::infer::{DefiningAnchor, InferCtxt, InferOk};
 use crate::traits;
 use hir::def_id::{DefId, LocalDefId};
@@ -153,22 +154,11 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     if let Some(OpaqueTyOrigin::TyAlias) =
                         did2.as_local().and_then(|did2| self.opaque_type_origin(did2, cause.span))
                     {
-                        self.tcx
-                                .sess
-                                .struct_span_err(
-                                    cause.span,
-                                    "opaque type's hidden type cannot be another opaque type from the same scope",
-                                )
-                                .span_label(cause.span, "one of the two opaque types used here has to be outside its defining scope")
-                                .span_note(
-                                    self.tcx.def_span(def_id),
-                                    "opaque type whose hidden type is being assigned",
-                                )
-                                .span_note(
-                                    self.tcx.def_span(did2),
-                                    "opaque type being used as hidden type",
-                                )
-                                .emit();
+                        self.tcx.sess.emit_err(OpaqueHiddenTypeDiag {
+                            span: cause.span,
+                            hidden_type: self.tcx.def_span(did2),
+                            opaque_type: self.tcx.def_span(def_id),
+                        });
                     }
                 }
                 Some(self.register_hidden_type(
diff --git a/compiler/rustc_infer/src/lib.rs b/compiler/rustc_infer/src/lib.rs
index 8f02a6cc4a14d..602a9ab13f377 100644
--- a/compiler/rustc_infer/src/lib.rs
+++ b/compiler/rustc_infer/src/lib.rs
@@ -34,5 +34,6 @@ extern crate tracing;
 #[macro_use]
 extern crate rustc_middle;
 
+mod errors;
 pub mod infer;
 pub mod traits;
diff --git a/compiler/rustc_lint/src/for_loop_over_fallibles.rs b/compiler/rustc_lint/src/for_loop_over_fallibles.rs
new file mode 100644
index 0000000000000..2253546b5d357
--- /dev/null
+++ b/compiler/rustc_lint/src/for_loop_over_fallibles.rs
@@ -0,0 +1,188 @@
+use crate::{LateContext, LateLintPass, LintContext};
+
+use hir::{Expr, Pat};
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_infer::traits::TraitEngine;
+use rustc_infer::{infer::TyCtxtInferExt, traits::ObligationCause};
+use rustc_middle::ty::{self, List};
+use rustc_span::{sym, Span};
+use rustc_trait_selection::traits::TraitEngineExt;
+
+declare_lint! {
+    /// The `for_loop_over_fallibles` lint checks for `for` loops over `Option` or `Result` values.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// let opt = Some(1);
+    /// for x in opt { /* ... */}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Both `Option` and `Result` implement `IntoIterator` trait, which allows using them in a `for` loop.
+    /// `for` loop over `Option` or `Result` will iterate either 0 (if the value is `None`/`Err(_)`)
+    /// or 1 time (if the value is `Some(_)`/`Ok(_)`). This is not very useful and is more clearly expressed
+    /// via `if let`.
+    ///
+    /// `for` loop can also be accidentally written with the intention to call a function multiple times,
+    /// while the function returns `Some(_)`, in these cases `while let` loop should be used instead.
+    ///
+    /// The "intended" use of `IntoIterator` implementations for `Option` and `Result` is passing them to
+    /// generic code that expects something implementing `IntoIterator`. For example using `.chain(option)`
+    /// to optionally add a value to an iterator.
+    pub FOR_LOOP_OVER_FALLIBLES,
+    Warn,
+    "for-looping over an `Option` or a `Result`, which is more clearly expressed as an `if let`"
+}
+
+declare_lint_pass!(ForLoopOverFallibles => [FOR_LOOP_OVER_FALLIBLES]);
+
+impl<'tcx> LateLintPass<'tcx> for ForLoopOverFallibles {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        let Some((pat, arg)) = extract_for_loop(expr) else { return };
+
+        let ty = cx.typeck_results().expr_ty(arg);
+
+        let &ty::Adt(adt, substs) = ty.kind() else { return };
+
+        let (article, ty, var) = match adt.did() {
+            did if cx.tcx.is_diagnostic_item(sym::Option, did) => ("an", "Option", "Some"),
+            did if cx.tcx.is_diagnostic_item(sym::Result, did) => ("a", "Result", "Ok"),
+            _ => return,
+        };
+
+        let msg = format!(
+            "for loop over {article} `{ty}`. This is more readably written as an `if let` statement",
+        );
+
+        cx.struct_span_lint(FOR_LOOP_OVER_FALLIBLES, arg.span, |diag| {
+            let mut warn = diag.build(msg);
+
+            if let Some(recv) = extract_iterator_next_call(cx, arg)
+            && let Ok(recv_snip) = cx.sess().source_map().span_to_snippet(recv.span)
+            {
+                warn.span_suggestion(
+                    recv.span.between(arg.span.shrink_to_hi()),
+                    format!("to iterate over `{recv_snip}` remove the call to `next`"),
+                    ".by_ref()",
+                    Applicability::MaybeIncorrect
+                );
+            } else {
+                warn.multipart_suggestion_verbose(
+                    format!("to check pattern in a loop use `while let`"),
+                    vec![
+                        // NB can't use `until` here because `expr.span` and `pat.span` have different syntax contexts
+                        (expr.span.with_hi(pat.span.lo()), format!("while let {var}(")),
+                        (pat.span.between(arg.span), format!(") = ")),
+                    ],
+                    Applicability::MaybeIncorrect
+                );
+            }
+
+            if suggest_question_mark(cx, adt, substs, expr.span) {
+                warn.span_suggestion(
+                    arg.span.shrink_to_hi(),
+                    "consider unwrapping the `Result` with `?` to iterate over its contents",
+                    "?",
+                    Applicability::MaybeIncorrect,
+                );
+            }
+
+            warn.multipart_suggestion_verbose(
+                "consider using `if let` to clear intent",
+                vec![
+                    // NB can't use `until` here because `expr.span` and `pat.span` have different syntax contexts
+                    (expr.span.with_hi(pat.span.lo()), format!("if let {var}(")),
+                    (pat.span.between(arg.span), format!(") = ")),
+                ],
+                Applicability::MaybeIncorrect,
+            );
+
+            warn.emit()
+        })
+    }
+}
+
+fn extract_for_loop<'tcx>(expr: &Expr<'tcx>) -> Option<(&'tcx Pat<'tcx>, &'tcx Expr<'tcx>)> {
+    if let hir::ExprKind::DropTemps(e) = expr.kind
+    && let hir::ExprKind::Match(iterexpr, [arm], hir::MatchSource::ForLoopDesugar) = e.kind
+    && let hir::ExprKind::Call(_, [arg]) = iterexpr.kind
+    && let hir::ExprKind::Loop(block, ..) = arm.body.kind
+    && let [stmt] = block.stmts
+    && let hir::StmtKind::Expr(e) = stmt.kind
+    && let hir::ExprKind::Match(_, [_, some_arm], _) = e.kind
+    && let hir::PatKind::Struct(_, [field], _) = some_arm.pat.kind
+    {
+        Some((field.pat, arg))
+    } else {
+        None
+    }
+}
+
+fn extract_iterator_next_call<'tcx>(
+    cx: &LateContext<'_>,
+    expr: &Expr<'tcx>,
+) -> Option<&'tcx Expr<'tcx>> {
+    // This won't work for `Iterator::next(iter)`, is this an issue?
+    if let hir::ExprKind::MethodCall(_, [recv], _) = expr.kind
+    && cx.typeck_results().type_dependent_def_id(expr.hir_id) == cx.tcx.lang_items().next_fn()
+    {
+        Some(recv)
+    } else {
+        return None
+    }
+}
+
+fn suggest_question_mark<'tcx>(
+    cx: &LateContext<'tcx>,
+    adt: ty::AdtDef<'tcx>,
+    substs: &List<ty::GenericArg<'tcx>>,
+    span: Span,
+) -> bool {
+    let Some(body_id) = cx.enclosing_body else { return false };
+    let Some(into_iterator_did) = cx.tcx.get_diagnostic_item(sym::IntoIterator) else { return false };
+
+    if !cx.tcx.is_diagnostic_item(sym::Result, adt.did()) {
+        return false;
+    }
+
+    // Check that the function/closure/constant we are in has a `Result` type.
+    // Otherwise suggesting using `?` may not be a good idea.
+    {
+        let ty = cx.typeck_results().expr_ty(&cx.tcx.hir().body(body_id).value);
+        let ty::Adt(ret_adt, ..) = ty.kind() else { return false };
+        if !cx.tcx.is_diagnostic_item(sym::Result, ret_adt.did()) {
+            return false;
+        }
+    }
+
+    let ty = substs.type_at(0);
+    let is_iterator = cx.tcx.infer_ctxt().enter(|infcx| {
+        let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
+
+        let cause = ObligationCause::new(
+            span,
+            body_id.hir_id,
+            rustc_infer::traits::ObligationCauseCode::MiscObligation,
+        );
+        fulfill_cx.register_bound(
+            &infcx,
+            ty::ParamEnv::empty(),
+            // Erase any region vids from the type, which may not be resolved
+            infcx.tcx.erase_regions(ty),
+            into_iterator_did,
+            cause,
+        );
+
+        // Select all, including ambiguous predicates
+        let errors = fulfill_cx.select_all_or_error(&infcx);
+
+        errors.is_empty()
+    });
+
+    is_iterator
+}
diff --git a/compiler/rustc_lint/src/lib.rs b/compiler/rustc_lint/src/lib.rs
index f34e062fd12a9..da97bb94a8d1a 100644
--- a/compiler/rustc_lint/src/lib.rs
+++ b/compiler/rustc_lint/src/lib.rs
@@ -49,6 +49,7 @@ mod early;
 mod enum_intrinsics_non_enums;
 mod errors;
 mod expect;
+mod for_loop_over_fallibles;
 pub mod hidden_unicode_codepoints;
 mod internal;
 mod late;
@@ -81,6 +82,7 @@ use rustc_span::Span;
 use array_into_iter::ArrayIntoIter;
 use builtin::*;
 use enum_intrinsics_non_enums::EnumIntrinsicsNonEnums;
+use for_loop_over_fallibles::*;
 use hidden_unicode_codepoints::*;
 use internal::*;
 use methods::*;
@@ -179,6 +181,7 @@ macro_rules! late_lint_mod_passes {
         $macro!(
             $args,
             [
+                ForLoopOverFallibles: ForLoopOverFallibles,
                 HardwiredLints: HardwiredLints,
                 ImproperCTypesDeclarations: ImproperCTypesDeclarations,
                 ImproperCTypesDefinitions: ImproperCTypesDefinitions,
diff --git a/compiler/rustc_macros/src/diagnostics/diagnostic_builder.rs b/compiler/rustc_macros/src/diagnostics/diagnostic_builder.rs
index 9df9fa4e9bf74..a4ccfcace1923 100644
--- a/compiler/rustc_macros/src/diagnostics/diagnostic_builder.rs
+++ b/compiler/rustc_macros/src/diagnostics/diagnostic_builder.rs
@@ -148,9 +148,9 @@ impl DiagnosticDeriveBuilder {
             // `#[help(..)]`/`#[note(..)]` when the user is specifying a alternative slug.
             Meta::List(MetaList { ref nested, .. }) => nested,
             // Subdiagnostics without spans can be applied to the type too, and these are just
-            // paths: `#[help]`, `#[note]` and `#[warn_]`
+            // paths: `#[help]`, `#[note]` and `#[warning]`
             Meta::Path(_) if !is_diag => {
-                let fn_name = if name == "warn_" {
+                let fn_name = if name == "warning" {
                     Ident::new("warn", attr.span())
                 } else {
                     Ident::new(name, attr.span())
@@ -163,12 +163,15 @@ impl DiagnosticDeriveBuilder {
         // Check the kind before doing any further processing so that there aren't misleading
         // "no kind specified" errors if there are failures later.
         match name {
-            "error" | "warning" | "lint" => throw_invalid_attr!(attr, &meta, |diag| {
-                diag.help("`error`, `warning` and `lint` have been replaced by `diag`")
+            "error" | "lint" => throw_invalid_attr!(attr, &meta, |diag| {
+                diag.help("`error` and `lint` have been replaced by `diag`")
             }),
-            "diag" | "help" | "note" | "warn_" => (),
+            "warn_" => throw_invalid_attr!(attr, &meta, |diag| {
+                diag.help("`warn_` have been replaced by `warning`")
+            }),
+            "diag" | "help" | "note" | "warning" => (),
             _ => throw_invalid_attr!(attr, &meta, |diag| {
-                diag.help("only `diag`, `help`, `note` and `warn_` are valid attributes")
+                diag.help("only `diag`, `help`, `note` and `warning` are valid attributes")
             }),
         }
 
@@ -180,7 +183,7 @@ impl DiagnosticDeriveBuilder {
             if !is_diag && nested_iter.next().is_some() {
                 throw_invalid_nested_attr!(attr, &nested_attr, |diag| {
                     diag.help(
-                        "`help`, `note` and `warn_` struct attributes can only have one argument",
+                        "`help`, `note` and `warning` struct attributes can only have one argument",
                     )
                 });
             }
@@ -348,12 +351,12 @@ impl DiagnosticDeriveBuilder {
                 report_error_if_not_applied_to_span(attr, &info)?;
                 Ok(self.add_spanned_subdiagnostic(binding, ident, parse_quote! { _subdiag::label }))
             }
-            "note" | "help" | "warn_" => {
+            "note" | "help" | "warning" => {
                 let warn_ident = Ident::new("warn", Span::call_site());
                 let (ident, path) = match name {
                     "note" => (ident, parse_quote! { _subdiag::note }),
                     "help" => (ident, parse_quote! { _subdiag::help }),
-                    "warn_" => (&warn_ident, parse_quote! { _subdiag::warn }),
+                    "warning" => (&warn_ident, parse_quote! { _subdiag::warn }),
                     _ => unreachable!(),
                 };
                 if type_matches_path(&info.ty, &["rustc_span", "Span"]) {
@@ -390,7 +393,7 @@ impl DiagnosticDeriveBuilder {
             "suggestion" | "suggestion_short" | "suggestion_hidden" | "suggestion_verbose" => {
                 return self.generate_inner_field_code_suggestion(attr, info);
             }
-            "label" | "help" | "note" | "warn_" => (),
+            "label" | "help" | "note" | "warning" => (),
             _ => throw_invalid_attr!(attr, &meta, |diag| {
                 diag.help(
                     "only `label`, `help`, `note`, `warn` or `suggestion{,_short,_hidden,_verbose}` are \
@@ -422,14 +425,14 @@ impl DiagnosticDeriveBuilder {
                 Ok(self.add_spanned_subdiagnostic(binding, ident, msg))
             }
             "note" | "help" if type_is_unit(&info.ty) => Ok(self.add_subdiagnostic(ident, msg)),
-            // `warn_` must be special-cased because the attribute `warn` already has meaning and
+            // `warning` must be special-cased because the attribute `warn` already has meaning and
             // so isn't used, despite the diagnostic API being named `warn`.
-            "warn_" if type_matches_path(&info.ty, &["rustc_span", "Span"]) => Ok(self
+            "warning" if type_matches_path(&info.ty, &["rustc_span", "Span"]) => Ok(self
                 .add_spanned_subdiagnostic(binding, &Ident::new("warn", Span::call_site()), msg)),
-            "warn_" if type_is_unit(&info.ty) => {
+            "warning" if type_is_unit(&info.ty) => {
                 Ok(self.add_subdiagnostic(&Ident::new("warn", Span::call_site()), msg))
             }
-            "note" | "help" | "warn_" => report_type_error(attr, "`Span` or `()`")?,
+            "note" | "help" | "warning" => report_type_error(attr, "`Span` or `()`")?,
             _ => unreachable!(),
         }
     }
diff --git a/compiler/rustc_macros/src/diagnostics/subdiagnostic.rs b/compiler/rustc_macros/src/diagnostics/subdiagnostic.rs
index edf4dbed9853e..666dbc23c287c 100644
--- a/compiler/rustc_macros/src/diagnostics/subdiagnostic.rs
+++ b/compiler/rustc_macros/src/diagnostics/subdiagnostic.rs
@@ -37,7 +37,7 @@ enum SubdiagnosticKind {
     Note,
     /// `#[help(...)]`
     Help,
-    /// `#[warn_(...)]`
+    /// `#[warning(...)]`
     Warn,
     /// `#[suggestion{,_short,_hidden,_verbose}]`
     Suggestion(SubdiagnosticSuggestionKind),
@@ -51,7 +51,7 @@ impl FromStr for SubdiagnosticKind {
             "label" => Ok(SubdiagnosticKind::Label),
             "note" => Ok(SubdiagnosticKind::Note),
             "help" => Ok(SubdiagnosticKind::Help),
-            "warn_" => Ok(SubdiagnosticKind::Warn),
+            "warning" => Ok(SubdiagnosticKind::Warn),
             "suggestion" => Ok(SubdiagnosticKind::Suggestion(SubdiagnosticSuggestionKind::Normal)),
             "suggestion_short" => {
                 Ok(SubdiagnosticKind::Suggestion(SubdiagnosticSuggestionKind::Short))
diff --git a/compiler/rustc_macros/src/lib.rs b/compiler/rustc_macros/src/lib.rs
index 87d7ab6ed517b..8faac8ef36a53 100644
--- a/compiler/rustc_macros/src/lib.rs
+++ b/compiler/rustc_macros/src/lib.rs
@@ -132,7 +132,7 @@ decl_derive!(
         diag,
         help,
         note,
-        warn_,
+        warning,
         // field attributes
         skip_arg,
         primary_span,
@@ -149,7 +149,7 @@ decl_derive!(
         diag,
         help,
         note,
-        warn_,
+        warning,
         // field attributes
         skip_arg,
         primary_span,
@@ -166,7 +166,7 @@ decl_derive!(
         label,
         help,
         note,
-        warn_,
+        warning,
         suggestion,
         suggestion_short,
         suggestion_hidden,
diff --git a/compiler/rustc_middle/src/mir/generic_graph.rs b/compiler/rustc_middle/src/mir/generic_graph.rs
index f3621cd99d344..d1f3561c02c5d 100644
--- a/compiler/rustc_middle/src/mir/generic_graph.rs
+++ b/compiler/rustc_middle/src/mir/generic_graph.rs
@@ -12,14 +12,14 @@ pub fn mir_fn_to_generic_graph<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'_>) -> Grap
 
     // Nodes
     let nodes: Vec<Node> = body
-        .basic_blocks()
+        .basic_blocks
         .iter_enumerated()
         .map(|(block, _)| bb_to_graph_node(block, body, dark_mode))
         .collect();
 
     // Edges
     let mut edges = Vec::new();
-    for (source, _) in body.basic_blocks().iter_enumerated() {
+    for (source, _) in body.basic_blocks.iter_enumerated() {
         let def_id = body.source.def_id();
         let terminator = body[source].terminator();
         let labels = terminator.kind.fmt_successor_labels();
diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index 9da9b4e91f647..f7a1e9b286488 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -331,11 +331,6 @@ impl<'tcx> Body<'tcx> {
         body
     }
 
-    #[inline]
-    pub fn basic_blocks(&self) -> &IndexVec<BasicBlock, BasicBlockData<'tcx>> {
-        &self.basic_blocks
-    }
-
     #[inline]
     pub fn basic_blocks_mut(&mut self) -> &mut IndexVec<BasicBlock, BasicBlockData<'tcx>> {
         self.basic_blocks.as_mut()
@@ -490,7 +485,7 @@ impl<'tcx> Index<BasicBlock> for Body<'tcx> {
 
     #[inline]
     fn index(&self, index: BasicBlock) -> &BasicBlockData<'tcx> {
-        &self.basic_blocks()[index]
+        &self.basic_blocks[index]
     }
 }
 
diff --git a/compiler/rustc_middle/src/mir/patch.rs b/compiler/rustc_middle/src/mir/patch.rs
index 8aa761aae8d90..24fe3b47256e2 100644
--- a/compiler/rustc_middle/src/mir/patch.rs
+++ b/compiler/rustc_middle/src/mir/patch.rs
@@ -19,7 +19,7 @@ pub struct MirPatch<'tcx> {
 impl<'tcx> MirPatch<'tcx> {
     pub fn new(body: &Body<'tcx>) -> Self {
         let mut result = MirPatch {
-            patch_map: IndexVec::from_elem(None, body.basic_blocks()),
+            patch_map: IndexVec::from_elem(None, &body.basic_blocks),
             new_blocks: vec![],
             new_statements: vec![],
             new_locals: vec![],
@@ -29,7 +29,7 @@ impl<'tcx> MirPatch<'tcx> {
         };
 
         // Check if we already have a resume block
-        for (bb, block) in body.basic_blocks().iter_enumerated() {
+        for (bb, block) in body.basic_blocks.iter_enumerated() {
             if let TerminatorKind::Resume = block.terminator().kind && block.statements.is_empty() {
                 result.resume_block = Some(bb);
                 break;
@@ -61,7 +61,7 @@ impl<'tcx> MirPatch<'tcx> {
     }
 
     pub fn terminator_loc(&self, body: &Body<'tcx>, bb: BasicBlock) -> Location {
-        let offset = match bb.index().checked_sub(body.basic_blocks().len()) {
+        let offset = match bb.index().checked_sub(body.basic_blocks.len()) {
             Some(index) => self.new_blocks[index].statements.len(),
             None => body[bb].statements.len(),
         };
@@ -129,7 +129,7 @@ impl<'tcx> MirPatch<'tcx> {
         debug!(
             "MirPatch: {} new blocks, starting from index {}",
             self.new_blocks.len(),
-            body.basic_blocks().len()
+            body.basic_blocks.len()
         );
         let bbs = if self.patch_map.is_empty() && self.new_blocks.is_empty() {
             body.basic_blocks.as_mut_preserves_cfg()
@@ -173,7 +173,7 @@ impl<'tcx> MirPatch<'tcx> {
     }
 
     pub fn source_info_for_location(&self, body: &Body<'tcx>, loc: Location) -> SourceInfo {
-        let data = match loc.block.index().checked_sub(body.basic_blocks().len()) {
+        let data = match loc.block.index().checked_sub(body.basic_blocks.len()) {
             Some(new) => &self.new_blocks[new],
             None => &body[loc.block],
         };
diff --git a/compiler/rustc_middle/src/mir/pretty.rs b/compiler/rustc_middle/src/mir/pretty.rs
index 0ce41337b910d..da6af89b09b9b 100644
--- a/compiler/rustc_middle/src/mir/pretty.rs
+++ b/compiler/rustc_middle/src/mir/pretty.rs
@@ -318,10 +318,10 @@ where
     F: FnMut(PassWhere, &mut dyn Write) -> io::Result<()>,
 {
     write_mir_intro(tcx, body, w)?;
-    for block in body.basic_blocks().indices() {
+    for block in body.basic_blocks.indices() {
         extra_data(PassWhere::BeforeBlock(block), w)?;
         write_basic_block(tcx, block, body, extra_data, w)?;
-        if block.index() + 1 != body.basic_blocks().len() {
+        if block.index() + 1 != body.basic_blocks.len() {
             writeln!(w)?;
         }
     }
diff --git a/compiler/rustc_middle/src/mir/spanview.rs b/compiler/rustc_middle/src/mir/spanview.rs
index 4418b848e512a..6e64a3b80c1fb 100644
--- a/compiler/rustc_middle/src/mir/spanview.rs
+++ b/compiler/rustc_middle/src/mir/spanview.rs
@@ -105,7 +105,7 @@ where
     }
     let body_span = hir_body.unwrap().value.span;
     let mut span_viewables = Vec::new();
-    for (bb, data) in body.basic_blocks().iter_enumerated() {
+    for (bb, data) in body.basic_blocks.iter_enumerated() {
         match spanview {
             MirSpanview::Statement => {
                 for (i, statement) in data.statements.iter().enumerate() {
diff --git a/compiler/rustc_middle/src/mir/traversal.rs b/compiler/rustc_middle/src/mir/traversal.rs
index 627dc32f37eb5..55b2c592795bf 100644
--- a/compiler/rustc_middle/src/mir/traversal.rs
+++ b/compiler/rustc_middle/src/mir/traversal.rs
@@ -37,7 +37,7 @@ impl<'a, 'tcx> Preorder<'a, 'tcx> {
 
         Preorder {
             body,
-            visited: BitSet::new_empty(body.basic_blocks().len()),
+            visited: BitSet::new_empty(body.basic_blocks.len()),
             worklist,
             root_is_start_block: root == START_BLOCK,
         }
@@ -71,7 +71,7 @@ impl<'a, 'tcx> Iterator for Preorder<'a, 'tcx> {
 
     fn size_hint(&self) -> (usize, Option<usize>) {
         // All the blocks, minus the number of blocks we've visited.
-        let upper = self.body.basic_blocks().len() - self.visited.count();
+        let upper = self.body.basic_blocks.len() - self.visited.count();
 
         let lower = if self.root_is_start_block {
             // We will visit all remaining blocks exactly once.
diff --git a/compiler/rustc_middle/src/mir/visit.rs b/compiler/rustc_middle/src/mir/visit.rs
index 4a85defb1ed84..7bd65f42e3f92 100644
--- a/compiler/rustc_middle/src/mir/visit.rs
+++ b/compiler/rustc_middle/src/mir/visit.rs
@@ -951,7 +951,7 @@ macro_rules! basic_blocks {
         $body.basic_blocks.as_mut_preserves_cfg()
     };
     ($body:ident,) => {
-        $body.basic_blocks()
+        $body.basic_blocks
     };
 }
 
diff --git a/compiler/rustc_middle/src/traits/mod.rs b/compiler/rustc_middle/src/traits/mod.rs
index 9b82320e556b3..ab7e5ba3a1067 100644
--- a/compiler/rustc_middle/src/traits/mod.rs
+++ b/compiler/rustc_middle/src/traits/mod.rs
@@ -469,6 +469,13 @@ impl<'tcx> ObligationCauseCode<'tcx> {
             _ => None,
         }
     }
+
+    pub fn peel_match_impls(&self) -> &Self {
+        match self {
+            MatchImpl(cause, _) => cause.code(),
+            _ => self,
+        }
+    }
 }
 
 // `ObligationCauseCode` is used a lot. Make sure it doesn't unintentionally get bigger.
diff --git a/compiler/rustc_mir_build/src/build/mod.rs b/compiler/rustc_mir_build/src/build/mod.rs
index 461c837f6df19..684b228e87fa9 100644
--- a/compiler/rustc_mir_build/src/build/mod.rs
+++ b/compiler/rustc_mir_build/src/build/mod.rs
@@ -272,7 +272,7 @@ fn mir_build(tcx: TyCtxt<'_>, def: ty::WithOptConstParam<LocalDefId>) -> Body<'_
         // by borrow checking.
         debug_assert!(
             !(body.local_decls.has_free_regions()
-                || body.basic_blocks().has_free_regions()
+                || body.basic_blocks.has_free_regions()
                 || body.var_debug_info.has_free_regions()
                 || body.yield_ty().has_free_regions()),
             "Unexpected free regions in MIR: {:?}",
diff --git a/compiler/rustc_mir_dataflow/src/framework/engine.rs b/compiler/rustc_mir_dataflow/src/framework/engine.rs
index 112204c7599be..bc75645e7c957 100644
--- a/compiler/rustc_mir_dataflow/src/framework/engine.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/engine.rs
@@ -111,9 +111,9 @@ where
         // Otherwise, compute and store the cumulative transfer function for each block.
 
         let identity = GenKillSet::identity(analysis.bottom_value(body).domain_size());
-        let mut trans_for_block = IndexVec::from_elem(identity, body.basic_blocks());
+        let mut trans_for_block = IndexVec::from_elem(identity, &body.basic_blocks);
 
-        for (block, block_data) in body.basic_blocks().iter_enumerated() {
+        for (block, block_data) in body.basic_blocks.iter_enumerated() {
             let trans = &mut trans_for_block[block];
             A::Direction::gen_kill_effects_in_block(&analysis, trans, block, block_data);
         }
@@ -147,7 +147,7 @@ where
         apply_trans_for_block: Option<Box<dyn Fn(BasicBlock, &mut A::Domain)>>,
     ) -> Self {
         let bottom_value = analysis.bottom_value(body);
-        let mut entry_sets = IndexVec::from_elem(bottom_value.clone(), body.basic_blocks());
+        let mut entry_sets = IndexVec::from_elem(bottom_value.clone(), &body.basic_blocks);
         analysis.initialize_start_block(body, &mut entry_sets[mir::START_BLOCK]);
 
         if A::Direction::IS_BACKWARD && entry_sets[mir::START_BLOCK] != bottom_value {
@@ -200,8 +200,7 @@ where
             ..
         } = self;
 
-        let mut dirty_queue: WorkQueue<BasicBlock> =
-            WorkQueue::with_none(body.basic_blocks().len());
+        let mut dirty_queue: WorkQueue<BasicBlock> = WorkQueue::with_none(body.basic_blocks.len());
 
         if A::Direction::IS_FORWARD {
             for (bb, _) in traversal::reverse_postorder(body) {
diff --git a/compiler/rustc_mir_dataflow/src/framework/graphviz.rs b/compiler/rustc_mir_dataflow/src/framework/graphviz.rs
index c94198c56a8d1..579fe68a14935 100644
--- a/compiler/rustc_mir_dataflow/src/framework/graphviz.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/graphviz.rs
@@ -108,12 +108,12 @@ where
     type Edge = CfgEdge;
 
     fn nodes(&self) -> dot::Nodes<'_, Self::Node> {
-        self.body.basic_blocks().indices().collect::<Vec<_>>().into()
+        self.body.basic_blocks.indices().collect::<Vec<_>>().into()
     }
 
     fn edges(&self) -> dot::Edges<'_, Self::Edge> {
         self.body
-            .basic_blocks()
+            .basic_blocks
             .indices()
             .flat_map(|bb| dataflow_successors(self.body, bb))
             .collect::<Vec<_>>()
diff --git a/compiler/rustc_mir_dataflow/src/framework/tests.rs b/compiler/rustc_mir_dataflow/src/framework/tests.rs
index d9461fd3abd81..17102454a88de 100644
--- a/compiler/rustc_mir_dataflow/src/framework/tests.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/tests.rs
@@ -100,9 +100,9 @@ impl<D: Direction> MockAnalysis<'_, D> {
 
     fn mock_entry_sets(&self) -> IndexVec<BasicBlock, BitSet<usize>> {
         let empty = self.bottom_value(self.body);
-        let mut ret = IndexVec::from_elem(empty, &self.body.basic_blocks());
+        let mut ret = IndexVec::from_elem(empty, &self.body.basic_blocks);
 
-        for (bb, _) in self.body.basic_blocks().iter_enumerated() {
+        for (bb, _) in self.body.basic_blocks.iter_enumerated() {
             ret[bb] = self.mock_entry_set(bb);
         }
 
@@ -169,7 +169,7 @@ impl<'tcx, D: Direction> AnalysisDomain<'tcx> for MockAnalysis<'tcx, D> {
     const NAME: &'static str = "mock";
 
     fn bottom_value(&self, body: &mir::Body<'tcx>) -> Self::Domain {
-        BitSet::new_empty(Self::BASIC_BLOCK_OFFSET + body.basic_blocks().len())
+        BitSet::new_empty(Self::BASIC_BLOCK_OFFSET + body.basic_blocks.len())
     }
 
     fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut Self::Domain) {
@@ -271,9 +271,7 @@ fn test_cursor<D: Direction>(analysis: MockAnalysis<'_, D>) {
     cursor.allow_unreachable();
 
     let every_target = || {
-        body.basic_blocks()
-            .iter_enumerated()
-            .flat_map(|(bb, _)| SeekTarget::iter_in_block(body, bb))
+        body.basic_blocks.iter_enumerated().flat_map(|(bb, _)| SeekTarget::iter_in_block(body, bb))
     };
 
     let mut seek_to_target = |targ| {
diff --git a/compiler/rustc_mir_dataflow/src/move_paths/builder.rs b/compiler/rustc_mir_dataflow/src/move_paths/builder.rs
index 116e5c1f3ce0a..c325838622399 100644
--- a/compiler/rustc_mir_dataflow/src/move_paths/builder.rs
+++ b/compiler/rustc_mir_dataflow/src/move_paths/builder.rs
@@ -243,7 +243,7 @@ pub(super) fn gather_moves<'tcx>(
 
     builder.gather_args();
 
-    for (bb, block) in body.basic_blocks().iter_enumerated() {
+    for (bb, block) in body.basic_blocks.iter_enumerated() {
         for (i, stmt) in block.statements.iter().enumerate() {
             let source = Location { block: bb, statement_index: i };
             builder.gather_statement(source, stmt);
diff --git a/compiler/rustc_mir_dataflow/src/move_paths/mod.rs b/compiler/rustc_mir_dataflow/src/move_paths/mod.rs
index a951c5b0b1c94..b36e268cf8bf7 100644
--- a/compiler/rustc_mir_dataflow/src/move_paths/mod.rs
+++ b/compiler/rustc_mir_dataflow/src/move_paths/mod.rs
@@ -217,7 +217,7 @@ where
     fn new(body: &Body<'_>) -> Self {
         LocationMap {
             map: body
-                .basic_blocks()
+                .basic_blocks
                 .iter()
                 .map(|block| vec![T::default(); block.statements.len() + 1])
                 .collect(),
diff --git a/compiler/rustc_mir_dataflow/src/rustc_peek.rs b/compiler/rustc_mir_dataflow/src/rustc_peek.rs
index 5fb7cb6584beb..7cae68efbecc3 100644
--- a/compiler/rustc_mir_dataflow/src/rustc_peek.rs
+++ b/compiler/rustc_mir_dataflow/src/rustc_peek.rs
@@ -101,7 +101,7 @@ pub fn sanity_check_via_rustc_peek<'tcx, A>(
 
     let mut cursor = ResultsCursor::new(body, results);
 
-    let peek_calls = body.basic_blocks().iter_enumerated().filter_map(|(bb, block_data)| {
+    let peek_calls = body.basic_blocks.iter_enumerated().filter_map(|(bb, block_data)| {
         PeekCall::from_terminator(tcx, block_data.terminator()).map(|call| (bb, block_data, call))
     });
 
diff --git a/compiler/rustc_mir_dataflow/src/storage.rs b/compiler/rustc_mir_dataflow/src/storage.rs
index c909648ea017e..e5a0e1d312eae 100644
--- a/compiler/rustc_mir_dataflow/src/storage.rs
+++ b/compiler/rustc_mir_dataflow/src/storage.rs
@@ -7,7 +7,7 @@ use rustc_middle::mir::{self, Local};
 pub fn always_storage_live_locals(body: &mir::Body<'_>) -> BitSet<Local> {
     let mut always_live_locals = BitSet::new_filled(body.local_decls.len());
 
-    for block in body.basic_blocks() {
+    for block in &*body.basic_blocks {
         for statement in &block.statements {
             use mir::StatementKind::{StorageDead, StorageLive};
             if let StorageLive(l) | StorageDead(l) = statement.kind {
diff --git a/compiler/rustc_mir_transform/src/abort_unwinding_calls.rs b/compiler/rustc_mir_transform/src/abort_unwinding_calls.rs
index 2502e8b603c3e..d8f85d2e37982 100644
--- a/compiler/rustc_mir_transform/src/abort_unwinding_calls.rs
+++ b/compiler/rustc_mir_transform/src/abort_unwinding_calls.rs
@@ -56,7 +56,7 @@ impl<'tcx> MirPass<'tcx> for AbortUnwindingCalls {
         // example.
         let mut calls_to_terminate = Vec::new();
         let mut cleanups_to_remove = Vec::new();
-        for (id, block) in body.basic_blocks().iter_enumerated() {
+        for (id, block) in body.basic_blocks.iter_enumerated() {
             if block.is_cleanup {
                 continue;
             }
diff --git a/compiler/rustc_mir_transform/src/add_call_guards.rs b/compiler/rustc_mir_transform/src/add_call_guards.rs
index f12c8560c0e82..30966d22e2f6d 100644
--- a/compiler/rustc_mir_transform/src/add_call_guards.rs
+++ b/compiler/rustc_mir_transform/src/add_call_guards.rs
@@ -45,7 +45,7 @@ impl AddCallGuards {
         // We need a place to store the new blocks generated
         let mut new_blocks = Vec::new();
 
-        let cur_len = body.basic_blocks().len();
+        let cur_len = body.basic_blocks.len();
 
         for block in body.basic_blocks_mut() {
             match block.terminator {
diff --git a/compiler/rustc_mir_transform/src/add_moves_for_packed_drops.rs b/compiler/rustc_mir_transform/src/add_moves_for_packed_drops.rs
index 8de0aad041cc3..ffb5d8c6d95f2 100644
--- a/compiler/rustc_mir_transform/src/add_moves_for_packed_drops.rs
+++ b/compiler/rustc_mir_transform/src/add_moves_for_packed_drops.rs
@@ -55,7 +55,7 @@ fn add_moves_for_packed_drops_patch<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>)
     let mut patch = MirPatch::new(body);
     let param_env = tcx.param_env(def_id);
 
-    for (bb, data) in body.basic_blocks().iter_enumerated() {
+    for (bb, data) in body.basic_blocks.iter_enumerated() {
         let loc = Location { block: bb, statement_index: data.statements.len() };
         let terminator = data.terminator();
 
diff --git a/compiler/rustc_mir_transform/src/const_goto.rs b/compiler/rustc_mir_transform/src/const_goto.rs
index 5acf939f06bf6..0a305a402095d 100644
--- a/compiler/rustc_mir_transform/src/const_goto.rs
+++ b/compiler/rustc_mir_transform/src/const_goto.rs
@@ -61,14 +61,14 @@ impl<'tcx> Visitor<'tcx> for ConstGotoOptimizationFinder<'_, 'tcx> {
         let _: Option<_> = try {
             let target = terminator.kind.as_goto()?;
             // We only apply this optimization if the last statement is a const assignment
-            let last_statement = self.body.basic_blocks()[location.block].statements.last()?;
+            let last_statement = self.body.basic_blocks[location.block].statements.last()?;
 
             if let (place, Rvalue::Use(Operand::Constant(_const))) =
                 last_statement.kind.as_assign()?
             {
                 // We found a constant being assigned to `place`.
                 // Now check that the target of this Goto switches on this place.
-                let target_bb = &self.body.basic_blocks()[target];
+                let target_bb = &self.body.basic_blocks[target];
 
                 // The `StorageDead(..)` statement does not affect the functionality of mir.
                 // We can move this part of the statement up to the predecessor.
diff --git a/compiler/rustc_mir_transform/src/const_prop.rs b/compiler/rustc_mir_transform/src/const_prop.rs
index 9f3a9d0b87826..5c9f4f57243b7 100644
--- a/compiler/rustc_mir_transform/src/const_prop.rs
+++ b/compiler/rustc_mir_transform/src/const_prop.rs
@@ -131,7 +131,7 @@ impl<'tcx> MirPass<'tcx> for ConstProp {
 
         let dummy_body = &Body::new(
             body.source,
-            body.basic_blocks().clone(),
+            (*body.basic_blocks).clone(),
             body.source_scopes.clone(),
             body.local_decls.clone(),
             Default::default(),
diff --git a/compiler/rustc_mir_transform/src/const_prop_lint.rs b/compiler/rustc_mir_transform/src/const_prop_lint.rs
index 1bc65721ea601..74b146833d03f 100644
--- a/compiler/rustc_mir_transform/src/const_prop_lint.rs
+++ b/compiler/rustc_mir_transform/src/const_prop_lint.rs
@@ -105,7 +105,7 @@ impl<'tcx> MirLint<'tcx> for ConstProp {
 
         let dummy_body = &Body::new(
             body.source,
-            body.basic_blocks().clone(),
+            (*body.basic_blocks).clone(),
             body.source_scopes.clone(),
             body.local_decls.clone(),
             Default::default(),
@@ -522,7 +522,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
 
 impl<'tcx> Visitor<'tcx> for ConstPropagator<'_, 'tcx> {
     fn visit_body(&mut self, body: &Body<'tcx>) {
-        for (bb, data) in body.basic_blocks().iter_enumerated() {
+        for (bb, data) in body.basic_blocks.iter_enumerated() {
             self.visit_basic_block_data(bb, data);
         }
     }
diff --git a/compiler/rustc_mir_transform/src/coverage/graph.rs b/compiler/rustc_mir_transform/src/coverage/graph.rs
index 759ea7cd32820..782129be088b6 100644
--- a/compiler/rustc_mir_transform/src/coverage/graph.rs
+++ b/compiler/rustc_mir_transform/src/coverage/graph.rs
@@ -713,7 +713,7 @@ impl<
 
         ShortCircuitPreorder {
             body,
-            visited: BitSet::new_empty(body.basic_blocks().len()),
+            visited: BitSet::new_empty(body.basic_blocks.len()),
             worklist,
             filtered_successors,
         }
@@ -747,7 +747,7 @@ impl<
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let size = self.body.basic_blocks().len() - self.visited.count();
+        let size = self.body.basic_blocks.len() - self.visited.count();
         (size, Some(size))
     }
 }
diff --git a/compiler/rustc_mir_transform/src/coverage/mod.rs b/compiler/rustc_mir_transform/src/coverage/mod.rs
index e72d016d7ac91..604810144887a 100644
--- a/compiler/rustc_mir_transform/src/coverage/mod.rs
+++ b/compiler/rustc_mir_transform/src/coverage/mod.rs
@@ -80,7 +80,7 @@ impl<'tcx> MirPass<'tcx> for InstrumentCoverage {
             return;
         }
 
-        match mir_body.basic_blocks()[mir::START_BLOCK].terminator().kind {
+        match mir_body.basic_blocks[mir::START_BLOCK].terminator().kind {
             TerminatorKind::Unreachable => {
                 trace!("InstrumentCoverage skipped for unreachable `START_BLOCK`");
                 return;
diff --git a/compiler/rustc_mir_transform/src/coverage/query.rs b/compiler/rustc_mir_transform/src/coverage/query.rs
index 9d02f58ae65ee..dc1e68b253e72 100644
--- a/compiler/rustc_mir_transform/src/coverage/query.rs
+++ b/compiler/rustc_mir_transform/src/coverage/query.rs
@@ -84,7 +84,7 @@ impl CoverageVisitor {
     }
 
     fn visit_body(&mut self, body: &Body<'_>) {
-        for bb_data in body.basic_blocks().iter() {
+        for bb_data in body.basic_blocks.iter() {
             for statement in bb_data.statements.iter() {
                 if let StatementKind::Coverage(box ref coverage) = statement.kind {
                     if is_inlined(body, statement) {
@@ -138,7 +138,7 @@ fn coverageinfo<'tcx>(tcx: TyCtxt<'tcx>, instance_def: ty::InstanceDef<'tcx>) ->
 
 fn covered_code_regions<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> Vec<&'tcx CodeRegion> {
     let body = mir_body(tcx, def_id);
-    body.basic_blocks()
+    body.basic_blocks
         .iter()
         .flat_map(|data| {
             data.statements.iter().filter_map(|statement| match statement.kind {
diff --git a/compiler/rustc_mir_transform/src/coverage/tests.rs b/compiler/rustc_mir_transform/src/coverage/tests.rs
index 6380f03528ae8..9c9ed5fa5105e 100644
--- a/compiler/rustc_mir_transform/src/coverage/tests.rs
+++ b/compiler/rustc_mir_transform/src/coverage/tests.rs
@@ -176,7 +176,7 @@ fn debug_basic_blocks<'tcx>(mir_body: &Body<'tcx>) -> String {
     format!(
         "{:?}",
         mir_body
-            .basic_blocks()
+            .basic_blocks
             .iter_enumerated()
             .map(|(bb, data)| {
                 let term = &data.terminator();
@@ -213,7 +213,7 @@ fn print_mir_graphviz(name: &str, mir_body: &Body<'_>) {
             "digraph {} {{\n{}\n}}",
             name,
             mir_body
-                .basic_blocks()
+                .basic_blocks
                 .iter_enumerated()
                 .map(|(bb, data)| {
                     format!(
@@ -653,7 +653,7 @@ fn test_traverse_coverage_with_loops() {
 
 fn synthesize_body_span_from_terminators(mir_body: &Body<'_>) -> Span {
     let mut some_span: Option<Span> = None;
-    for (_, data) in mir_body.basic_blocks().iter_enumerated() {
+    for (_, data) in mir_body.basic_blocks.iter_enumerated() {
         let term_span = data.terminator().source_info.span;
         if let Some(span) = some_span.as_mut() {
             *span = span.to(term_span);
diff --git a/compiler/rustc_mir_transform/src/deduplicate_blocks.rs b/compiler/rustc_mir_transform/src/deduplicate_blocks.rs
index d1977ed49fe15..909116a77f54f 100644
--- a/compiler/rustc_mir_transform/src/deduplicate_blocks.rs
+++ b/compiler/rustc_mir_transform/src/deduplicate_blocks.rs
@@ -58,7 +58,7 @@ fn find_duplicates(body: &Body<'_>) -> FxHashMap<BasicBlock, BasicBlock> {
     let mut duplicates = FxHashMap::default();
 
     let bbs_to_go_through =
-        body.basic_blocks().iter_enumerated().filter(|(_, bbd)| !bbd.is_cleanup).count();
+        body.basic_blocks.iter_enumerated().filter(|(_, bbd)| !bbd.is_cleanup).count();
 
     let mut same_hashes =
         FxHashMap::with_capacity_and_hasher(bbs_to_go_through, Default::default());
@@ -71,8 +71,7 @@ fn find_duplicates(body: &Body<'_>) -> FxHashMap<BasicBlock, BasicBlock> {
     // When we see bb1, we see that it is a duplicate of bb3, and therefore insert it in the duplicates list
     // with replacement bb3.
     // When the duplicates are removed, we will end up with only bb3.
-    for (bb, bbd) in body.basic_blocks().iter_enumerated().rev().filter(|(_, bbd)| !bbd.is_cleanup)
-    {
+    for (bb, bbd) in body.basic_blocks.iter_enumerated().rev().filter(|(_, bbd)| !bbd.is_cleanup) {
         // Basic blocks can get really big, so to avoid checking for duplicates in basic blocks
         // that are unlikely to have duplicates, we stop early. The early bail number has been
         // found experimentally by eprintln while compiling the crates in the rustc-perf suite.
diff --git a/compiler/rustc_mir_transform/src/dest_prop.rs b/compiler/rustc_mir_transform/src/dest_prop.rs
index 33572068f5cd9..da55510920e12 100644
--- a/compiler/rustc_mir_transform/src/dest_prop.rs
+++ b/compiler/rustc_mir_transform/src/dest_prop.rs
@@ -150,7 +150,7 @@ impl<'tcx> MirPass<'tcx> for DestinationPropagation {
             def_id,
             body.local_decls.len(),
             relevant,
-            body.basic_blocks().len()
+            body.basic_blocks.len()
         );
         if relevant > MAX_LOCALS {
             warn!(
@@ -159,11 +159,11 @@ impl<'tcx> MirPass<'tcx> for DestinationPropagation {
             );
             return;
         }
-        if body.basic_blocks().len() > MAX_BLOCKS {
+        if body.basic_blocks.len() > MAX_BLOCKS {
             warn!(
                 "too many blocks in {:?} ({}, max is {}), not optimizing",
                 def_id,
-                body.basic_blocks().len(),
+                body.basic_blocks.len(),
                 MAX_BLOCKS
             );
             return;
diff --git a/compiler/rustc_mir_transform/src/early_otherwise_branch.rs b/compiler/rustc_mir_transform/src/early_otherwise_branch.rs
index dba42f7aff033..32e738bbcea44 100644
--- a/compiler/rustc_mir_transform/src/early_otherwise_branch.rs
+++ b/compiler/rustc_mir_transform/src/early_otherwise_branch.rs
@@ -104,8 +104,8 @@ impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch {
         let mut should_cleanup = false;
 
         // Also consider newly generated bbs in the same pass
-        for i in 0..body.basic_blocks().len() {
-            let bbs = body.basic_blocks();
+        for i in 0..body.basic_blocks.len() {
+            let bbs = &*body.basic_blocks;
             let parent = BasicBlock::from_usize(i);
             let Some(opt_data) = evaluate_candidate(tcx, body, parent) else {
                 continue
@@ -316,7 +316,7 @@ fn evaluate_candidate<'tcx>(
     body: &Body<'tcx>,
     parent: BasicBlock,
 ) -> Option<OptimizationData<'tcx>> {
-    let bbs = body.basic_blocks();
+    let bbs = &body.basic_blocks;
     let TerminatorKind::SwitchInt {
         targets,
         switch_ty: parent_ty,
diff --git a/compiler/rustc_mir_transform/src/elaborate_drops.rs b/compiler/rustc_mir_transform/src/elaborate_drops.rs
index 9c1fcbaa69d6b..61118ecc8ed51 100644
--- a/compiler/rustc_mir_transform/src/elaborate_drops.rs
+++ b/compiler/rustc_mir_transform/src/elaborate_drops.rs
@@ -89,13 +89,13 @@ fn find_dead_unwinds<'tcx>(
     debug!("find_dead_unwinds({:?})", body.span);
     // We only need to do this pass once, because unwind edges can only
     // reach cleanup blocks, which can't have unwind edges themselves.
-    let mut dead_unwinds = BitSet::new_empty(body.basic_blocks().len());
+    let mut dead_unwinds = BitSet::new_empty(body.basic_blocks.len());
     let mut flow_inits = MaybeInitializedPlaces::new(tcx, body, &env)
         .into_engine(tcx, body)
         .pass_name("find_dead_unwinds")
         .iterate_to_fixpoint()
         .into_results_cursor(body);
-    for (bb, bb_data) in body.basic_blocks().iter_enumerated() {
+    for (bb, bb_data) in body.basic_blocks.iter_enumerated() {
         let place = match bb_data.terminator().kind {
             TerminatorKind::Drop { ref place, unwind: Some(_), .. }
             | TerminatorKind::DropAndReplace { ref place, unwind: Some(_), .. } => {
@@ -303,7 +303,7 @@ impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
     }
 
     fn collect_drop_flags(&mut self) {
-        for (bb, data) in self.body.basic_blocks().iter_enumerated() {
+        for (bb, data) in self.body.basic_blocks.iter_enumerated() {
             let terminator = data.terminator();
             let place = match terminator.kind {
                 TerminatorKind::Drop { ref place, .. }
@@ -358,7 +358,7 @@ impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
     }
 
     fn elaborate_drops(&mut self) {
-        for (bb, data) in self.body.basic_blocks().iter_enumerated() {
+        for (bb, data) in self.body.basic_blocks.iter_enumerated() {
             let loc = Location { block: bb, statement_index: data.statements.len() };
             let terminator = data.terminator();
 
@@ -515,7 +515,7 @@ impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
     }
 
     fn drop_flags_for_fn_rets(&mut self) {
-        for (bb, data) in self.body.basic_blocks().iter_enumerated() {
+        for (bb, data) in self.body.basic_blocks.iter_enumerated() {
             if let TerminatorKind::Call {
                 destination, target: Some(tgt), cleanup: Some(_), ..
             } = data.terminator().kind
@@ -550,7 +550,7 @@ impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
         // drop flags by themselves, to avoid the drop flags being
         // clobbered before they are read.
 
-        for (bb, data) in self.body.basic_blocks().iter_enumerated() {
+        for (bb, data) in self.body.basic_blocks.iter_enumerated() {
             debug!("drop_flags_for_locs({:?})", data);
             for i in 0..(data.statements.len() + 1) {
                 debug!("drop_flag_for_locs: stmt {}", i);
diff --git a/compiler/rustc_mir_transform/src/ffi_unwind_calls.rs b/compiler/rustc_mir_transform/src/ffi_unwind_calls.rs
index 7728fdaffb0dc..7522a50a8c643 100644
--- a/compiler/rustc_mir_transform/src/ffi_unwind_calls.rs
+++ b/compiler/rustc_mir_transform/src/ffi_unwind_calls.rs
@@ -65,7 +65,7 @@ fn has_ffi_unwind_calls(tcx: TyCtxt<'_>, local_def_id: LocalDefId) -> bool {
 
     let mut tainted = false;
 
-    for block in body.basic_blocks() {
+    for block in body.basic_blocks.iter() {
         if block.is_cleanup {
             continue;
         }
diff --git a/compiler/rustc_mir_transform/src/generator.rs b/compiler/rustc_mir_transform/src/generator.rs
index 5b0d9900c0fb5..c260611b40712 100644
--- a/compiler/rustc_mir_transform/src/generator.rs
+++ b/compiler/rustc_mir_transform/src/generator.rs
@@ -490,12 +490,12 @@ fn locals_live_across_suspend_points<'tcx>(
         .iterate_to_fixpoint()
         .into_results_cursor(body_ref);
 
-    let mut storage_liveness_map = IndexVec::from_elem(None, body.basic_blocks());
+    let mut storage_liveness_map = IndexVec::from_elem(None, &body.basic_blocks);
     let mut live_locals_at_suspension_points = Vec::new();
     let mut source_info_at_suspension_points = Vec::new();
     let mut live_locals_at_any_suspension_point = BitSet::new_empty(body.local_decls.len());
 
-    for (block, data) in body.basic_blocks().iter_enumerated() {
+    for (block, data) in body.basic_blocks.iter_enumerated() {
         if let TerminatorKind::Yield { .. } = data.terminator().kind {
             let loc = Location { block, statement_index: data.statements.len() };
 
@@ -704,7 +704,7 @@ impl<'mir, 'tcx> rustc_mir_dataflow::ResultsVisitor<'mir, 'tcx>
 impl StorageConflictVisitor<'_, '_, '_> {
     fn apply_state(&mut self, flow_state: &BitSet<Local>, loc: Location) {
         // Ignore unreachable blocks.
-        if self.body.basic_blocks()[loc.block].terminator().kind == TerminatorKind::Unreachable {
+        if self.body.basic_blocks[loc.block].terminator().kind == TerminatorKind::Unreachable {
             return;
         }
 
@@ -886,7 +886,7 @@ fn elaborate_generator_drops<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
 
     let mut elaborator = DropShimElaborator { body, patch: MirPatch::new(body), tcx, param_env };
 
-    for (block, block_data) in body.basic_blocks().iter_enumerated() {
+    for (block, block_data) in body.basic_blocks.iter_enumerated() {
         let (target, unwind, source_info) = match block_data.terminator() {
             Terminator { source_info, kind: TerminatorKind::Drop { place, target, unwind } } => {
                 if let Some(local) = place.as_local() {
@@ -991,7 +991,7 @@ fn insert_panic_block<'tcx>(
     body: &mut Body<'tcx>,
     message: AssertMessage<'tcx>,
 ) -> BasicBlock {
-    let assert_block = BasicBlock::new(body.basic_blocks().len());
+    let assert_block = BasicBlock::new(body.basic_blocks.len());
     let term = TerminatorKind::Assert {
         cond: Operand::Constant(Box::new(Constant {
             span: body.span,
@@ -1021,7 +1021,7 @@ fn can_return<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, param_env: ty::ParamEn
     }
 
     // If there's a return terminator the function may return.
-    for block in body.basic_blocks() {
+    for block in body.basic_blocks.iter() {
         if let TerminatorKind::Return = block.terminator().kind {
             return true;
         }
@@ -1038,7 +1038,7 @@ fn can_unwind<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>) -> bool {
     }
 
     // Unwinds can only start at certain terminators.
-    for block in body.basic_blocks() {
+    for block in body.basic_blocks.iter() {
         match block.terminator().kind {
             // These never unwind.
             TerminatorKind::Goto { .. }
diff --git a/compiler/rustc_mir_transform/src/inline.rs b/compiler/rustc_mir_transform/src/inline.rs
index d7d2984018804..ba00f16308eae 100644
--- a/compiler/rustc_mir_transform/src/inline.rs
+++ b/compiler/rustc_mir_transform/src/inline.rs
@@ -95,7 +95,7 @@ fn inline<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) -> bool {
         history: Vec::new(),
         changed: false,
     };
-    let blocks = BasicBlock::new(0)..body.basic_blocks().next_index();
+    let blocks = BasicBlock::new(0)..body.basic_blocks.next_index();
     this.process_blocks(body, blocks);
     this.changed
 }
@@ -217,9 +217,9 @@ impl<'tcx> Inliner<'tcx> {
             }
         }
 
-        let old_blocks = caller_body.basic_blocks().next_index();
+        let old_blocks = caller_body.basic_blocks.next_index();
         self.inline_call(caller_body, &callsite, callee_body);
-        let new_blocks = old_blocks..caller_body.basic_blocks().next_index();
+        let new_blocks = old_blocks..caller_body.basic_blocks.next_index();
 
         Ok(new_blocks)
     }
@@ -409,14 +409,14 @@ impl<'tcx> Inliner<'tcx> {
         // Give a bonus functions with a small number of blocks,
         // We normally have two or three blocks for even
         // very small functions.
-        if callee_body.basic_blocks().len() <= 3 {
+        if callee_body.basic_blocks.len() <= 3 {
             threshold += threshold / 4;
         }
         debug!("    final inline threshold = {}", threshold);
 
         // FIXME: Give a bonus to functions with only a single caller
         let diverges = matches!(
-            callee_body.basic_blocks()[START_BLOCK].terminator().kind,
+            callee_body.basic_blocks[START_BLOCK].terminator().kind,
             TerminatorKind::Unreachable | TerminatorKind::Call { target: None, .. }
         );
         if diverges && !matches!(callee_attrs.inline, InlineAttr::Always) {
@@ -434,13 +434,13 @@ impl<'tcx> Inliner<'tcx> {
 
         // Traverse the MIR manually so we can account for the effects of inlining on the CFG.
         let mut work_list = vec![START_BLOCK];
-        let mut visited = BitSet::new_empty(callee_body.basic_blocks().len());
+        let mut visited = BitSet::new_empty(callee_body.basic_blocks.len());
         while let Some(bb) = work_list.pop() {
             if !visited.insert(bb.index()) {
                 continue;
             }
 
-            let blk = &callee_body.basic_blocks()[bb];
+            let blk = &callee_body.basic_blocks[bb];
             checker.visit_basic_block_data(bb, blk);
 
             let term = blk.terminator();
@@ -541,7 +541,7 @@ impl<'tcx> Inliner<'tcx> {
                     args: &args,
                     new_locals: Local::new(caller_body.local_decls.len())..,
                     new_scopes: SourceScope::new(caller_body.source_scopes.len())..,
-                    new_blocks: BasicBlock::new(caller_body.basic_blocks().len())..,
+                    new_blocks: BasicBlock::new(caller_body.basic_blocks.len())..,
                     destination: dest,
                     callsite_scope: caller_body.source_scopes[callsite.source_info.scope].clone(),
                     callsite,
diff --git a/compiler/rustc_mir_transform/src/inline/cycle.rs b/compiler/rustc_mir_transform/src/inline/cycle.rs
index 7810218fd6744..b027f94925d2f 100644
--- a/compiler/rustc_mir_transform/src/inline/cycle.rs
+++ b/compiler/rustc_mir_transform/src/inline/cycle.rs
@@ -153,7 +153,7 @@ pub(crate) fn mir_inliner_callees<'tcx>(
         _ => tcx.instance_mir(instance),
     };
     let mut calls = FxIndexSet::default();
-    for bb_data in body.basic_blocks() {
+    for bb_data in body.basic_blocks.iter() {
         let terminator = bb_data.terminator();
         if let TerminatorKind::Call { func, .. } = &terminator.kind {
             let ty = func.ty(&body.local_decls, tcx);
diff --git a/compiler/rustc_mir_transform/src/multiple_return_terminators.rs b/compiler/rustc_mir_transform/src/multiple_return_terminators.rs
index 22b6dead99c50..3957cd92c4e39 100644
--- a/compiler/rustc_mir_transform/src/multiple_return_terminators.rs
+++ b/compiler/rustc_mir_transform/src/multiple_return_terminators.rs
@@ -15,7 +15,7 @@ impl<'tcx> MirPass<'tcx> for MultipleReturnTerminators {
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         // find basic blocks with no statement and a return terminator
-        let mut bbs_simple_returns = BitSet::new_empty(body.basic_blocks().len());
+        let mut bbs_simple_returns = BitSet::new_empty(body.basic_blocks.len());
         let def_id = body.source.def_id();
         let bbs = body.basic_blocks_mut();
         for idx in bbs.indices() {
diff --git a/compiler/rustc_mir_transform/src/normalize_array_len.rs b/compiler/rustc_mir_transform/src/normalize_array_len.rs
index c0217a105414b..a159e61717823 100644
--- a/compiler/rustc_mir_transform/src/normalize_array_len.rs
+++ b/compiler/rustc_mir_transform/src/normalize_array_len.rs
@@ -21,10 +21,10 @@ impl<'tcx> MirPass<'tcx> for NormalizeArrayLen {
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         // early returns for edge cases of highly unrolled functions
-        if body.basic_blocks().len() > MAX_NUM_BLOCKS {
+        if body.basic_blocks.len() > MAX_NUM_BLOCKS {
             return;
         }
-        if body.local_decls().len() > MAX_NUM_LOCALS {
+        if body.local_decls.len() > MAX_NUM_LOCALS {
             return;
         }
         normalize_array_len_calls(tcx, body)
diff --git a/compiler/rustc_mir_transform/src/nrvo.rs b/compiler/rustc_mir_transform/src/nrvo.rs
index 42d732730eccc..4291e81c78c25 100644
--- a/compiler/rustc_mir_transform/src/nrvo.rs
+++ b/compiler/rustc_mir_transform/src/nrvo.rs
@@ -89,7 +89,7 @@ fn local_eligible_for_nrvo(body: &mut mir::Body<'_>) -> Option<Local> {
     }
 
     let mut copied_to_return_place = None;
-    for block in body.basic_blocks().indices() {
+    for block in body.basic_blocks.indices() {
         // Look for blocks with a `Return` terminator.
         if !matches!(body[block].terminator().kind, mir::TerminatorKind::Return) {
             continue;
@@ -122,7 +122,7 @@ fn find_local_assigned_to_return_place(
     body: &mut mir::Body<'_>,
 ) -> Option<Local> {
     let mut block = start;
-    let mut seen = HybridBitSet::new_empty(body.basic_blocks().len());
+    let mut seen = HybridBitSet::new_empty(body.basic_blocks.len());
 
     // Iterate as long as `block` has exactly one predecessor that we have not yet visited.
     while seen.insert(block) {
diff --git a/compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs b/compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs
index 5c441c5b19492..41a0bfac41aed 100644
--- a/compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs
+++ b/compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs
@@ -94,7 +94,7 @@ impl RemoveNoopLandingPads {
 
         let mut jumps_folded = 0;
         let mut landing_pads_removed = 0;
-        let mut nop_landing_pads = BitSet::new_empty(body.basic_blocks().len());
+        let mut nop_landing_pads = BitSet::new_empty(body.basic_blocks.len());
 
         // This is a post-order traversal, so that if A post-dominates B
         // then A will be visited before B.
diff --git a/compiler/rustc_mir_transform/src/remove_uninit_drops.rs b/compiler/rustc_mir_transform/src/remove_uninit_drops.rs
index 96b715402e772..78b6f714a9b0b 100644
--- a/compiler/rustc_mir_transform/src/remove_uninit_drops.rs
+++ b/compiler/rustc_mir_transform/src/remove_uninit_drops.rs
@@ -35,7 +35,7 @@ impl<'tcx> MirPass<'tcx> for RemoveUninitDrops {
             .into_results_cursor(body);
 
         let mut to_remove = vec![];
-        for (bb, block) in body.basic_blocks().iter_enumerated() {
+        for (bb, block) in body.basic_blocks.iter_enumerated() {
             let terminator = block.terminator();
             let (TerminatorKind::Drop { place, .. } | TerminatorKind::DropAndReplace { place, .. })
                 = &terminator.kind
diff --git a/compiler/rustc_mir_transform/src/separate_const_switch.rs b/compiler/rustc_mir_transform/src/separate_const_switch.rs
index 925eb10a1f75f..190f9c1ac158c 100644
--- a/compiler/rustc_mir_transform/src/separate_const_switch.rs
+++ b/compiler/rustc_mir_transform/src/separate_const_switch.rs
@@ -62,7 +62,7 @@ impl<'tcx> MirPass<'tcx> for SeparateConstSwitch {
 pub fn separate_const_switch(body: &mut Body<'_>) -> usize {
     let mut new_blocks: SmallVec<[(BasicBlock, BasicBlock); 6]> = SmallVec::new();
     let predecessors = body.basic_blocks.predecessors();
-    'block_iter: for (block_id, block) in body.basic_blocks().iter_enumerated() {
+    'block_iter: for (block_id, block) in body.basic_blocks.iter_enumerated() {
         if let TerminatorKind::SwitchInt {
             discr: Operand::Copy(switch_place) | Operand::Move(switch_place),
             ..
@@ -90,7 +90,7 @@ pub fn separate_const_switch(body: &mut Body<'_>) -> usize {
 
                 let mut predecessors_left = predecessors[block_id].len();
                 'predec_iter: for predecessor_id in predecessors[block_id].iter().copied() {
-                    let predecessor = &body.basic_blocks()[predecessor_id];
+                    let predecessor = &body.basic_blocks[predecessor_id];
 
                     // First we make sure the predecessor jumps
                     // in a reasonable way
diff --git a/compiler/rustc_mir_transform/src/simplify.rs b/compiler/rustc_mir_transform/src/simplify.rs
index 7a6ca917d0ffd..bed48db959a53 100644
--- a/compiler/rustc_mir_transform/src/simplify.rs
+++ b/compiler/rustc_mir_transform/src/simplify.rs
@@ -74,7 +74,7 @@ pub struct CfgSimplifier<'a, 'tcx> {
 
 impl<'a, 'tcx> CfgSimplifier<'a, 'tcx> {
     pub fn new(body: &'a mut Body<'tcx>) -> Self {
-        let mut pred_count = IndexVec::from_elem(0u32, body.basic_blocks());
+        let mut pred_count = IndexVec::from_elem(0u32, &body.basic_blocks);
 
         // we can't use mir.predecessors() here because that counts
         // dead blocks, which we don't want to.
@@ -263,7 +263,7 @@ impl<'a, 'tcx> CfgSimplifier<'a, 'tcx> {
 
 pub fn remove_dead_blocks<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
     let reachable = traversal::reachable_as_bitset(body);
-    let num_blocks = body.basic_blocks().len();
+    let num_blocks = body.basic_blocks.len();
     if num_blocks == reachable.count() {
         return;
     }
diff --git a/compiler/rustc_mir_transform/src/simplify_comparison_integral.rs b/compiler/rustc_mir_transform/src/simplify_comparison_integral.rs
index bbfaace7041ec..321d8c63b6e02 100644
--- a/compiler/rustc_mir_transform/src/simplify_comparison_integral.rs
+++ b/compiler/rustc_mir_transform/src/simplify_comparison_integral.rs
@@ -151,7 +151,7 @@ struct OptimizationFinder<'a, 'tcx> {
 impl<'tcx> OptimizationFinder<'_, 'tcx> {
     fn find_optimizations(&self) -> Vec<OptimizationInfo<'tcx>> {
         self.body
-            .basic_blocks()
+            .basic_blocks
             .iter_enumerated()
             .filter_map(|(bb_idx, bb)| {
                 // find switch
diff --git a/compiler/rustc_mir_transform/src/simplify_try.rs b/compiler/rustc_mir_transform/src/simplify_try.rs
index d52f1261b23a2..baeb620ef2403 100644
--- a/compiler/rustc_mir_transform/src/simplify_try.rs
+++ b/compiler/rustc_mir_transform/src/simplify_try.rs
@@ -596,7 +596,7 @@ struct SimplifyBranchSameOptimizationFinder<'a, 'tcx> {
 impl<'tcx> SimplifyBranchSameOptimizationFinder<'_, 'tcx> {
     fn find(&self) -> Vec<SimplifyBranchSameOptimization> {
         self.body
-            .basic_blocks()
+            .basic_blocks
             .iter_enumerated()
             .filter_map(|(bb_idx, bb)| {
                 let (discr_switched_on, targets_and_values) = match &bb.terminator().kind {
@@ -632,7 +632,7 @@ impl<'tcx> SimplifyBranchSameOptimizationFinder<'_, 'tcx> {
 
                 let mut iter_bbs_reachable = targets_and_values
                     .iter()
-                    .map(|target_and_value| (target_and_value, &self.body.basic_blocks()[target_and_value.target]))
+                    .map(|target_and_value| (target_and_value, &self.body.basic_blocks[target_and_value.target]))
                     .filter(|(_, bb)| {
                         // Reaching `unreachable` is UB so assume it doesn't happen.
                         bb.terminator().kind != TerminatorKind::Unreachable
diff --git a/compiler/rustc_mir_transform/src/uninhabited_enum_branching.rs b/compiler/rustc_mir_transform/src/uninhabited_enum_branching.rs
index 30be64f5b2f2e..96ea15f1b8097 100644
--- a/compiler/rustc_mir_transform/src/uninhabited_enum_branching.rs
+++ b/compiler/rustc_mir_transform/src/uninhabited_enum_branching.rs
@@ -79,7 +79,7 @@ fn ensure_otherwise_unreachable<'tcx>(
     targets: &SwitchTargets,
 ) -> Option<BasicBlockData<'tcx>> {
     let otherwise = targets.otherwise();
-    let bb = &body.basic_blocks()[otherwise];
+    let bb = &body.basic_blocks[otherwise];
     if bb.terminator().kind == TerminatorKind::Unreachable
         && bb.statements.iter().all(|s| matches!(&s.kind, StatementKind::StorageDead(_)))
     {
@@ -102,10 +102,10 @@ impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         trace!("UninhabitedEnumBranching starting for {:?}", body.source);
 
-        for bb in body.basic_blocks().indices() {
+        for bb in body.basic_blocks.indices() {
             trace!("processing block {:?}", bb);
 
-            let Some(discriminant_ty) = get_switched_on_type(&body.basic_blocks()[bb], tcx, body) else {
+            let Some(discriminant_ty) = get_switched_on_type(&body.basic_blocks[bb], tcx, body) else {
                 continue;
             };
 
diff --git a/compiler/rustc_monomorphize/src/partitioning/mod.rs b/compiler/rustc_monomorphize/src/partitioning/mod.rs
index ff2d386932858..43319dc5862e6 100644
--- a/compiler/rustc_monomorphize/src/partitioning/mod.rs
+++ b/compiler/rustc_monomorphize/src/partitioning/mod.rs
@@ -481,7 +481,7 @@ fn codegened_and_inlined_items<'tcx>(tcx: TyCtxt<'tcx>, (): ()) -> &'tcx DefIdSe
                     continue;
                 }
                 let body = tcx.instance_mir(instance.def);
-                for block in body.basic_blocks() {
+                for block in body.basic_blocks.iter() {
                     for statement in &block.statements {
                         let mir::StatementKind::Coverage(_) = statement.kind else { continue };
                         let scope = statement.source_info.scope;
diff --git a/compiler/rustc_passes/src/errors.rs b/compiler/rustc_passes/src/errors.rs
index 383982013d9e7..901f56ad96d17 100644
--- a/compiler/rustc_passes/src/errors.rs
+++ b/compiler/rustc_passes/src/errors.rs
@@ -28,7 +28,7 @@ pub struct IgnoredInlineAttrFnProto;
 
 #[derive(LintDiagnostic)]
 #[diag(passes::inline_ignored_constants)]
-#[warn_]
+#[warning]
 #[note]
 pub struct IgnoredInlineAttrConstants;
 
@@ -347,7 +347,7 @@ pub struct MustNotSuspend {
 
 #[derive(LintDiagnostic)]
 #[diag(passes::cold)]
-#[warn_]
+#[warning]
 pub struct Cold {
     #[label]
     pub span: Span,
@@ -355,7 +355,7 @@ pub struct Cold {
 
 #[derive(LintDiagnostic)]
 #[diag(passes::link)]
-#[warn_]
+#[warning]
 pub struct Link {
     #[label]
     pub span: Option<Span>,
@@ -363,7 +363,7 @@ pub struct Link {
 
 #[derive(LintDiagnostic)]
 #[diag(passes::link_name)]
-#[warn_]
+#[warning]
 pub struct LinkName<'a> {
     #[help]
     pub attr_span: Option<Span>,
@@ -449,7 +449,7 @@ pub struct RustcDirtyClean {
 
 #[derive(LintDiagnostic)]
 #[diag(passes::link_section)]
-#[warn_]
+#[warning]
 pub struct LinkSection {
     #[label]
     pub span: Span,
@@ -457,7 +457,7 @@ pub struct LinkSection {
 
 #[derive(LintDiagnostic)]
 #[diag(passes::no_mangle_foreign)]
-#[warn_]
+#[warning]
 #[note]
 pub struct NoMangleForeign {
     #[label]
@@ -469,7 +469,7 @@ pub struct NoMangleForeign {
 
 #[derive(LintDiagnostic)]
 #[diag(passes::no_mangle)]
-#[warn_]
+#[warning]
 pub struct NoMangle {
     #[label]
     pub span: Span,
@@ -617,7 +617,7 @@ pub struct UnusedDuplicate {
     pub this: Span,
     #[note]
     pub other: Span,
-    #[warn_]
+    #[warning]
     pub warning: Option<()>,
 }
 
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index 70fac83325a9c..e4af7022239b8 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -1506,13 +1506,28 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                 .emit();
             }
             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
-                self.report_mismatched_consts(
+                let mut diag = self.report_mismatched_consts(
                     &error.obligation.cause,
                     expected_found.expected,
                     expected_found.found,
                     err.clone(),
-                )
-                .emit();
+                );
+                let code = error.obligation.cause.code().peel_derives().peel_match_impls();
+                if let ObligationCauseCode::BindingObligation(..)
+                | ObligationCauseCode::ItemObligation(..)
+                | ObligationCauseCode::ExprBindingObligation(..)
+                | ObligationCauseCode::ExprItemObligation(..) = code
+                {
+                    self.note_obligation_cause_code(
+                        &mut diag,
+                        &error.obligation.predicate,
+                        error.obligation.param_env,
+                        code,
+                        &mut vec![],
+                        &mut Default::default(),
+                    );
+                }
+                diag.emit();
             }
         }
     }
diff --git a/compiler/rustc_ty_utils/src/ty.rs b/compiler/rustc_ty_utils/src/ty.rs
index db0d45b86fc03..acfeefb4d12d5 100644
--- a/compiler/rustc_ty_utils/src/ty.rs
+++ b/compiler/rustc_ty_utils/src/ty.rs
@@ -348,7 +348,7 @@ fn instance_def_size_estimate<'tcx>(
     match instance_def {
         InstanceDef::Item(..) | InstanceDef::DropGlue(..) => {
             let mir = tcx.instance_mir(instance_def);
-            mir.basic_blocks().iter().map(|bb| bb.statements.len() + 1).sum()
+            mir.basic_blocks.iter().map(|bb| bb.statements.len() + 1).sum()
         }
         // Estimate the size of other compiler-generated shims to be 1.
         _ => 1,
diff --git a/library/core/tests/option.rs b/library/core/tests/option.rs
index 9f5e537dcefc0..84eb4fc0aa329 100644
--- a/library/core/tests/option.rs
+++ b/library/core/tests/option.rs
@@ -57,6 +57,7 @@ fn test_get_resource() {
 }
 
 #[test]
+#[cfg_attr(not(bootstrap), allow(for_loop_over_fallibles))]
 fn test_option_dance() {
     let x = Some(());
     let mut y = Some(5);
diff --git a/library/std/src/io/mod.rs b/library/std/src/io/mod.rs
index a9fa7b0a6aca0..f244a23abe25d 100644
--- a/library/std/src/io/mod.rs
+++ b/library/std/src/io/mod.rs
@@ -265,6 +265,8 @@ pub use self::buffered::WriterPanicked;
 #[unstable(feature = "internal_output_capture", issue = "none")]
 #[doc(no_inline, hidden)]
 pub use self::stdio::set_output_capture;
+#[unstable(feature = "is_terminal", issue = "98070")]
+pub use self::stdio::IsTerminal;
 #[unstable(feature = "print_internals", issue = "none")]
 pub use self::stdio::{_eprint, _print};
 #[stable(feature = "rust1", since = "1.0.0")]
diff --git a/library/std/src/io/stdio.rs b/library/std/src/io/stdio.rs
index 4d3736f79146c..0c2b02c3bc360 100644
--- a/library/std/src/io/stdio.rs
+++ b/library/std/src/io/stdio.rs
@@ -7,6 +7,7 @@ use crate::io::prelude::*;
 
 use crate::cell::{Cell, RefCell};
 use crate::fmt;
+use crate::fs::File;
 use crate::io::{self, BufReader, IoSlice, IoSliceMut, LineWriter, Lines};
 use crate::pin::Pin;
 use crate::sync::atomic::{AtomicBool, Ordering};
@@ -1016,6 +1017,34 @@ where
     }
 }
 
+/// Trait to determine if a descriptor/handle refers to a terminal/tty.
+#[unstable(feature = "is_terminal", issue = "98070")]
+pub trait IsTerminal: crate::sealed::Sealed {
+    /// Returns `true` if the descriptor/handle refers to a terminal/tty.
+    ///
+    /// On platforms where Rust does not know how to detect a terminal yet, this will return
+    /// `false`. This will also return `false` if an unexpected error occurred, such as from
+    /// passing an invalid file descriptor.
+    fn is_terminal(&self) -> bool;
+}
+
+macro_rules! impl_is_terminal {
+    ($($t:ty),*$(,)?) => {$(
+        #[unstable(feature = "sealed", issue = "none")]
+        impl crate::sealed::Sealed for $t {}
+
+        #[unstable(feature = "is_terminal", issue = "98070")]
+        impl IsTerminal for $t {
+            #[inline]
+            fn is_terminal(&self) -> bool {
+                crate::sys::io::is_terminal(self)
+            }
+        }
+    )*}
+}
+
+impl_is_terminal!(File, Stdin, StdinLock<'_>, Stdout, StdoutLock<'_>, Stderr, StderrLock<'_>);
+
 #[unstable(
     feature = "print_internals",
     reason = "implementation detail which may disappear or be replaced at any time",
diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index c918c9ba818e0..eba8e975472a4 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -252,6 +252,7 @@
 #![feature(dropck_eyepatch)]
 #![feature(exhaustive_patterns)]
 #![feature(intra_doc_pointers)]
+#![feature(is_terminal)]
 #![cfg_attr(bootstrap, feature(label_break_value))]
 #![feature(lang_items)]
 #![feature(let_else)]
diff --git a/library/std/src/net/parser.rs b/library/std/src/net/parser.rs
index 069b660998559..a38031c48c862 100644
--- a/library/std/src/net/parser.rs
+++ b/library/std/src/net/parser.rs
@@ -39,8 +39,8 @@ struct Parser<'a> {
 }
 
 impl<'a> Parser<'a> {
-    fn new(input: &'a str) -> Parser<'a> {
-        Parser { state: input.as_bytes() }
+    fn new(input: &'a [u8]) -> Parser<'a> {
+        Parser { state: input }
     }
 
     /// Run a parser, and restore the pre-parse state if it fails.
@@ -273,32 +273,106 @@ impl<'a> Parser<'a> {
     }
 }
 
+impl IpAddr {
+    /// Parse an IP address from a slice of bytes.
+    ///
+    /// ```
+    /// #![feature(addr_parse_ascii)]
+    ///
+    /// use std::net::{IpAddr, Ipv4Addr, Ipv6Addr};
+    ///
+    /// let localhost_v4 = IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1));
+    /// let localhost_v6 = IpAddr::V6(Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 1));
+    ///
+    /// assert_eq!(IpAddr::parse_ascii(b"127.0.0.1"), Ok(localhost_v4));
+    /// assert_eq!(IpAddr::parse_ascii(b"::1"), Ok(localhost_v6));
+    /// ```
+    #[unstable(feature = "addr_parse_ascii", issue = "101035")]
+    pub fn parse_ascii(b: &[u8]) -> Result<Self, AddrParseError> {
+        Parser::new(b).parse_with(|p| p.read_ip_addr(), AddrKind::Ip)
+    }
+}
+
 #[stable(feature = "ip_addr", since = "1.7.0")]
 impl FromStr for IpAddr {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<IpAddr, AddrParseError> {
-        Parser::new(s).parse_with(|p| p.read_ip_addr(), AddrKind::Ip)
+        Self::parse_ascii(s.as_bytes())
     }
 }
 
-#[stable(feature = "rust1", since = "1.0.0")]
-impl FromStr for Ipv4Addr {
-    type Err = AddrParseError;
-    fn from_str(s: &str) -> Result<Ipv4Addr, AddrParseError> {
+impl Ipv4Addr {
+    /// Parse an IPv4 address from a slice of bytes.
+    ///
+    /// ```
+    /// #![feature(addr_parse_ascii)]
+    ///
+    /// use std::net::Ipv4Addr;
+    ///
+    /// let localhost = Ipv4Addr::new(127, 0, 0, 1);
+    ///
+    /// assert_eq!(Ipv4Addr::parse_ascii(b"127.0.0.1"), Ok(localhost));
+    /// ```
+    #[unstable(feature = "addr_parse_ascii", issue = "101035")]
+    pub fn parse_ascii(b: &[u8]) -> Result<Self, AddrParseError> {
         // don't try to parse if too long
-        if s.len() > 15 {
+        if b.len() > 15 {
             Err(AddrParseError(AddrKind::Ipv4))
         } else {
-            Parser::new(s).parse_with(|p| p.read_ipv4_addr(), AddrKind::Ipv4)
+            Parser::new(b).parse_with(|p| p.read_ipv4_addr(), AddrKind::Ipv4)
         }
     }
 }
 
+#[stable(feature = "rust1", since = "1.0.0")]
+impl FromStr for Ipv4Addr {
+    type Err = AddrParseError;
+    fn from_str(s: &str) -> Result<Ipv4Addr, AddrParseError> {
+        Self::parse_ascii(s.as_bytes())
+    }
+}
+
+impl Ipv6Addr {
+    /// Parse an IPv6 address from a slice of bytes.
+    ///
+    /// ```
+    /// #![feature(addr_parse_ascii)]
+    ///
+    /// use std::net::Ipv6Addr;
+    ///
+    /// let localhost = Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 1);
+    ///
+    /// assert_eq!(Ipv6Addr::parse_ascii(b"::1"), Ok(localhost));
+    /// ```
+    #[unstable(feature = "addr_parse_ascii", issue = "101035")]
+    pub fn parse_ascii(b: &[u8]) -> Result<Self, AddrParseError> {
+        Parser::new(b).parse_with(|p| p.read_ipv6_addr(), AddrKind::Ipv6)
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl FromStr for Ipv6Addr {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<Ipv6Addr, AddrParseError> {
-        Parser::new(s).parse_with(|p| p.read_ipv6_addr(), AddrKind::Ipv6)
+        Self::parse_ascii(s.as_bytes())
+    }
+}
+
+impl SocketAddrV4 {
+    /// Parse an IPv4 socket address from a slice of bytes.
+    ///
+    /// ```
+    /// #![feature(addr_parse_ascii)]
+    ///
+    /// use std::net::{Ipv4Addr, SocketAddrV4};
+    ///
+    /// let socket = SocketAddrV4::new(Ipv4Addr::new(127, 0, 0, 1), 8080);
+    ///
+    /// assert_eq!(SocketAddrV4::parse_ascii(b"127.0.0.1:8080"), Ok(socket));
+    /// ```
+    #[unstable(feature = "addr_parse_ascii", issue = "101035")]
+    pub fn parse_ascii(b: &[u8]) -> Result<Self, AddrParseError> {
+        Parser::new(b).parse_with(|p| p.read_socket_addr_v4(), AddrKind::SocketV4)
     }
 }
 
@@ -306,7 +380,25 @@ impl FromStr for Ipv6Addr {
 impl FromStr for SocketAddrV4 {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<SocketAddrV4, AddrParseError> {
-        Parser::new(s).parse_with(|p| p.read_socket_addr_v4(), AddrKind::SocketV4)
+        Self::parse_ascii(s.as_bytes())
+    }
+}
+
+impl SocketAddrV6 {
+    /// Parse an IPv6 socket address from a slice of bytes.
+    ///
+    /// ```
+    /// #![feature(addr_parse_ascii)]
+    ///
+    /// use std::net::{Ipv6Addr, SocketAddrV6};
+    ///
+    /// let socket = SocketAddrV6::new(Ipv6Addr::new(0x2001, 0xdb8, 0, 0, 0, 0, 0, 1), 8080, 0, 0);
+    ///
+    /// assert_eq!(SocketAddrV6::parse_ascii(b"[2001:db8::1]:8080"), Ok(socket));
+    /// ```
+    #[unstable(feature = "addr_parse_ascii", issue = "101035")]
+    pub fn parse_ascii(b: &[u8]) -> Result<Self, AddrParseError> {
+        Parser::new(b).parse_with(|p| p.read_socket_addr_v6(), AddrKind::SocketV6)
     }
 }
 
@@ -314,7 +406,27 @@ impl FromStr for SocketAddrV4 {
 impl FromStr for SocketAddrV6 {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<SocketAddrV6, AddrParseError> {
-        Parser::new(s).parse_with(|p| p.read_socket_addr_v6(), AddrKind::SocketV6)
+        Self::parse_ascii(s.as_bytes())
+    }
+}
+
+impl SocketAddr {
+    /// Parse a socket address from a slice of bytes.
+    ///
+    /// ```
+    /// #![feature(addr_parse_ascii)]
+    ///
+    /// use std::net::{IpAddr, Ipv4Addr, Ipv6Addr, SocketAddr};
+    ///
+    /// let socket_v4 = SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1)), 8080);
+    /// let socket_v6 = SocketAddr::new(IpAddr::V6(Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 1)), 8080);
+    ///
+    /// assert_eq!(SocketAddr::parse_ascii(b"127.0.0.1:8080"), Ok(socket_v4));
+    /// assert_eq!(SocketAddr::parse_ascii(b"[::1]:8080"), Ok(socket_v6));
+    /// ```
+    #[unstable(feature = "addr_parse_ascii", issue = "101035")]
+    pub fn parse_ascii(b: &[u8]) -> Result<Self, AddrParseError> {
+        Parser::new(b).parse_with(|p| p.read_socket_addr(), AddrKind::Socket)
     }
 }
 
@@ -322,7 +434,7 @@ impl FromStr for SocketAddrV6 {
 impl FromStr for SocketAddr {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<SocketAddr, AddrParseError> {
-        Parser::new(s).parse_with(|p| p.read_socket_addr(), AddrKind::Socket)
+        Self::parse_ascii(s.as_bytes())
     }
 }
 
diff --git a/library/std/src/os/fd/owned.rs b/library/std/src/os/fd/owned.rs
index a463bc41db7aa..2475c99923336 100644
--- a/library/std/src/os/fd/owned.rs
+++ b/library/std/src/os/fd/owned.rs
@@ -191,6 +191,23 @@ impl fmt::Debug for OwnedFd {
     }
 }
 
+macro_rules! impl_is_terminal {
+    ($($t:ty),*$(,)?) => {$(
+        #[unstable(feature = "sealed", issue = "none")]
+        impl crate::sealed::Sealed for $t {}
+
+        #[unstable(feature = "is_terminal", issue = "98070")]
+        impl crate::io::IsTerminal for $t {
+            #[inline]
+            fn is_terminal(&self) -> bool {
+                crate::sys::io::is_terminal(self)
+            }
+        }
+    )*}
+}
+
+impl_is_terminal!(BorrowedFd<'_>, OwnedFd);
+
 /// A trait to borrow the file descriptor from an underlying object.
 ///
 /// This is only available on unix platforms and must be imported in order to
diff --git a/library/std/src/os/mod.rs b/library/std/src/os/mod.rs
index 6fbaa42c76846..65e53d9e22e15 100644
--- a/library/std/src/os/mod.rs
+++ b/library/std/src/os/mod.rs
@@ -147,4 +147,4 @@ pub mod solid;
 pub mod vxworks;
 
 #[cfg(any(unix, target_os = "wasi", doc))]
-mod fd;
+pub(crate) mod fd;
diff --git a/library/std/src/os/unix/net/datagram.rs b/library/std/src/os/unix/net/datagram.rs
index 8008acfd1c96f..f758f88d0a370 100644
--- a/library/std/src/os/unix/net/datagram.rs
+++ b/library/std/src/os/unix/net/datagram.rs
@@ -838,6 +838,31 @@ impl UnixDatagram {
         self.0.passcred()
     }
 
+    /// Set the id of the socket for network filtering purpose
+    ///
+    #[cfg_attr(
+        any(target_os = "linux", target_os = "freebsd", target_os = "openbsd"),
+        doc = "```no_run"
+    )]
+    #[cfg_attr(
+        not(any(target_os = "linux", target_os = "freebsd", target_os = "openbsd")),
+        doc = "```ignore"
+    )]
+    /// #![feature(unix_set_mark)]
+    /// use std::os::unix::net::UnixDatagram;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let sock = UnixDatagram::unbound()?;
+    ///     sock.set_mark(32)?;
+    ///     Ok(())
+    /// }
+    /// ```
+    #[cfg(any(doc, target_os = "linux", target_os = "freebsd", target_os = "openbsd",))]
+    #[unstable(feature = "unix_set_mark", issue = "96467")]
+    pub fn set_mark(&self, mark: u32) -> io::Result<()> {
+        self.0.set_mark(mark)
+    }
+
     /// Returns the value of the `SO_ERROR` option.
     ///
     /// # Examples
diff --git a/library/std/src/os/unix/net/stream.rs b/library/std/src/os/unix/net/stream.rs
index cc3a885879345..dff8f6e856750 100644
--- a/library/std/src/os/unix/net/stream.rs
+++ b/library/std/src/os/unix/net/stream.rs
@@ -427,6 +427,31 @@ impl UnixStream {
         self.0.passcred()
     }
 
+    /// Set the id of the socket for network filtering purpose
+    ///
+    #[cfg_attr(
+        any(target_os = "linux", target_os = "freebsd", target_os = "openbsd"),
+        doc = "```no_run"
+    )]
+    #[cfg_attr(
+        not(any(target_os = "linux", target_os = "freebsd", target_os = "openbsd")),
+        doc = "```ignore"
+    )]
+    /// #![feature(unix_set_mark)]
+    /// use std::os::unix::net::UnixStream;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let sock = UnixStream::connect("/tmp/sock")?;
+    ///     sock.set_mark(32)?;
+    ///     Ok(())
+    /// }
+    /// ```
+    #[cfg(any(doc, target_os = "linux", target_os = "freebsd", target_os = "openbsd",))]
+    #[unstable(feature = "unix_set_mark", issue = "96467")]
+    pub fn set_mark(&self, mark: u32) -> io::Result<()> {
+        self.0.set_mark(mark)
+    }
+
     /// Returns the value of the `SO_ERROR` option.
     ///
     /// # Examples
diff --git a/library/std/src/os/windows/io/handle.rs b/library/std/src/os/windows/io/handle.rs
index 16cc8fa2783ee..1dfecc57338a7 100644
--- a/library/std/src/os/windows/io/handle.rs
+++ b/library/std/src/os/windows/io/handle.rs
@@ -384,6 +384,23 @@ impl fmt::Debug for OwnedHandle {
     }
 }
 
+macro_rules! impl_is_terminal {
+    ($($t:ty),*$(,)?) => {$(
+        #[unstable(feature = "sealed", issue = "none")]
+        impl crate::sealed::Sealed for $t {}
+
+        #[unstable(feature = "is_terminal", issue = "98070")]
+        impl crate::io::IsTerminal for $t {
+            #[inline]
+            fn is_terminal(&self) -> bool {
+                crate::sys::io::is_terminal(self)
+            }
+        }
+    )*}
+}
+
+impl_is_terminal!(BorrowedHandle<'_>, OwnedHandle);
+
 /// A trait to borrow the handle from an underlying object.
 #[stable(feature = "io_safety", since = "1.63.0")]
 pub trait AsHandle {
diff --git a/library/std/src/sys/unix/io.rs b/library/std/src/sys/unix/io.rs
index deb5ee76bd035..8d189059d3641 100644
--- a/library/std/src/sys/unix/io.rs
+++ b/library/std/src/sys/unix/io.rs
@@ -1,4 +1,6 @@
 use crate::marker::PhantomData;
+use crate::os::fd::owned::AsFd;
+use crate::os::fd::raw::AsRawFd;
 use crate::slice;
 
 use libc::{c_void, iovec};
@@ -74,3 +76,8 @@ impl<'a> IoSliceMut<'a> {
         unsafe { slice::from_raw_parts_mut(self.vec.iov_base as *mut u8, self.vec.iov_len) }
     }
 }
+
+pub fn is_terminal(fd: &impl AsFd) -> bool {
+    let fd = fd.as_fd();
+    unsafe { libc::isatty(fd.as_raw_fd()) != 0 }
+}
diff --git a/library/std/src/sys/unix/net.rs b/library/std/src/sys/unix/net.rs
index 462a45b01ab49..004c5f4b40d91 100644
--- a/library/std/src/sys/unix/net.rs
+++ b/library/std/src/sys/unix/net.rs
@@ -427,6 +427,17 @@ impl Socket {
         self.0.set_nonblocking(nonblocking)
     }
 
+    #[cfg(any(target_os = "linux", target_os = "freebsd", target_os = "openbsd"))]
+    pub fn set_mark(&self, mark: u32) -> io::Result<()> {
+        #[cfg(target_os = "linux")]
+        let option = libc::SO_MARK;
+        #[cfg(target_os = "freebsd")]
+        let option = libc::SO_USER_COOKIE;
+        #[cfg(target_os = "openbsd")]
+        let option = libc::SO_RTABLE;
+        setsockopt(self, libc::SOL_SOCKET, option, mark as libc::c_int)
+    }
+
     pub fn take_error(&self) -> io::Result<Option<io::Error>> {
         let raw: c_int = getsockopt(self, libc::SOL_SOCKET, libc::SO_ERROR)?;
         if raw == 0 { Ok(None) } else { Ok(Some(io::Error::from_raw_os_error(raw as i32))) }
diff --git a/library/std/src/sys/unsupported/io.rs b/library/std/src/sys/unsupported/io.rs
index d5f475b4310fd..82610ffab7e1e 100644
--- a/library/std/src/sys/unsupported/io.rs
+++ b/library/std/src/sys/unsupported/io.rs
@@ -45,3 +45,7 @@ impl<'a> IoSliceMut<'a> {
         self.0
     }
 }
+
+pub fn is_terminal<T>(_: &T) -> bool {
+    false
+}
diff --git a/library/std/src/sys/wasi/io.rs b/library/std/src/sys/wasi/io.rs
index ee017d13a4ca0..bd9f8872333e2 100644
--- a/library/std/src/sys/wasi/io.rs
+++ b/library/std/src/sys/wasi/io.rs
@@ -1,6 +1,8 @@
 #![deny(unsafe_op_in_unsafe_fn)]
 
 use crate::marker::PhantomData;
+use crate::os::fd::owned::AsFd;
+use crate::os::fd::raw::AsRawFd;
 use crate::slice;
 
 #[derive(Copy, Clone)]
@@ -71,3 +73,8 @@ impl<'a> IoSliceMut<'a> {
         unsafe { slice::from_raw_parts_mut(self.vec.buf as *mut u8, self.vec.buf_len) }
     }
 }
+
+pub fn is_terminal(fd: &impl AsFd) -> bool {
+    let fd = fd.as_fd();
+    unsafe { libc::isatty(fd.as_raw_fd()) != 0 }
+}
diff --git a/library/std/src/sys/windows/c.rs b/library/std/src/sys/windows/c.rs
index ef3f6a9ba1755..09ca02afea241 100644
--- a/library/std/src/sys/windows/c.rs
+++ b/library/std/src/sys/windows/c.rs
@@ -125,6 +125,8 @@ pub const SECURITY_SQOS_PRESENT: DWORD = 0x00100000;
 
 pub const FIONBIO: c_ulong = 0x8004667e;
 
+pub const MAX_PATH: usize = 260;
+
 #[repr(C)]
 #[derive(Copy)]
 pub struct WIN32_FIND_DATAW {
@@ -521,6 +523,12 @@ pub struct SYMBOLIC_LINK_REPARSE_BUFFER {
     pub PathBuffer: WCHAR,
 }
 
+#[repr(C)]
+pub struct FILE_NAME_INFO {
+    pub FileNameLength: DWORD,
+    pub FileName: [WCHAR; 1],
+}
+
 #[repr(C)]
 pub struct MOUNT_POINT_REPARSE_BUFFER {
     pub SubstituteNameOffset: c_ushort,
diff --git a/library/std/src/sys/windows/io.rs b/library/std/src/sys/windows/io.rs
index fb06df1f80cda..7e7518b6d8905 100644
--- a/library/std/src/sys/windows/io.rs
+++ b/library/std/src/sys/windows/io.rs
@@ -1,6 +1,10 @@
 use crate::marker::PhantomData;
+use crate::mem::size_of;
+use crate::os::windows::io::{AsHandle, AsRawHandle, BorrowedHandle};
 use crate::slice;
 use crate::sys::c;
+use core;
+use libc;
 
 #[derive(Copy, Clone)]
 #[repr(transparent)]
@@ -78,3 +82,66 @@ impl<'a> IoSliceMut<'a> {
         unsafe { slice::from_raw_parts_mut(self.vec.buf as *mut u8, self.vec.len as usize) }
     }
 }
+
+pub fn is_terminal(h: &impl AsHandle) -> bool {
+    unsafe { handle_is_console(h.as_handle()) }
+}
+
+unsafe fn handle_is_console(handle: BorrowedHandle<'_>) -> bool {
+    let handle = handle.as_raw_handle();
+
+    // A null handle means the process has no console.
+    if handle.is_null() {
+        return false;
+    }
+
+    let mut out = 0;
+    if c::GetConsoleMode(handle, &mut out) != 0 {
+        // False positives aren't possible. If we got a console then we definitely have a console.
+        return true;
+    }
+
+    // At this point, we *could* have a false negative. We can determine that this is a true
+    // negative if we can detect the presence of a console on any of the standard I/O streams. If
+    // another stream has a console, then we know we're in a Windows console and can therefore
+    // trust the negative.
+    for std_handle in [c::STD_INPUT_HANDLE, c::STD_OUTPUT_HANDLE, c::STD_ERROR_HANDLE] {
+        let std_handle = c::GetStdHandle(std_handle);
+        if !std_handle.is_null()
+            && std_handle != handle
+            && c::GetConsoleMode(std_handle, &mut out) != 0
+        {
+            return false;
+        }
+    }
+
+    // Otherwise, we fall back to an msys hack to see if we can detect the presence of a pty.
+    msys_tty_on(handle)
+}
+
+unsafe fn msys_tty_on(handle: c::HANDLE) -> bool {
+    let size = size_of::<c::FILE_NAME_INFO>() + c::MAX_PATH * size_of::<c::WCHAR>();
+    let mut name_info_bytes = vec![0u8; size];
+    let res = c::GetFileInformationByHandleEx(
+        handle,
+        c::FileNameInfo,
+        name_info_bytes.as_mut_ptr() as *mut libc::c_void,
+        size as u32,
+    );
+    if res == 0 {
+        return false;
+    }
+    let name_info: &c::FILE_NAME_INFO = &*(name_info_bytes.as_ptr() as *const c::FILE_NAME_INFO);
+    let name_len = name_info.FileNameLength as usize / 2;
+    // Offset to get the `FileName` field.
+    let name_ptr = name_info_bytes.as_ptr().offset(size_of::<c::DWORD>() as isize).cast::<u16>();
+    let s = core::slice::from_raw_parts(name_ptr, name_len);
+    let name = String::from_utf16_lossy(s);
+    // This checks whether 'pty' exists in the file name, which indicates that
+    // a pseudo-terminal is attached. To mitigate against false positives
+    // (e.g., an actual file name that contains 'pty'), we also require that
+    // either the strings 'msys-' or 'cygwin-' are in the file name as well.)
+    let is_msys = name.contains("msys-") || name.contains("cygwin-");
+    let is_pty = name.contains("-pty");
+    is_msys && is_pty
+}
diff --git a/library/test/src/cli.rs b/library/test/src/cli.rs
index f981b9c495476..8be32183fe780 100644
--- a/library/test/src/cli.rs
+++ b/library/test/src/cli.rs
@@ -3,9 +3,9 @@
 use std::env;
 use std::path::PathBuf;
 
-use super::helpers::isatty;
 use super::options::{ColorConfig, Options, OutputFormat, RunIgnored};
 use super::time::TestTimeOptions;
+use std::io::{self, IsTerminal};
 
 #[derive(Debug)]
 pub struct TestOpts {
@@ -32,7 +32,7 @@ pub struct TestOpts {
 impl TestOpts {
     pub fn use_color(&self) -> bool {
         match self.color {
-            ColorConfig::AutoColor => !self.nocapture && isatty::stdout_isatty(),
+            ColorConfig::AutoColor => !self.nocapture && io::stdout().is_terminal(),
             ColorConfig::AlwaysColor => true,
             ColorConfig::NeverColor => false,
         }
diff --git a/library/test/src/helpers/isatty.rs b/library/test/src/helpers/isatty.rs
deleted file mode 100644
index 874ecc3764572..0000000000000
--- a/library/test/src/helpers/isatty.rs
+++ /dev/null
@@ -1,32 +0,0 @@
-//! Helper module which provides a function to test
-//! if stdout is a tty.
-
-cfg_if::cfg_if! {
-    if #[cfg(unix)] {
-        pub fn stdout_isatty() -> bool {
-            unsafe { libc::isatty(libc::STDOUT_FILENO) != 0 }
-        }
-    } else if #[cfg(windows)] {
-        pub fn stdout_isatty() -> bool {
-            type DWORD = u32;
-            type BOOL = i32;
-            type HANDLE = *mut u8;
-            type LPDWORD = *mut u32;
-            const STD_OUTPUT_HANDLE: DWORD = -11i32 as DWORD;
-            extern "system" {
-                fn GetStdHandle(which: DWORD) -> HANDLE;
-                fn GetConsoleMode(hConsoleHandle: HANDLE, lpMode: LPDWORD) -> BOOL;
-            }
-            unsafe {
-                let handle = GetStdHandle(STD_OUTPUT_HANDLE);
-                let mut out = 0;
-                GetConsoleMode(handle, &mut out) != 0
-            }
-        }
-    } else {
-        // FIXME: Implement isatty on SGX
-        pub fn stdout_isatty() -> bool {
-            false
-        }
-    }
-}
diff --git a/library/test/src/helpers/mod.rs b/library/test/src/helpers/mod.rs
index 049cadf86a6d0..6f366a911e8cd 100644
--- a/library/test/src/helpers/mod.rs
+++ b/library/test/src/helpers/mod.rs
@@ -3,6 +3,5 @@
 
 pub mod concurrency;
 pub mod exit_code;
-pub mod isatty;
 pub mod metrics;
 pub mod shuffle;
diff --git a/library/test/src/lib.rs b/library/test/src/lib.rs
index 3b7193adcc758..f595e2a74d4ad 100644
--- a/library/test/src/lib.rs
+++ b/library/test/src/lib.rs
@@ -17,6 +17,7 @@
 #![doc(test(attr(deny(warnings))))]
 #![feature(bench_black_box)]
 #![feature(internal_output_capture)]
+#![feature(is_terminal)]
 #![feature(staged_api)]
 #![feature(process_exitcode_internals)]
 #![feature(test)]
diff --git a/src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.rs b/src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.rs
index aaa8caa64f343..b1f557cb94de8 100644
--- a/src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.rs
+++ b/src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.rs
@@ -549,7 +549,7 @@ struct ErrorWithMultiSpan {
 
 #[derive(SessionDiagnostic)]
 #[diag(typeck::ambiguous_lifetime_bound, code = "E0123")]
-#[warn_]
+#[warning]
 struct ErrorWithWarn {
     val: String,
 }
@@ -562,11 +562,11 @@ struct ErrorWithWarn {
 struct ErrorAttribute {}
 
 #[derive(SessionDiagnostic)]
-#[warning(typeck::ambiguous_lifetime_bound, code = "E0123")]
-//~^ ERROR `#[warning(...)]` is not a valid attribute
+#[warn_(typeck::ambiguous_lifetime_bound, code = "E0123")]
+//~^ ERROR `#[warn_(...)]` is not a valid attribute
 //~| ERROR diagnostic slug not specified
-//~| ERROR cannot find attribute `warning` in this scope
-struct WarningAttribute {}
+//~| ERROR cannot find attribute `warn_` in this scope
+struct WarnAttribute {}
 
 #[derive(SessionDiagnostic)]
 #[lint(typeck::ambiguous_lifetime_bound, code = "E0123")]
diff --git a/src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.stderr b/src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.stderr
index 866b1a1de999c..621c59f448951 100644
--- a/src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.stderr
+++ b/src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.stderr
@@ -21,7 +21,7 @@ error: `#[nonsense(...)]` is not a valid attribute
 LL | #[nonsense(typeck::ambiguous_lifetime_bound, code = "E0123")]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: only `diag`, `help`, `note` and `warn_` are valid attributes
+   = help: only `diag`, `help`, `note` and `warning` are valid attributes
 
 error: diagnostic slug not specified
   --> $DIR/diagnostic-derive.rs:53:1
@@ -329,7 +329,7 @@ error: `#[error(...)]` is not a valid attribute
 LL | #[error(typeck::ambiguous_lifetime_bound, code = "E0123")]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: `error`, `warning` and `lint` have been replaced by `diag`
+   = help: `error` and `lint` have been replaced by `diag`
 
 error: diagnostic slug not specified
   --> $DIR/diagnostic-derive.rs:558:1
@@ -343,23 +343,23 @@ LL | | struct ErrorAttribute {}
    |
    = help: specify the slug as the first argument to the `#[diag(...)]` attribute, such as `#[diag(typeck::example_error)]`
 
-error: `#[warning(...)]` is not a valid attribute
+error: `#[warn_(...)]` is not a valid attribute
   --> $DIR/diagnostic-derive.rs:565:1
    |
-LL | #[warning(typeck::ambiguous_lifetime_bound, code = "E0123")]
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | #[warn_(typeck::ambiguous_lifetime_bound, code = "E0123")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: `error`, `warning` and `lint` have been replaced by `diag`
+   = help: `warn_` have been replaced by `warning`
 
 error: diagnostic slug not specified
   --> $DIR/diagnostic-derive.rs:565:1
    |
-LL | / #[warning(typeck::ambiguous_lifetime_bound, code = "E0123")]
+LL | / #[warn_(typeck::ambiguous_lifetime_bound, code = "E0123")]
 LL | |
 LL | |
 LL | |
-LL | | struct WarningAttribute {}
-   | |__________________________^
+LL | | struct WarnAttribute {}
+   | |_______________________^
    |
    = help: specify the slug as the first argument to the `#[diag(...)]` attribute, such as `#[diag(typeck::example_error)]`
 
@@ -369,7 +369,7 @@ error: `#[lint(...)]` is not a valid attribute
 LL | #[lint(typeck::ambiguous_lifetime_bound, code = "E0123")]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: `error`, `warning` and `lint` have been replaced by `diag`
+   = help: `error` and `lint` have been replaced by `diag`
 
 error: diagnostic slug not specified
   --> $DIR/diagnostic-derive.rs:572:1
@@ -389,7 +389,7 @@ error: `#[lint(...)]` is not a valid attribute
 LL | #[lint(typeck::ambiguous_lifetime_bound, code = "E0123")]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: `error`, `warning` and `lint` have been replaced by `diag`
+   = help: `error` and `lint` have been replaced by `diag`
 
 error: diagnostic slug not specified
   --> $DIR/diagnostic-derive.rs:579:1
@@ -421,11 +421,11 @@ error: cannot find attribute `error` in this scope
 LL | #[error(typeck::ambiguous_lifetime_bound, code = "E0123")]
    |   ^^^^^
 
-error: cannot find attribute `warning` in this scope
+error: cannot find attribute `warn_` in this scope
   --> $DIR/diagnostic-derive.rs:565:3
    |
-LL | #[warning(typeck::ambiguous_lifetime_bound, code = "E0123")]
-   |   ^^^^^^^
+LL | #[warn_(typeck::ambiguous_lifetime_bound, code = "E0123")]
+   |   ^^^^^ help: a built-in attribute with a similar name exists: `warn`
 
 error: cannot find attribute `lint` in this scope
   --> $DIR/diagnostic-derive.rs:572:3
diff --git a/src/test/ui-fulldeps/session-diagnostic/subdiagnostic-derive.rs b/src/test/ui-fulldeps/session-diagnostic/subdiagnostic-derive.rs
index 16da25c402b57..ddfc0d3365df0 100644
--- a/src/test/ui-fulldeps/session-diagnostic/subdiagnostic-derive.rs
+++ b/src/test/ui-fulldeps/session-diagnostic/subdiagnostic-derive.rs
@@ -510,12 +510,11 @@ enum AX {
 }
 
 #[derive(SessionSubdiagnostic)]
-#[warn_(parser::add_paren)]
-struct AY {
-}
+#[warning(parser::add_paren)]
+struct AY {}
 
 #[derive(SessionSubdiagnostic)]
-#[warn_(parser::add_paren)]
+#[warning(parser::add_paren)]
 struct AZ {
     #[primary_span]
     span: Span,
diff --git a/src/test/ui/const-generics/defaults/generic-expr-default-concrete.stderr b/src/test/ui/const-generics/defaults/generic-expr-default-concrete.stderr
index e8826ce4335e7..61b3551182c90 100644
--- a/src/test/ui/const-generics/defaults/generic-expr-default-concrete.stderr
+++ b/src/test/ui/const-generics/defaults/generic-expr-default-concrete.stderr
@@ -4,8 +4,8 @@ error[E0308]: mismatched types
 LL |     Foo::<10, 12>
    |     ^^^^^^^^^^^^^ expected `11`, found `12`
    |
-   = note: expected type `11`
-              found type `12`
+   = note: expected constant `11`
+              found constant `12`
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/const-generics/defaults/generic-expr-default-mismatched-types.stderr b/src/test/ui/const-generics/defaults/generic-expr-default-mismatched-types.stderr
index d5a3071b77d15..e83f89a60333f 100644
--- a/src/test/ui/const-generics/defaults/generic-expr-default-mismatched-types.stderr
+++ b/src/test/ui/const-generics/defaults/generic-expr-default-mismatched-types.stderr
@@ -4,8 +4,8 @@ error[E0308]: mismatched types
 LL |     Foo::<N, { N + 2 }>
    |     ^^^^^^^^^^^^^^^^^^^ expected `{ N + 1 }`, found `{ N + 2 }`
    |
-   = note: expected type `{ N + 1 }`
-              found type `{ N + 2 }`
+   = note: expected constant `{ N + 1 }`
+              found constant `{ N + 2 }`
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/const-generics/generic_const_exprs/abstract-const-as-cast-3.stderr b/src/test/ui/const-generics/generic_const_exprs/abstract-const-as-cast-3.stderr
index 615dc875f67a3..ada1050d35f35 100644
--- a/src/test/ui/const-generics/generic_const_exprs/abstract-const-as-cast-3.stderr
+++ b/src/test/ui/const-generics/generic_const_exprs/abstract-const-as-cast-3.stderr
@@ -22,8 +22,13 @@ error[E0308]: mismatched types
 LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as u128 }>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `{ N as u128 }`, found `{ O as u128 }`
    |
-   = note: expected type `{ N as u128 }`
-              found type `{ O as u128 }`
+   = note: expected constant `{ N as u128 }`
+              found constant `{ O as u128 }`
+note: required by a bound in `use_trait_impl::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:14:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl::assert_impl`
 
 error: unconstrained generic constant
   --> $DIR/abstract-const-as-cast-3.rs:20:19
@@ -49,8 +54,13 @@ error[E0308]: mismatched types
 LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as _ }>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `{ N as _ }`, found `{ O as u128 }`
    |
-   = note: expected type `{ N as _ }`
-              found type `{ O as u128 }`
+   = note: expected constant `{ N as _ }`
+              found constant `{ O as u128 }`
+note: required by a bound in `use_trait_impl::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:14:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl::assert_impl`
 
 error[E0308]: mismatched types
   --> $DIR/abstract-const-as-cast-3.rs:23:5
@@ -58,8 +68,13 @@ error[E0308]: mismatched types
 LL |     assert_impl::<HasCastInTraitImpl<13, { 12 as u128 }>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `12`, found `13`
    |
-   = note: expected type `12`
-              found type `13`
+   = note: expected constant `12`
+              found constant `13`
+note: required by a bound in `use_trait_impl::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:14:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl::assert_impl`
 
 error[E0308]: mismatched types
   --> $DIR/abstract-const-as-cast-3.rs:25:5
@@ -67,8 +82,13 @@ error[E0308]: mismatched types
 LL |     assert_impl::<HasCastInTraitImpl<14, 13>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `13`, found `14`
    |
-   = note: expected type `13`
-              found type `14`
+   = note: expected constant `13`
+              found constant `14`
+note: required by a bound in `use_trait_impl::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:14:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl::assert_impl`
 
 error: unconstrained generic constant
   --> $DIR/abstract-const-as-cast-3.rs:35:19
@@ -94,8 +114,13 @@ error[E0308]: mismatched types
 LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as u128 }>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `{ N as u128 }`, found `{ O as u128 }`
    |
-   = note: expected type `{ N as u128 }`
-              found type `{ O as u128 }`
+   = note: expected constant `{ N as u128 }`
+              found constant `{ O as u128 }`
+note: required by a bound in `use_trait_impl_2::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:32:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl_2::assert_impl`
 
 error: unconstrained generic constant
   --> $DIR/abstract-const-as-cast-3.rs:38:19
@@ -121,8 +146,13 @@ error[E0308]: mismatched types
 LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as _ }>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `{ N as _ }`, found `{ O as u128 }`
    |
-   = note: expected type `{ N as _ }`
-              found type `{ O as u128 }`
+   = note: expected constant `{ N as _ }`
+              found constant `{ O as u128 }`
+note: required by a bound in `use_trait_impl_2::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:32:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl_2::assert_impl`
 
 error[E0308]: mismatched types
   --> $DIR/abstract-const-as-cast-3.rs:41:5
@@ -130,8 +160,13 @@ error[E0308]: mismatched types
 LL |     assert_impl::<HasCastInTraitImpl<13, { 12 as u128 }>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `12`, found `13`
    |
-   = note: expected type `12`
-              found type `13`
+   = note: expected constant `12`
+              found constant `13`
+note: required by a bound in `use_trait_impl_2::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:32:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl_2::assert_impl`
 
 error[E0308]: mismatched types
   --> $DIR/abstract-const-as-cast-3.rs:43:5
@@ -139,8 +174,13 @@ error[E0308]: mismatched types
 LL |     assert_impl::<HasCastInTraitImpl<14, 13>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `13`, found `14`
    |
-   = note: expected type `13`
-              found type `14`
+   = note: expected constant `13`
+              found constant `14`
+note: required by a bound in `use_trait_impl_2::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:32:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl_2::assert_impl`
 
 error: aborting due to 12 previous errors
 
diff --git a/src/test/ui/const-generics/generic_const_exprs/different-fn.stderr b/src/test/ui/const-generics/generic_const_exprs/different-fn.stderr
index 2aeb9b961ffde..83a2f3740b146 100644
--- a/src/test/ui/const-generics/generic_const_exprs/different-fn.stderr
+++ b/src/test/ui/const-generics/generic_const_exprs/different-fn.stderr
@@ -4,8 +4,8 @@ error[E0308]: mismatched types
 LL |     [0; size_of::<Foo<T>>()]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^ expected `size_of::<T>()`, found `size_of::<Foo<T>>()`
    |
-   = note: expected type `size_of::<T>()`
-              found type `size_of::<Foo<T>>()`
+   = note: expected constant `size_of::<T>()`
+              found constant `size_of::<Foo<T>>()`
 
 error: unconstrained generic constant
   --> $DIR/different-fn.rs:10:9
diff --git a/src/test/ui/const-generics/generic_const_exprs/issue-62504.full.stderr b/src/test/ui/const-generics/generic_const_exprs/issue-62504.full.stderr
index f2ae361dc81d9..0742db398c9c4 100644
--- a/src/test/ui/const-generics/generic_const_exprs/issue-62504.full.stderr
+++ b/src/test/ui/const-generics/generic_const_exprs/issue-62504.full.stderr
@@ -4,8 +4,8 @@ error[E0308]: mismatched types
 LL |         ArrayHolder([0; Self::SIZE])
    |                     ^^^^^^^^^^^^^^^ expected `X`, found `Self::SIZE`
    |
-   = note: expected type `X`
-              found type `Self::SIZE`
+   = note: expected constant `X`
+              found constant `Self::SIZE`
 
 error: unconstrained generic constant
   --> $DIR/issue-62504.rs:18:25
diff --git a/src/test/ui/const-generics/generic_const_exprs/issue-72819-generic-in-const-eval.full.stderr b/src/test/ui/const-generics/generic_const_exprs/issue-72819-generic-in-const-eval.full.stderr
index d536f6fd1d557..f2fddfbfbb52a 100644
--- a/src/test/ui/const-generics/generic_const_exprs/issue-72819-generic-in-const-eval.full.stderr
+++ b/src/test/ui/const-generics/generic_const_exprs/issue-72819-generic-in-const-eval.full.stderr
@@ -4,8 +4,15 @@ error[E0308]: mismatched types
 LL |     let x: Arr<{usize::MAX}> = Arr {};
    |            ^^^^^^^^^^^^^^^^^ expected `false`, found `true`
    |
-   = note: expected type `false`
-              found type `true`
+   = note: expected constant `false`
+              found constant `true`
+note: required by a bound in `Arr`
+  --> $DIR/issue-72819-generic-in-const-eval.rs:8:39
+   |
+LL | struct Arr<const N: usize>
+   |        --- required by a bound in this
+LL | where Assert::<{N < usize::MAX / 2}>: IsTrue,
+   |                                       ^^^^^^ required by this bound in `Arr`
 
 error[E0308]: mismatched types
   --> $DIR/issue-72819-generic-in-const-eval.rs:20:32
@@ -13,8 +20,15 @@ error[E0308]: mismatched types
 LL |     let x: Arr<{usize::MAX}> = Arr {};
    |                                ^^^ expected `false`, found `true`
    |
-   = note: expected type `false`
-              found type `true`
+   = note: expected constant `false`
+              found constant `true`
+note: required by a bound in `Arr`
+  --> $DIR/issue-72819-generic-in-const-eval.rs:8:39
+   |
+LL | struct Arr<const N: usize>
+   |        --- required by a bound in this
+LL | where Assert::<{N < usize::MAX / 2}>: IsTrue,
+   |                                       ^^^^^^ required by this bound in `Arr`
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/const-generics/generic_const_exprs/issue-83765.stderr b/src/test/ui/const-generics/generic_const_exprs/issue-83765.stderr
index 0332e82fe0727..b693023f125a4 100644
--- a/src/test/ui/const-generics/generic_const_exprs/issue-83765.stderr
+++ b/src/test/ui/const-generics/generic_const_exprs/issue-83765.stderr
@@ -4,8 +4,8 @@ error[E0308]: method not compatible with trait
 LL |     fn size(&self) -> [usize; DIM] {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `Self::DIM`, found `DIM`
    |
-   = note: expected type `Self::DIM`
-              found type `DIM`
+   = note: expected constant `Self::DIM`
+              found constant `DIM`
 
 error: unconstrained generic constant
   --> $DIR/issue-83765.rs:32:24
@@ -26,8 +26,8 @@ error[E0308]: mismatched types
 LL |         self.reference.size()
    |         ^^^^^^^^^^^^^^^^^^^^^ expected `DIM`, found `Self::DIM`
    |
-   = note: expected type `DIM`
-              found type `Self::DIM`
+   = note: expected constant `DIM`
+              found constant `Self::DIM`
 
 error: aborting due to 3 previous errors
 
diff --git a/src/test/ui/const-generics/generic_const_exprs/issue-85848.stderr b/src/test/ui/const-generics/generic_const_exprs/issue-85848.stderr
index 808b305c680f3..09bcb0860b71b 100644
--- a/src/test/ui/const-generics/generic_const_exprs/issue-85848.stderr
+++ b/src/test/ui/const-generics/generic_const_exprs/issue-85848.stderr
@@ -54,8 +54,8 @@ error[E0308]: mismatched types
 LL |     writes_to_specific_path(&cap);
    |     ^^^^^^^^^^^^^^^^^^^^^^^ expected `true`, found `{ contains::<T, U>() }`
    |
-   = note: expected type `true`
-              found type `{ contains::<T, U>() }`
+   = note: expected constant `true`
+              found constant `{ contains::<T, U>() }`
 
 error: aborting due to 3 previous errors
 
diff --git a/src/test/ui/const-generics/generic_const_exprs/obligation-cause.rs b/src/test/ui/const-generics/generic_const_exprs/obligation-cause.rs
new file mode 100644
index 0000000000000..e7c8e4f667d05
--- /dev/null
+++ b/src/test/ui/const-generics/generic_const_exprs/obligation-cause.rs
@@ -0,0 +1,24 @@
+#![allow(incomplete_features)]
+#![feature(generic_const_exprs)]
+
+trait True {}
+
+struct Is<const V: bool>;
+
+impl True for Is<true> {}
+
+fn g<T>()
+//~^ NOTE required by a bound in this
+where
+    Is<{ std::mem::size_of::<T>() == 0 }>: True,
+    //~^ NOTE required by a bound in `g`
+    //~| NOTE required by this bound in `g`
+{
+}
+
+fn main() {
+    g::<usize>();
+    //~^ ERROR mismatched types
+    //~| NOTE expected `false`, found `true`
+    //~| NOTE expected constant `false`
+}
diff --git a/src/test/ui/const-generics/generic_const_exprs/obligation-cause.stderr b/src/test/ui/const-generics/generic_const_exprs/obligation-cause.stderr
new file mode 100644
index 0000000000000..a253ec676f716
--- /dev/null
+++ b/src/test/ui/const-generics/generic_const_exprs/obligation-cause.stderr
@@ -0,0 +1,20 @@
+error[E0308]: mismatched types
+  --> $DIR/obligation-cause.rs:20:5
+   |
+LL |     g::<usize>();
+   |     ^^^^^^^^^^ expected `false`, found `true`
+   |
+   = note: expected constant `false`
+              found constant `true`
+note: required by a bound in `g`
+  --> $DIR/obligation-cause.rs:13:44
+   |
+LL | fn g<T>()
+   |    - required by a bound in this
+...
+LL |     Is<{ std::mem::size_of::<T>() == 0 }>: True,
+   |                                            ^^^^ required by this bound in `g`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/const-generics/issues/issue-73260.stderr b/src/test/ui/const-generics/issues/issue-73260.stderr
index f1fc50e6e5914..7670032e5b758 100644
--- a/src/test/ui/const-generics/issues/issue-73260.stderr
+++ b/src/test/ui/const-generics/issues/issue-73260.stderr
@@ -4,8 +4,16 @@ error[E0308]: mismatched types
 LL |     let x: Arr<{usize::MAX}> = Arr {};
    |            ^^^^^^^^^^^^^^^^^ expected `false`, found `true`
    |
-   = note: expected type `false`
-              found type `true`
+   = note: expected constant `false`
+              found constant `true`
+note: required by a bound in `Arr`
+  --> $DIR/issue-73260.rs:6:37
+   |
+LL | struct Arr<const N: usize>
+   |        --- required by a bound in this
+LL | where
+LL |     Assert::<{N < usize::MAX / 2}>: IsTrue,
+   |                                     ^^^^^^ required by this bound in `Arr`
 
 error[E0308]: mismatched types
   --> $DIR/issue-73260.rs:16:32
@@ -13,8 +21,16 @@ error[E0308]: mismatched types
 LL |     let x: Arr<{usize::MAX}> = Arr {};
    |                                ^^^ expected `false`, found `true`
    |
-   = note: expected type `false`
-              found type `true`
+   = note: expected constant `false`
+              found constant `true`
+note: required by a bound in `Arr`
+  --> $DIR/issue-73260.rs:6:37
+   |
+LL | struct Arr<const N: usize>
+   |        --- required by a bound in this
+LL | where
+LL |     Assert::<{N < usize::MAX / 2}>: IsTrue,
+   |                                     ^^^^^^ required by this bound in `Arr`
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/const-generics/issues/issue-79674.stderr b/src/test/ui/const-generics/issues/issue-79674.stderr
index 8c029289cbb0d..02b48b55f8b38 100644
--- a/src/test/ui/const-generics/issues/issue-79674.stderr
+++ b/src/test/ui/const-generics/issues/issue-79674.stderr
@@ -4,8 +4,16 @@ error[E0308]: mismatched types
 LL |     requires_distinct("str", 12);
    |     ^^^^^^^^^^^^^^^^^ expected `true`, found `false`
    |
-   = note: expected type `true`
-              found type `false`
+   = note: expected constant `true`
+              found constant `false`
+note: required by a bound in `requires_distinct`
+  --> $DIR/issue-79674.rs:23:37
+   |
+LL | fn requires_distinct<A, B>(_a: A, _b: B) where
+   |    ----------------- required by a bound in this
+LL |     A: MiniTypeId, B: MiniTypeId,
+LL |     Lift<{is_same_type::<A, B>()}>: IsFalse {}
+   |                                     ^^^^^^^ required by this bound in `requires_distinct`
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/const-generics/types-mismatch-const-args.full.stderr b/src/test/ui/const-generics/types-mismatch-const-args.full.stderr
index 486506239ddfd..b6a22df74369a 100644
--- a/src/test/ui/const-generics/types-mismatch-const-args.full.stderr
+++ b/src/test/ui/const-generics/types-mismatch-const-args.full.stderr
@@ -4,8 +4,8 @@ error[E0308]: mismatched types
 LL |     let _: A<'a, u32, {2u32}, {3u32}> = A::<'a, u32, {2u32 + 2u32}, {3u32}> { data: PhantomData };
    |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `2`, found `4`
    |
-   = note: expected type `2`
-              found type `4`
+   = note: expected constant `2`
+              found constant `4`
 
 error[E0308]: mismatched types
   --> $DIR/types-mismatch-const-args.rs:16:41
diff --git a/src/test/ui/drop/dropck_legal_cycles.rs b/src/test/ui/drop/dropck_legal_cycles.rs
index 27a599315dc1c..6a0fe7784fbcc 100644
--- a/src/test/ui/drop/dropck_legal_cycles.rs
+++ b/src/test/ui/drop/dropck_legal_cycles.rs
@@ -1017,7 +1017,7 @@ impl<'a> Children<'a> for HM<'a> {
         where C: Context + PrePost<Self>, Self: Sized
     {
         if let Some(ref hm) = self.contents.get() {
-            for (k, v) in hm.iter().nth(index / 2) {
+            if let Some((k, v)) = hm.iter().nth(index / 2) {
                 [k, v][index % 2].descend_into_self(context);
             }
         }
@@ -1032,7 +1032,7 @@ impl<'a> Children<'a> for VD<'a> {
         where C: Context + PrePost<Self>, Self: Sized
     {
         if let Some(ref vd) = self.contents.get() {
-            for r in vd.iter().nth(index) {
+            if let Some(r) = vd.iter().nth(index) {
                 r.descend_into_self(context);
             }
         }
@@ -1047,7 +1047,7 @@ impl<'a> Children<'a> for VM<'a> {
         where C: Context + PrePost<VM<'a>>
     {
         if let Some(ref vd) = self.contents.get() {
-            for (_idx, r) in vd.iter().nth(index) {
+            if let Some((_idx, r)) = vd.iter().nth(index) {
                 r.descend_into_self(context);
             }
         }
@@ -1062,7 +1062,7 @@ impl<'a> Children<'a> for LL<'a> {
         where C: Context + PrePost<LL<'a>>
     {
         if let Some(ref ll) = self.contents.get() {
-            for r in ll.iter().nth(index) {
+            if let Some(r) = ll.iter().nth(index) {
                 r.descend_into_self(context);
             }
         }
@@ -1077,7 +1077,7 @@ impl<'a> Children<'a> for BH<'a> {
         where C: Context + PrePost<BH<'a>>
     {
         if let Some(ref bh) = self.contents.get() {
-            for r in bh.iter().nth(index) {
+            if let Some(r) = bh.iter().nth(index) {
                 r.descend_into_self(context);
             }
         }
@@ -1092,7 +1092,7 @@ impl<'a> Children<'a> for BTM<'a> {
         where C: Context + PrePost<BTM<'a>>
     {
         if let Some(ref bh) = self.contents.get() {
-            for (k, v) in bh.iter().nth(index / 2) {
+            if let Some((k, v)) = bh.iter().nth(index / 2) {
                 [k, v][index % 2].descend_into_self(context);
             }
         }
@@ -1107,7 +1107,7 @@ impl<'a> Children<'a> for BTS<'a> {
         where C: Context + PrePost<BTS<'a>>
     {
         if let Some(ref bh) = self.contents.get() {
-            for r in bh.iter().nth(index) {
+            if let Some(r) = bh.iter().nth(index) {
                 r.descend_into_self(context);
             }
         }
diff --git a/src/test/ui/issues/issue-30371.rs b/src/test/ui/issues/issue-30371.rs
index a1ae9a36bc1d7..880558eb5b697 100644
--- a/src/test/ui/issues/issue-30371.rs
+++ b/src/test/ui/issues/issue-30371.rs
@@ -1,5 +1,6 @@
 // run-pass
 #![allow(unreachable_code)]
+#![allow(for_loop_over_fallibles)]
 #![deny(unused_variables)]
 
 fn main() {
diff --git a/src/test/ui/lint/for_loop_over_fallibles.rs b/src/test/ui/lint/for_loop_over_fallibles.rs
new file mode 100644
index 0000000000000..43d71c2e808a9
--- /dev/null
+++ b/src/test/ui/lint/for_loop_over_fallibles.rs
@@ -0,0 +1,43 @@
+// check-pass
+
+fn main() {
+    // Common
+    for _ in Some(1) {}
+    //~^ WARN for loop over an `Option`. This is more readably written as an `if let` statement
+    //~| HELP to check pattern in a loop use `while let`
+    //~| HELP consider using `if let` to clear intent
+    for _ in Ok::<_, ()>(1) {}
+    //~^ WARN for loop over a `Result`. This is more readably written as an `if let` statement
+    //~| HELP to check pattern in a loop use `while let`
+    //~| HELP consider using `if let` to clear intent
+
+    // `Iterator::next` specific
+    for _ in [0; 0].iter().next() {}
+    //~^ WARN for loop over an `Option`. This is more readably written as an `if let` statement
+    //~| HELP to iterate over `[0; 0].iter()` remove the call to `next`
+    //~| HELP consider using `if let` to clear intent
+
+    // `Result<impl Iterator, _>`, but function doesn't return `Result`
+    for _ in Ok::<_, ()>([0; 0].iter()) {}
+    //~^ WARN for loop over a `Result`. This is more readably written as an `if let` statement
+    //~| HELP to check pattern in a loop use `while let`
+    //~| HELP consider using `if let` to clear intent
+}
+
+fn _returns_result() -> Result<(), ()> {
+    // `Result<impl Iterator, _>`
+    for _ in Ok::<_, ()>([0; 0].iter()) {}
+    //~^ WARN for loop over a `Result`. This is more readably written as an `if let` statement
+    //~| HELP to check pattern in a loop use `while let`
+    //~| HELP consider unwrapping the `Result` with `?` to iterate over its contents
+    //~| HELP consider using `if let` to clear intent
+
+    // `Result<impl IntoIterator>`
+    for _ in Ok::<_, ()>([0; 0]) {}
+    //~^ WARN for loop over a `Result`. This is more readably written as an `if let` statement
+    //~| HELP to check pattern in a loop use `while let`
+    //~| HELP consider unwrapping the `Result` with `?` to iterate over its contents
+    //~| HELP consider using `if let` to clear intent
+
+    Ok(())
+}
diff --git a/src/test/ui/lint/for_loop_over_fallibles.stderr b/src/test/ui/lint/for_loop_over_fallibles.stderr
new file mode 100644
index 0000000000000..56e3126dc09a4
--- /dev/null
+++ b/src/test/ui/lint/for_loop_over_fallibles.stderr
@@ -0,0 +1,101 @@
+warning: for loop over an `Option`. This is more readably written as an `if let` statement
+  --> $DIR/for_loop_over_fallibles.rs:5:14
+   |
+LL |     for _ in Some(1) {}
+   |              ^^^^^^^
+   |
+   = note: `#[warn(for_loop_over_fallibles)]` on by default
+help: to check pattern in a loop use `while let`
+   |
+LL |     while let Some(_) = Some(1) {}
+   |     ~~~~~~~~~~~~~~~ ~~~
+help: consider using `if let` to clear intent
+   |
+LL |     if let Some(_) = Some(1) {}
+   |     ~~~~~~~~~~~~ ~~~
+
+warning: for loop over a `Result`. This is more readably written as an `if let` statement
+  --> $DIR/for_loop_over_fallibles.rs:9:14
+   |
+LL |     for _ in Ok::<_, ()>(1) {}
+   |              ^^^^^^^^^^^^^^
+   |
+help: to check pattern in a loop use `while let`
+   |
+LL |     while let Ok(_) = Ok::<_, ()>(1) {}
+   |     ~~~~~~~~~~~~~ ~~~
+help: consider using `if let` to clear intent
+   |
+LL |     if let Ok(_) = Ok::<_, ()>(1) {}
+   |     ~~~~~~~~~~ ~~~
+
+warning: for loop over an `Option`. This is more readably written as an `if let` statement
+  --> $DIR/for_loop_over_fallibles.rs:15:14
+   |
+LL |     for _ in [0; 0].iter().next() {}
+   |              ^^^^^^^^^^^^^^^^^^^^
+   |
+help: to iterate over `[0; 0].iter()` remove the call to `next`
+   |
+LL |     for _ in [0; 0].iter().by_ref() {}
+   |                           ~~~~~~~~~
+help: consider using `if let` to clear intent
+   |
+LL |     if let Some(_) = [0; 0].iter().next() {}
+   |     ~~~~~~~~~~~~ ~~~
+
+warning: for loop over a `Result`. This is more readably written as an `if let` statement
+  --> $DIR/for_loop_over_fallibles.rs:21:14
+   |
+LL |     for _ in Ok::<_, ()>([0; 0].iter()) {}
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: to check pattern in a loop use `while let`
+   |
+LL |     while let Ok(_) = Ok::<_, ()>([0; 0].iter()) {}
+   |     ~~~~~~~~~~~~~ ~~~
+help: consider using `if let` to clear intent
+   |
+LL |     if let Ok(_) = Ok::<_, ()>([0; 0].iter()) {}
+   |     ~~~~~~~~~~ ~~~
+
+warning: for loop over a `Result`. This is more readably written as an `if let` statement
+  --> $DIR/for_loop_over_fallibles.rs:29:14
+   |
+LL |     for _ in Ok::<_, ()>([0; 0].iter()) {}
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: to check pattern in a loop use `while let`
+   |
+LL |     while let Ok(_) = Ok::<_, ()>([0; 0].iter()) {}
+   |     ~~~~~~~~~~~~~ ~~~
+help: consider unwrapping the `Result` with `?` to iterate over its contents
+   |
+LL |     for _ in Ok::<_, ()>([0; 0].iter())? {}
+   |                                        +
+help: consider using `if let` to clear intent
+   |
+LL |     if let Ok(_) = Ok::<_, ()>([0; 0].iter()) {}
+   |     ~~~~~~~~~~ ~~~
+
+warning: for loop over a `Result`. This is more readably written as an `if let` statement
+  --> $DIR/for_loop_over_fallibles.rs:36:14
+   |
+LL |     for _ in Ok::<_, ()>([0; 0]) {}
+   |              ^^^^^^^^^^^^^^^^^^^
+   |
+help: to check pattern in a loop use `while let`
+   |
+LL |     while let Ok(_) = Ok::<_, ()>([0; 0]) {}
+   |     ~~~~~~~~~~~~~ ~~~
+help: consider unwrapping the `Result` with `?` to iterate over its contents
+   |
+LL |     for _ in Ok::<_, ()>([0; 0])? {}
+   |                                 +
+help: consider using `if let` to clear intent
+   |
+LL |     if let Ok(_) = Ok::<_, ()>([0; 0]) {}
+   |     ~~~~~~~~~~ ~~~
+
+warning: 6 warnings emitted
+
diff --git a/src/tools/clippy/clippy_lints/src/redundant_clone.rs b/src/tools/clippy/clippy_lints/src/redundant_clone.rs
index eddca60457574..9fd86331ec755 100644
--- a/src/tools/clippy/clippy_lints/src/redundant_clone.rs
+++ b/src/tools/clippy/clippy_lints/src/redundant_clone.rs
@@ -105,7 +105,7 @@ impl<'tcx> LateLintPass<'tcx> for RedundantClone {
             vis.into_map(cx, maybe_storage_live_result)
         };
 
-        for (bb, bbdata) in mir.basic_blocks().iter_enumerated() {
+        for (bb, bbdata) in mir.basic_blocks.iter_enumerated() {
             let terminator = bbdata.terminator();
 
             if terminator.source_info.span.from_expansion() {
@@ -186,7 +186,7 @@ impl<'tcx> LateLintPass<'tcx> for RedundantClone {
                     unwrap_or_continue!(find_stmt_assigns_to(cx, mir, pred_arg, true, ps[0]));
                 let loc = mir::Location {
                     block: bb,
-                    statement_index: mir.basic_blocks()[bb].statements.len(),
+                    statement_index: mir.basic_blocks[bb].statements.len(),
                 };
 
                 // This can be turned into `res = move local` if `arg` and `cloned` are not borrowed
@@ -310,7 +310,7 @@ fn find_stmt_assigns_to<'tcx>(
     by_ref: bool,
     bb: mir::BasicBlock,
 ) -> Option<(mir::Local, CannotMoveOut)> {
-    let rvalue = mir.basic_blocks()[bb].statements.iter().rev().find_map(|stmt| {
+    let rvalue = mir.basic_blocks[bb].statements.iter().rev().find_map(|stmt| {
         if let mir::StatementKind::Assign(box (mir::Place { local, .. }, v)) = &stmt.kind {
             return if *local == to_local { Some(v) } else { None };
         }
diff --git a/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs b/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs
index 3bf75bcbee83e..74c222bbcbeb9 100644
--- a/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs
+++ b/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs
@@ -55,7 +55,7 @@ pub fn is_min_const_fn<'a, 'tcx>(tcx: TyCtxt<'tcx>, body: &'a Body<'tcx>, msrv:
         body.local_decls.iter().next().unwrap().source_info.span,
     )?;
 
-    for bb in body.basic_blocks() {
+    for bb in body.basic_blocks.iter() {
         check_terminator(tcx, body, bb.terminator(), msrv)?;
         for stmt in &bb.statements {
             check_statement(tcx, body, def_id, stmt)?;
diff --git a/src/tools/clippy/tests/ui/for_loop_unfixable.rs b/src/tools/clippy/tests/ui/for_loop_unfixable.rs
index efcaffce24ea4..203656fa4d6c2 100644
--- a/src/tools/clippy/tests/ui/for_loop_unfixable.rs
+++ b/src/tools/clippy/tests/ui/for_loop_unfixable.rs
@@ -8,6 +8,7 @@
     clippy::for_kv_map
 )]
 #[allow(clippy::linkedlist, clippy::unnecessary_mut_passed, clippy::similar_names)]
+#[allow(for_loop_over_fallibles)]
 fn main() {
     let vec = vec![1, 2, 3, 4];
 
diff --git a/src/tools/clippy/tests/ui/for_loop_unfixable.stderr b/src/tools/clippy/tests/ui/for_loop_unfixable.stderr
index f769b4bdc9411..50a86eaa68f7d 100644
--- a/src/tools/clippy/tests/ui/for_loop_unfixable.stderr
+++ b/src/tools/clippy/tests/ui/for_loop_unfixable.stderr
@@ -1,5 +1,5 @@
 error: you are iterating over `Iterator::next()` which is an Option; this will compile but is probably not what you want
-  --> $DIR/for_loop_unfixable.rs:14:15
+  --> $DIR/for_loop_unfixable.rs:15:15
    |
 LL |     for _v in vec.iter().next() {}
    |               ^^^^^^^^^^^^^^^^^
diff --git a/src/tools/clippy/tests/ui/for_loops_over_fallibles.rs b/src/tools/clippy/tests/ui/for_loops_over_fallibles.rs
index 3390111d0a8fe..3c7733e665356 100644
--- a/src/tools/clippy/tests/ui/for_loops_over_fallibles.rs
+++ b/src/tools/clippy/tests/ui/for_loops_over_fallibles.rs
@@ -1,4 +1,5 @@
 #![warn(clippy::for_loops_over_fallibles)]
+#![allow(for_loop_over_fallibles)]
 
 fn for_loops_over_fallibles() {
     let option = Some(1);
diff --git a/src/tools/clippy/tests/ui/for_loops_over_fallibles.stderr b/src/tools/clippy/tests/ui/for_loops_over_fallibles.stderr
index 8c8c022243aeb..5f5a81d24cfde 100644
--- a/src/tools/clippy/tests/ui/for_loops_over_fallibles.stderr
+++ b/src/tools/clippy/tests/ui/for_loops_over_fallibles.stderr
@@ -1,5 +1,5 @@
 error: for loop over `option`, which is an `Option`. This is more readably written as an `if let` statement
-  --> $DIR/for_loops_over_fallibles.rs:9:14
+  --> $DIR/for_loops_over_fallibles.rs:10:14
    |
 LL |     for x in option {
    |              ^^^^^^
@@ -8,7 +8,7 @@ LL |     for x in option {
    = help: consider replacing `for x in option` with `if let Some(x) = option`
 
 error: for loop over `option`, which is an `Option`. This is more readably written as an `if let` statement
-  --> $DIR/for_loops_over_fallibles.rs:14:14
+  --> $DIR/for_loops_over_fallibles.rs:15:14
    |
 LL |     for x in option.iter() {
    |              ^^^^^^
@@ -16,7 +16,7 @@ LL |     for x in option.iter() {
    = help: consider replacing `for x in option.iter()` with `if let Some(x) = option`
 
 error: for loop over `result`, which is a `Result`. This is more readably written as an `if let` statement
-  --> $DIR/for_loops_over_fallibles.rs:19:14
+  --> $DIR/for_loops_over_fallibles.rs:20:14
    |
 LL |     for x in result {
    |              ^^^^^^
@@ -24,7 +24,7 @@ LL |     for x in result {
    = help: consider replacing `for x in result` with `if let Ok(x) = result`
 
 error: for loop over `result`, which is a `Result`. This is more readably written as an `if let` statement
-  --> $DIR/for_loops_over_fallibles.rs:24:14
+  --> $DIR/for_loops_over_fallibles.rs:25:14
    |
 LL |     for x in result.iter_mut() {
    |              ^^^^^^
@@ -32,7 +32,7 @@ LL |     for x in result.iter_mut() {
    = help: consider replacing `for x in result.iter_mut()` with `if let Ok(x) = result`
 
 error: for loop over `result`, which is a `Result`. This is more readably written as an `if let` statement
-  --> $DIR/for_loops_over_fallibles.rs:29:14
+  --> $DIR/for_loops_over_fallibles.rs:30:14
    |
 LL |     for x in result.into_iter() {
    |              ^^^^^^
@@ -40,7 +40,7 @@ LL |     for x in result.into_iter() {
    = help: consider replacing `for x in result.into_iter()` with `if let Ok(x) = result`
 
 error: for loop over `option.ok_or("x not found")`, which is a `Result`. This is more readably written as an `if let` statement
-  --> $DIR/for_loops_over_fallibles.rs:33:14
+  --> $DIR/for_loops_over_fallibles.rs:34:14
    |
 LL |     for x in option.ok_or("x not found") {
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -48,7 +48,7 @@ LL |     for x in option.ok_or("x not found") {
    = help: consider replacing `for x in option.ok_or("x not found")` with `if let Ok(x) = option.ok_or("x not found")`
 
 error: you are iterating over `Iterator::next()` which is an Option; this will compile but is probably not what you want
-  --> $DIR/for_loops_over_fallibles.rs:39:14
+  --> $DIR/for_loops_over_fallibles.rs:40:14
    |
 LL |     for x in v.iter().next() {
    |              ^^^^^^^^^^^^^^^
@@ -56,7 +56,7 @@ LL |     for x in v.iter().next() {
    = note: `#[deny(clippy::iter_next_loop)]` on by default
 
 error: for loop over `v.iter().next().and(Some(0))`, which is an `Option`. This is more readably written as an `if let` statement
-  --> $DIR/for_loops_over_fallibles.rs:44:14
+  --> $DIR/for_loops_over_fallibles.rs:45:14
    |
 LL |     for x in v.iter().next().and(Some(0)) {
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -64,7 +64,7 @@ LL |     for x in v.iter().next().and(Some(0)) {
    = help: consider replacing `for x in v.iter().next().and(Some(0))` with `if let Some(x) = v.iter().next().and(Some(0))`
 
 error: for loop over `v.iter().next().ok_or("x not found")`, which is a `Result`. This is more readably written as an `if let` statement
-  --> $DIR/for_loops_over_fallibles.rs:48:14
+  --> $DIR/for_loops_over_fallibles.rs:49:14
    |
 LL |     for x in v.iter().next().ok_or("x not found") {
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -72,7 +72,7 @@ LL |     for x in v.iter().next().ok_or("x not found") {
    = help: consider replacing `for x in v.iter().next().ok_or("x not found")` with `if let Ok(x) = v.iter().next().ok_or("x not found")`
 
 error: this loop never actually loops
-  --> $DIR/for_loops_over_fallibles.rs:60:5
+  --> $DIR/for_loops_over_fallibles.rs:61:5
    |
 LL | /     while let Some(x) = option {
 LL | |         println!("{}", x);
@@ -83,7 +83,7 @@ LL | |     }
    = note: `#[deny(clippy::never_loop)]` on by default
 
 error: this loop never actually loops
-  --> $DIR/for_loops_over_fallibles.rs:66:5
+  --> $DIR/for_loops_over_fallibles.rs:67:5
    |
 LL | /     while let Ok(x) = result {
 LL | |         println!("{}", x);