Rollup of 7 pull requests

compiler/rustc_hir_analysis/src/collect.rs

@@ -18,7 +18,9 @@ use rustc_ast::Recovered;
 use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
 use rustc_data_structures::unord::UnordMap;
-use rustc_errors::{struct_span_code_err, Applicability, Diag, ErrorGuaranteed, StashKey, E0228};
+use rustc_errors::{
+    struct_span_code_err, Applicability, Diag, DiagCtxtHandle, ErrorGuaranteed, StashKey, E0228,
+};
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::intravisit::{self, walk_generics, Visitor};
@@ -161,7 +163,7 @@ pub struct CollectItemTypesVisitor<'tcx> {
 /// and suggest adding type parameters in the appropriate place, taking into consideration any and
 /// all already existing generic type parameters to avoid suggesting a name that is already in use.
 pub(crate) fn placeholder_type_error<'tcx>(
-    tcx: TyCtxt<'tcx>,
+    cx: &dyn HirTyLowerer<'tcx>,
     generics: Option<&hir::Generics<'_>>,
     placeholder_types: Vec<Span>,
     suggest: bool,
@@ -172,21 +174,21 @@ pub(crate) fn placeholder_type_error<'tcx>(
         return;
     }
 
-    placeholder_type_error_diag(tcx, generics, placeholder_types, vec![], suggest, hir_ty, kind)
+    placeholder_type_error_diag(cx, generics, placeholder_types, vec![], suggest, hir_ty, kind)
         .emit();
 }
 
-pub(crate) fn placeholder_type_error_diag<'tcx>(
-    tcx: TyCtxt<'tcx>,
+pub(crate) fn placeholder_type_error_diag<'cx, 'tcx>(
+    cx: &'cx dyn HirTyLowerer<'tcx>,
     generics: Option<&hir::Generics<'_>>,
     placeholder_types: Vec<Span>,
     additional_spans: Vec<Span>,
     suggest: bool,
     hir_ty: Option<&hir::Ty<'_>>,
     kind: &'static str,
-) -> Diag<'tcx> {
+) -> Diag<'cx> {
     if placeholder_types.is_empty() {
-        return bad_placeholder(tcx, additional_spans, kind);
+        return bad_placeholder(cx, additional_spans, kind);
     }
 
     let params = generics.map(|g| g.params).unwrap_or_default();
@@ -210,7 +212,7 @@ pub(crate) fn placeholder_type_error_diag<'tcx>(
     }
 
     let mut err =
-        bad_placeholder(tcx, placeholder_types.into_iter().chain(additional_spans).collect(), kind);
+        bad_placeholder(cx, placeholder_types.into_iter().chain(additional_spans).collect(), kind);
 
     // Suggest, but only if it is not a function in const or static
     if suggest {
@@ -224,7 +226,7 @@ pub(crate) fn placeholder_type_error_diag<'tcx>(
 
             // Check if parent is const or static
             is_const_or_static = matches!(
-                tcx.parent_hir_node(hir_ty.hir_id),
+                cx.tcx().parent_hir_node(hir_ty.hir_id),
                 Node::Item(&hir::Item {
                     kind: hir::ItemKind::Const(..) | hir::ItemKind::Static(..),
                     ..
@@ -267,7 +269,16 @@ fn reject_placeholder_type_signatures_in_item<'tcx>(
     let mut visitor = HirPlaceholderCollector::default();
     visitor.visit_item(item);
 
-    placeholder_type_error(tcx, Some(generics), visitor.0, suggest, None, item.kind.descr());
+    let icx = ItemCtxt::new(tcx, item.owner_id.def_id);
+
+    placeholder_type_error(
+        icx.lowerer(),
+        Some(generics),
+        visitor.0,
+        suggest,
+        None,
+        item.kind.descr(),
+    );
 }
 
 impl<'tcx> Visitor<'tcx> for CollectItemTypesVisitor<'tcx> {
@@ -329,15 +340,15 @@ impl<'tcx> Visitor<'tcx> for CollectItemTypesVisitor<'tcx> {
 ///////////////////////////////////////////////////////////////////////////
 // Utility types and common code for the above passes.
 
-fn bad_placeholder<'tcx>(
-    tcx: TyCtxt<'tcx>,
+fn bad_placeholder<'cx, 'tcx>(
+    cx: &'cx dyn HirTyLowerer<'tcx>,
     mut spans: Vec<Span>,
     kind: &'static str,
-) -> Diag<'tcx> {
+) -> Diag<'cx> {
     let kind = if kind.ends_with('s') { format!("{kind}es") } else { format!("{kind}s") };
 
     spans.sort();
-    tcx.dcx().create_err(errors::PlaceholderNotAllowedItemSignatures { spans, kind })
+    cx.dcx().create_err(errors::PlaceholderNotAllowedItemSignatures { spans, kind })
 }
 
 impl<'tcx> ItemCtxt<'tcx> {
@@ -370,21 +381,24 @@ impl<'tcx> HirTyLowerer<'tcx> for ItemCtxt<'tcx> {
         self.tcx
     }
 
+    fn dcx(&self) -> DiagCtxtHandle<'_> {
+        self.tcx.dcx().taintable_handle(&self.tainted_by_errors)
+    }
+
     fn item_def_id(&self) -> LocalDefId {
         self.item_def_id
     }
 
     fn re_infer(&self, span: Span, reason: RegionInferReason<'_>) -> ty::Region<'tcx> {
         if let RegionInferReason::BorrowedObjectLifetimeDefault = reason {
             let e = struct_span_code_err!(
-                self.tcx().dcx(),
+                self.dcx(),
                 span,
                 E0228,
                 "the lifetime bound for this object type cannot be deduced \
                 from context; please supply an explicit bound"
             )
             .emit();
-            self.set_tainted_by_errors(e);
             ty::Region::new_error(self.tcx(), e)
         } else {
             // This indicates an illegal lifetime in a non-assoc-trait position
@@ -509,10 +523,6 @@ impl<'tcx> HirTyLowerer<'tcx> for ItemCtxt<'tcx> {
         None
     }
 
-    fn set_tainted_by_errors(&self, err: ErrorGuaranteed) {
-        self.tainted_by_errors.set(Some(err));
-    }
-
     fn lower_fn_sig(
         &self,
         decl: &hir::FnDecl<'tcx>,
@@ -570,7 +580,7 @@ impl<'tcx> HirTyLowerer<'tcx> for ItemCtxt<'tcx> {
             // `ident_span` to not emit an error twice when we have `fn foo(_: fn() -> _)`.
 
             let mut diag = crate::collect::placeholder_type_error_diag(
-                tcx,
+                self,
                 generics,
                 visitor.0,
                 infer_replacements.iter().map(|(s, _)| *s).collect(),
@@ -590,7 +600,7 @@ impl<'tcx> HirTyLowerer<'tcx> for ItemCtxt<'tcx> {
                 );
             }
 
-            self.set_tainted_by_errors(diag.emit());
+            diag.emit();
         }
 
         (input_tys, output_ty)
@@ -639,6 +649,7 @@ fn lower_item(tcx: TyCtxt<'_>, item_id: hir::ItemId) {
     let it = tcx.hir().item(item_id);
     debug!(item = %it.ident, id = %it.hir_id());
     let def_id = item_id.owner_id.def_id;
+    let icx = ItemCtxt::new(tcx, def_id);
 
     match &it.kind {
         // These don't define types.
@@ -663,7 +674,7 @@ fn lower_item(tcx: TyCtxt<'_>, item_id: hir::ItemId) {
                         let mut visitor = HirPlaceholderCollector::default();
                         visitor.visit_foreign_item(item);
                         placeholder_type_error(
-                            tcx,
+                            icx.lowerer(),
                             None,
                             visitor.0,
                             false,
@@ -742,7 +753,14 @@ fn lower_item(tcx: TyCtxt<'_>, item_id: hir::ItemId) {
             if !ty.is_suggestable_infer_ty() {
                 let mut visitor = HirPlaceholderCollector::default();
                 visitor.visit_item(it);
-                placeholder_type_error(tcx, None, visitor.0, false, None, it.kind.descr());
+                placeholder_type_error(
+                    icx.lowerer(),
+                    None,
+                    visitor.0,
+                    false,
+                    None,
+                    it.kind.descr(),
+                );
             }
         }
 
@@ -760,6 +778,7 @@ fn lower_trait_item(tcx: TyCtxt<'_>, trait_item_id: hir::TraitItemId) {
     let trait_item = tcx.hir().trait_item(trait_item_id);
     let def_id = trait_item_id.owner_id;
     tcx.ensure().generics_of(def_id);
+    let icx = ItemCtxt::new(tcx, def_id.def_id);
 
     match trait_item.kind {
         hir::TraitItemKind::Fn(..) => {
@@ -776,7 +795,14 @@ fn lower_trait_item(tcx: TyCtxt<'_>, trait_item_id: hir::TraitItemId) {
                 // Account for `const C: _;`.
                 let mut visitor = HirPlaceholderCollector::default();
                 visitor.visit_trait_item(trait_item);
-                placeholder_type_error(tcx, None, visitor.0, false, None, "associated constant");
+                placeholder_type_error(
+                    icx.lowerer(),
+                    None,
+                    visitor.0,
+                    false,
+                    None,
+                    "associated constant",
+                );
             }
         }
 
@@ -787,7 +813,7 @@ fn lower_trait_item(tcx: TyCtxt<'_>, trait_item_id: hir::TraitItemId) {
             // Account for `type T = _;`.
             let mut visitor = HirPlaceholderCollector::default();
             visitor.visit_trait_item(trait_item);
-            placeholder_type_error(tcx, None, visitor.0, false, None, "associated type");
+            placeholder_type_error(icx.lowerer(), None, visitor.0, false, None, "associated type");
         }
 
         hir::TraitItemKind::Type(_, None) => {
@@ -798,7 +824,7 @@ fn lower_trait_item(tcx: TyCtxt<'_>, trait_item_id: hir::TraitItemId) {
             let mut visitor = HirPlaceholderCollector::default();
             visitor.visit_trait_item(trait_item);
 
-            placeholder_type_error(tcx, None, visitor.0, false, None, "associated type");
+            placeholder_type_error(icx.lowerer(), None, visitor.0, false, None, "associated type");
         }
     };
 
@@ -811,6 +837,7 @@ fn lower_impl_item(tcx: TyCtxt<'_>, impl_item_id: hir::ImplItemId) {
     tcx.ensure().type_of(def_id);
     tcx.ensure().predicates_of(def_id);
     let impl_item = tcx.hir().impl_item(impl_item_id);
+    let icx = ItemCtxt::new(tcx, def_id.def_id);
     match impl_item.kind {
         hir::ImplItemKind::Fn(..) => {
             tcx.ensure().codegen_fn_attrs(def_id);
@@ -821,14 +848,21 @@ fn lower_impl_item(tcx: TyCtxt<'_>, impl_item_id: hir::ImplItemId) {
             let mut visitor = HirPlaceholderCollector::default();
             visitor.visit_impl_item(impl_item);
 
-            placeholder_type_error(tcx, None, visitor.0, false, None, "associated type");
+            placeholder_type_error(icx.lowerer(), None, visitor.0, false, None, "associated type");
         }
         hir::ImplItemKind::Const(ty, _) => {
             // Account for `const T: _ = ..;`
             if !ty.is_suggestable_infer_ty() {
                 let mut visitor = HirPlaceholderCollector::default();
                 visitor.visit_impl_item(impl_item);
-                placeholder_type_error(tcx, None, visitor.0, false, None, "associated constant");
+                placeholder_type_error(
+                    icx.lowerer(),
+                    None,
+                    visitor.0,
+                    false,
+                    None,
+                    "associated constant",
+                );
             }
         }
     }
@@ -1385,7 +1419,7 @@ fn fn_sig(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::EarlyBinder<'_, ty::PolyFn
             ..
         })
         | Item(hir::Item { kind: ItemKind::Fn(sig, generics, _), .. }) => {
-            infer_return_ty_for_fn_sig(tcx, sig, generics, def_id, &icx)
+            infer_return_ty_for_fn_sig(sig, generics, def_id, &icx)
         }
 
         ImplItem(hir::ImplItem { kind: ImplItemKind::Fn(sig, _), generics, .. }) => {
@@ -1402,7 +1436,7 @@ fn fn_sig(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::EarlyBinder<'_, ty::PolyFn
                     None,
                 )
             } else {
-                infer_return_ty_for_fn_sig(tcx, sig, generics, def_id, &icx)
+                infer_return_ty_for_fn_sig(sig, generics, def_id, &icx)
             }
         }
 
@@ -1455,12 +1489,12 @@ fn fn_sig(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::EarlyBinder<'_, ty::PolyFn
 }
 
 fn infer_return_ty_for_fn_sig<'tcx>(
-    tcx: TyCtxt<'tcx>,
     sig: &hir::FnSig<'tcx>,
     generics: &hir::Generics<'_>,
     def_id: LocalDefId,
     icx: &ItemCtxt<'tcx>,
 ) -> ty::PolyFnSig<'tcx> {
+    let tcx = icx.tcx;
     let hir_id = tcx.local_def_id_to_hir_id(def_id);
 
     match sig.decl.output.get_infer_ret_ty() {
@@ -1492,7 +1526,7 @@ fn infer_return_ty_for_fn_sig<'tcx>(
             let mut visitor = HirPlaceholderCollector::default();
             visitor.visit_ty(ty);
 
-            let mut diag = bad_placeholder(tcx, visitor.0, "return type");
+            let mut diag = bad_placeholder(icx.lowerer(), visitor.0, "return type");
             let ret_ty = fn_sig.output();
             // Don't leak types into signatures unless they're nameable!
             // For example, if a function returns itself, we don't want that

compiler/rustc_hir_analysis/src/collect/type_of.rs

@@ -12,6 +12,7 @@ use rustc_span::symbol::Ident;
 use rustc_span::{Span, DUMMY_SP};
 
 use crate::errors::TypeofReservedKeywordUsed;
+use crate::hir_ty_lowering::HirTyLowerer;
 
 use super::bad_placeholder;
 use super::ItemCtxt;
@@ -357,7 +358,7 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::EarlyBinder<'_
                 .and_then(|body_id| {
                     ty.is_suggestable_infer_ty().then(|| {
                         infer_placeholder_type(
-                            tcx,
+                            icx.lowerer(),
                             def_id,
                             body_id,
                             ty.span,
@@ -381,7 +382,7 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::EarlyBinder<'_
             ImplItemKind::Const(ty, body_id) => {
                 if ty.is_suggestable_infer_ty() {
                     infer_placeholder_type(
-                        tcx,
+                        icx.lowerer(),
                         def_id,
                         body_id,
                         ty.span,
@@ -405,7 +406,7 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::EarlyBinder<'_
             ItemKind::Static(ty, .., body_id) => {
                 if ty.is_suggestable_infer_ty() {
                     infer_placeholder_type(
-                        tcx,
+                        icx.lowerer(),
                         def_id,
                         body_id,
                         ty.span,
@@ -418,7 +419,14 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::EarlyBinder<'_
             }
             ItemKind::Const(ty, _, body_id) => {
                 if ty.is_suggestable_infer_ty() {
-                    infer_placeholder_type(tcx, def_id, body_id, ty.span, item.ident, "constant")
+                    infer_placeholder_type(
+                        icx.lowerer(),
+                        def_id,
+                        body_id,
+                        ty.span,
+                        item.ident,
+                        "constant",
+                    )
                 } else {
                     icx.lower_ty(ty)
                 }
@@ -559,21 +567,22 @@ pub(super) fn type_of_opaque(
     }
 }
 
-fn infer_placeholder_type<'a>(
-    tcx: TyCtxt<'a>,
+fn infer_placeholder_type<'tcx>(
+    cx: &dyn HirTyLowerer<'tcx>,
     def_id: LocalDefId,
     body_id: hir::BodyId,
     span: Span,
     item_ident: Ident,
     kind: &'static str,
-) -> Ty<'a> {
+) -> Ty<'tcx> {
+    let tcx = cx.tcx();
     let ty = tcx.diagnostic_only_typeck(def_id).node_type(body_id.hir_id);
 
     // If this came from a free `const` or `static mut?` item,
     // then the user may have written e.g. `const A = 42;`.
     // In this case, the parser has stashed a diagnostic for
     // us to improve in typeck so we do that now.
-    let guar = tcx
+    let guar = cx
         .dcx()
         .try_steal_modify_and_emit_err(span, StashKey::ItemNoType, |err| {
             if !ty.references_error() {
@@ -602,7 +611,7 @@ fn infer_placeholder_type<'a>(
             }
         })
         .unwrap_or_else(|| {
-            let mut diag = bad_placeholder(tcx, vec![span], kind);
+            let mut diag = bad_placeholder(cx, vec![span], kind);
 
             if !ty.references_error() {
                 if let Some(ty) = ty.make_suggestable(tcx, false, None) {

compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs

@@ -76,7 +76,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         }
 
         if unbounds.len() > 1 {
-            tcx.dcx().emit_err(errors::MultipleRelaxedDefaultBounds {
+            self.dcx().emit_err(errors::MultipleRelaxedDefaultBounds {
                 spans: unbounds.iter().map(|ptr| ptr.span).collect(),
             });
         }
@@ -90,7 +90,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 continue;
             }
             // There was a `?Trait` bound, but it was not `?Sized`; warn.
-            tcx.dcx().span_warn(
+            self.dcx().span_warn(
                 unbound.span,
                 "relaxing a default bound only does something for `?Sized`; \
                 all other traits are not bound by default",
@@ -310,7 +310,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         duplicates
             .entry(assoc_item.def_id)
             .and_modify(|prev_span| {
-                tcx.dcx().emit_err(errors::ValueOfAssociatedStructAlreadySpecified {
+                self.dcx().emit_err(errors::ValueOfAssociatedStructAlreadySpecified {
                     span: constraint.span,
                     prev_span: *prev_span,
                     item_name: constraint.ident,
@@ -338,7 +338,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         .into(),
                         ty::GenericParamDefKind::Type { .. } => {
                             let guar = *emitted_bad_param_err.get_or_insert_with(|| {
-                                tcx.dcx().emit_err(
+                                self.dcx().emit_err(
                                     crate::errors::ReturnTypeNotationIllegalParam::Type {
                                         span: path_span,
                                         param_span: tcx.def_span(param.def_id),
@@ -349,7 +349,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         }
                         ty::GenericParamDefKind::Const { .. } => {
                             let guar = *emitted_bad_param_err.get_or_insert_with(|| {
-                                tcx.dcx().emit_err(
+                                self.dcx().emit_err(
                                     crate::errors::ReturnTypeNotationIllegalParam::Const {
                                         span: path_span,
                                         param_span: tcx.def_span(param.def_id),
@@ -371,7 +371,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             {
                 alias_ty.into()
             } else {
-                return Err(tcx.dcx().emit_err(crate::errors::ReturnTypeNotationOnNonRpitit {
+                return Err(self.dcx().emit_err(crate::errors::ReturnTypeNotationOnNonRpitit {
                     span: constraint.span,
                     ty: tcx.liberate_late_bound_regions(assoc_item.def_id, output),
                     fn_span: tcx.hir().span_if_local(assoc_item.def_id),
@@ -417,7 +417,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 let ty = alias_term
                     .map_bound(|alias| tcx.type_of(alias.def_id).instantiate(tcx, alias.args));
                 let ty =
-                    check_assoc_const_binding_type(tcx, constraint.ident, ty, constraint.hir_id);
+                    check_assoc_const_binding_type(self, constraint.ident, ty, constraint.hir_id);
                 tcx.feed_anon_const_type(anon_const.def_id, ty::EarlyBinder::bind(ty));
             }
 
@@ -426,7 +426,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
         match constraint.kind {
             hir::AssocItemConstraintKind::Equality { .. } if let ty::AssocKind::Fn = assoc_kind => {
-                return Err(tcx.dcx().emit_err(crate::errors::ReturnTypeNotationEqualityBound {
+                return Err(self.dcx().emit_err(crate::errors::ReturnTypeNotationEqualityBound {
                     span: constraint.span,
                 }));
             }
@@ -462,7 +462,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     late_bound_in_term,
                     |br_name| {
                         struct_span_code_err!(
-                            tcx.dcx(),
+                            self.dcx(),
                             constraint.span,
                             E0582,
                             "binding for associated type `{}` references {}, \
@@ -519,7 +519,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 ///
 /// [^1]: <https://github.com/rust-lang/project-const-generics/issues/28>.
 fn check_assoc_const_binding_type<'tcx>(
-    tcx: TyCtxt<'tcx>,
+    cx: &dyn HirTyLowerer<'tcx>,
     assoc_const: Ident,
     ty: ty::Binder<'tcx, Ty<'tcx>>,
     hir_id: hir::HirId,
@@ -536,13 +536,14 @@ fn check_assoc_const_binding_type<'tcx>(
     }
 
     let mut collector = GenericParamAndBoundVarCollector {
-        tcx,
+        cx,
         params: Default::default(),
         vars: Default::default(),
         depth: ty::INNERMOST,
     };
     let mut guar = ty.visit_with(&mut collector).break_value();
 
+    let tcx = cx.tcx();
     let ty_note = ty
         .make_suggestable(tcx, false, None)
         .map(|ty| crate::errors::TyOfAssocConstBindingNote { assoc_const, ty });
@@ -556,7 +557,7 @@ fn check_assoc_const_binding_type<'tcx>(
     for index in collector.params {
         let param = generics.param_at(index as _, tcx);
         let is_self_param = param.name == rustc_span::symbol::kw::SelfUpper;
-        guar.get_or_insert(tcx.dcx().emit_err(crate::errors::ParamInTyOfAssocConstBinding {
+        guar.get_or_insert(cx.dcx().emit_err(crate::errors::ParamInTyOfAssocConstBinding {
             span: assoc_const.span,
             assoc_const,
             param_name: param.name,
@@ -574,7 +575,7 @@ fn check_assoc_const_binding_type<'tcx>(
         }));
     }
     for (var_def_id, var_name) in collector.vars {
-        guar.get_or_insert(tcx.dcx().emit_err(
+        guar.get_or_insert(cx.dcx().emit_err(
             crate::errors::EscapingBoundVarInTyOfAssocConstBinding {
                 span: assoc_const.span,
                 assoc_const,
@@ -590,14 +591,14 @@ fn check_assoc_const_binding_type<'tcx>(
     Ty::new_error(tcx, guar)
 }
 
-struct GenericParamAndBoundVarCollector<'tcx> {
-    tcx: TyCtxt<'tcx>,
+struct GenericParamAndBoundVarCollector<'a, 'tcx> {
+    cx: &'a dyn HirTyLowerer<'tcx>,
     params: FxIndexSet<u32>,
     vars: FxIndexSet<(DefId, Symbol)>,
     depth: ty::DebruijnIndex,
 }
 
-impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for GenericParamAndBoundVarCollector<'tcx> {
+impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for GenericParamAndBoundVarCollector<'_, 'tcx> {
     type Result = ControlFlow<ErrorGuaranteed>;
 
     fn visit_binder<T: TypeVisitable<TyCtxt<'tcx>>>(
@@ -620,7 +621,7 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for GenericParamAndBoundVarCollector<'tcx>
                     ty::BoundTyKind::Param(def_id, name) => (def_id, name),
                     ty::BoundTyKind::Anon => {
                         let reported = self
-                            .tcx
+                            .cx
                             .dcx()
                             .delayed_bug(format!("unexpected anon bound ty: {:?}", bt.var));
                         return ControlFlow::Break(reported);
@@ -643,7 +644,7 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for GenericParamAndBoundVarCollector<'tcx>
                     ty::BrNamed(def_id, name) => (def_id, name),
                     ty::BrAnon | ty::BrEnv => {
                         let guar = self
-                            .tcx
+                            .cx
                             .dcx()
                             .delayed_bug(format!("unexpected bound region kind: {:?}", br.kind));
                         return ControlFlow::Break(guar);
@@ -661,7 +662,7 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for GenericParamAndBoundVarCollector<'tcx>
                 self.params.insert(param.index);
             }
             ty::ConstKind::Bound(db, ty::BoundVar { .. }) if db >= self.depth => {
-                let guar = self.tcx.dcx().delayed_bug("unexpected escaping late-bound const var");
+                let guar = self.cx.dcx().delayed_bug("unexpected escaping late-bound const var");
                 return ControlFlow::Break(guar);
             }
             _ if ct.has_param() || ct.has_bound_vars() => return ct.super_visit_with(self),

compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

@@ -46,7 +46,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             return;
         }
 
-        self.tcx().dcx().emit_err(MissingTypeParams {
+        self.dcx().emit_err(MissingTypeParams {
             span,
             def_span: self.tcx().def_span(def_id),
             span_snippet: self.tcx().sess.source_map().span_to_snippet(span).ok(),
@@ -109,7 +109,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
         if is_impl {
             let trait_name = self.tcx().def_path_str(trait_def_id);
-            self.tcx().dcx().emit_err(ManualImplementation { span, trait_name });
+            self.dcx().emit_err(ManualImplementation { span, trait_name });
         }
     }
 
@@ -156,7 +156,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
         if is_dummy {
             err.label = Some(errors::AssocItemNotFoundLabel::NotFound { span });
-            return tcx.dcx().emit_err(err);
+            return self.dcx().emit_err(err);
         }
 
         let all_candidate_names: Vec<_> = all_candidates()
@@ -174,7 +174,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 assoc_kind: assoc_kind_str,
                 suggested_name,
             });
-            return tcx.dcx().emit_err(err);
+            return self.dcx().emit_err(err);
         }
 
         // If we didn't find a good item in the supertraits (or couldn't get
@@ -239,10 +239,10 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                             assoc_kind: assoc_kind_str,
                             suggested_name,
                         });
-                        return tcx.dcx().emit_err(err);
+                        return self.dcx().emit_err(err);
                     }
 
-                    let mut err = tcx.dcx().create_err(err);
+                    let mut err = self.dcx().create_err(err);
                     if suggest_constraining_type_param(
                         tcx,
                         generics,
@@ -264,7 +264,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     }
                     return err.emit();
                 }
-                return tcx.dcx().emit_err(err);
+                return self.dcx().emit_err(err);
             }
         }
 
@@ -291,7 +291,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             err.label = Some(errors::AssocItemNotFoundLabel::NotFound { span: assoc_name.span });
         }
 
-        tcx.dcx().emit_err(err)
+        self.dcx().emit_err(err)
     }
 
     fn complain_about_assoc_kind_mismatch(
@@ -347,7 +347,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             (ident.span, None, assoc_kind, assoc_item.kind)
         };
 
-        tcx.dcx().emit_err(errors::AssocKindMismatch {
+        self.dcx().emit_err(errors::AssocKindMismatch {
             span,
             expected: super::assoc_kind_str(expected),
             got: super::assoc_kind_str(got),
@@ -366,8 +366,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         traits: &[String],
         name: Symbol,
     ) -> ErrorGuaranteed {
-        let mut err =
-            struct_span_code_err!(self.tcx().dcx(), span, E0223, "ambiguous associated type");
+        let mut err = struct_span_code_err!(self.dcx(), span, E0223, "ambiguous associated type");
         if self
             .tcx()
             .resolutions(())
@@ -463,9 +462,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 }
             }
         }
-        let reported = err.emit();
-        self.set_tainted_by_errors(reported);
-        reported
+        err.emit()
     }
 
     pub(crate) fn complain_about_ambiguous_inherent_assoc_ty(
@@ -475,16 +472,14 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         span: Span,
     ) -> ErrorGuaranteed {
         let mut err = struct_span_code_err!(
-            self.tcx().dcx(),
+            self.dcx(),
             name.span,
             E0034,
             "multiple applicable items in scope"
         );
         err.span_label(name.span, format!("multiple `{name}` found"));
         self.note_ambiguous_inherent_assoc_ty(&mut err, candidates, span);
-        let reported = err.emit();
-        self.set_tainted_by_errors(reported);
-        reported
+        err.emit()
     }
 
     // FIXME(fmease): Heavily adapted from `rustc_hir_typeck::method::suggest`. Deduplicate.
@@ -576,7 +571,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             };
 
             let mut err = struct_span_code_err!(
-                tcx.dcx(),
+                self.dcx(),
                 name.span,
                 E0220,
                 "associated type `{name}` not found for `{self_ty}` in the current scope"
@@ -662,7 +657,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         bounds.sort();
         bounds.dedup();
 
-        let mut err = tcx.dcx().struct_span_err(
+        let mut err = self.dcx().struct_span_err(
             name.span,
             format!("the associated type `{name}` exists for `{self_ty}`, but its trait bounds were not satisfied")
         );
@@ -829,7 +824,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
         trait_bound_spans.sort();
         let mut err = struct_span_code_err!(
-            tcx.dcx(),
+            self.dcx(),
             trait_bound_spans,
             E0191,
             "the value of the associated type{} {} must be specified",
@@ -974,7 +969,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             }
         }
 
-        self.set_tainted_by_errors(err.emit());
+        err.emit();
     }
 
     /// On ambiguous associated type, look for an associated function whose name matches the
@@ -1011,17 +1006,14 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 .filter_by_name_unhygienic(Symbol::intern(&name))
                 .next()
         {
-            let reported =
-                struct_span_code_err!(tcx.dcx(), span, E0223, "ambiguous associated type")
-                    .with_span_suggestion_verbose(
-                        ident2.span.to(ident3.span),
-                        format!("there is an associated function with a similar name: `{name}`"),
-                        name,
-                        Applicability::MaybeIncorrect,
-                    )
-                    .emit();
-            self.set_tainted_by_errors(reported);
-            Err(reported)
+            Err(struct_span_code_err!(self.dcx(), span, E0223, "ambiguous associated type")
+                .with_span_suggestion_verbose(
+                    ident2.span.to(ident3.span),
+                    format!("there is an associated function with a similar name: `{name}`"),
+                    name,
+                    Applicability::MaybeIncorrect,
+                )
+                .emit())
         } else {
             Ok(())
         }
@@ -1120,7 +1112,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         let last_span = *arg_spans.last().unwrap();
         let span: MultiSpan = arg_spans.into();
         let mut err = struct_span_code_err!(
-            self.tcx().dcx(),
+            self.dcx(),
             span,
             E0109,
             "{kind} arguments are not allowed on {this_type}",
@@ -1130,20 +1122,17 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             err.span_label(span, format!("not allowed on {what}"));
         }
         generics_args_err_extend(self.tcx(), segments, &mut err, err_extend);
-        let reported = err.emit();
-        self.set_tainted_by_errors(reported);
-        reported
+        err.emit()
     }
 
     pub fn report_trait_object_addition_traits_error(
         &self,
         regular_traits: &Vec<TraitAliasExpansionInfo<'_>>,
     ) -> ErrorGuaranteed {
-        let tcx = self.tcx();
         let first_trait = &regular_traits[0];
         let additional_trait = &regular_traits[1];
         let mut err = struct_span_code_err!(
-            tcx.dcx(),
+            self.dcx(),
             additional_trait.bottom().1,
             E0225,
             "only auto traits can be used as additional traits in a trait object"
@@ -1169,9 +1158,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
              for more information on them, visit \
              <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>",
         );
-        let reported = err.emit();
-        self.set_tainted_by_errors(reported);
-        reported
+        err.emit()
     }
 
     pub fn report_trait_object_with_no_traits_error(
@@ -1185,20 +1172,19 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             .map(|&(trait_ref, _)| trait_ref.def_id())
             .find(|&trait_ref| tcx.is_trait_alias(trait_ref))
             .map(|trait_ref| tcx.def_span(trait_ref));
-        let reported =
-            tcx.dcx().emit_err(TraitObjectDeclaredWithNoTraits { span, trait_alias_span });
-        self.set_tainted_by_errors(reported);
-        reported
+
+        self.dcx().emit_err(TraitObjectDeclaredWithNoTraits { span, trait_alias_span })
     }
 }
 
 /// Emit an error for the given associated item constraint.
 pub fn prohibit_assoc_item_constraint(
-    tcx: TyCtxt<'_>,
+    cx: &dyn HirTyLowerer<'_>,
     constraint: &hir::AssocItemConstraint<'_>,
     segment: Option<(DefId, &hir::PathSegment<'_>, Span)>,
 ) -> ErrorGuaranteed {
-    let mut err = tcx.dcx().create_err(AssocItemConstraintsNotAllowedHere {
+    let tcx = cx.tcx();
+    let mut err = cx.dcx().create_err(AssocItemConstraintsNotAllowedHere {
         span: constraint.span,
         fn_trait_expansion: if let Some((_, segment, span)) = segment
             && segment.args().parenthesized == hir::GenericArgsParentheses::ParenSugar

compiler/rustc_hir_analysis/src/hir_ty_lowering/generics.rs

@@ -1,4 +1,4 @@
-use super::IsMethodCall;
+use super::{HirTyLowerer, IsMethodCall};
 use crate::errors::wrong_number_of_generic_args::{GenericArgsInfo, WrongNumberOfGenericArgs};
 use crate::hir_ty_lowering::{
     errors::prohibit_assoc_item_constraint, ExplicitLateBound, GenericArgCountMismatch,
@@ -13,7 +13,7 @@ use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::DefId;
 use rustc_hir::GenericArg;
 use rustc_middle::ty::{
-    self, GenericArgsRef, GenericParamDef, GenericParamDefKind, IsSuggestable, Ty, TyCtxt,
+    self, GenericArgsRef, GenericParamDef, GenericParamDefKind, IsSuggestable, Ty,
 };
 use rustc_session::lint::builtin::LATE_BOUND_LIFETIME_ARGUMENTS;
 use rustc_span::symbol::{kw, sym};
@@ -22,15 +22,16 @@ use smallvec::SmallVec;
 /// Report an error that a generic argument did not match the generic parameter that was
 /// expected.
 fn generic_arg_mismatch_err(
-    tcx: TyCtxt<'_>,
+    cx: &dyn HirTyLowerer<'_>,
     arg: &GenericArg<'_>,
     param: &GenericParamDef,
     possible_ordering_error: bool,
     help: Option<String>,
 ) -> ErrorGuaranteed {
+    let tcx = cx.tcx();
     let sess = tcx.sess;
     let mut err = struct_span_code_err!(
-        tcx.dcx(),
+        cx.dcx(),
         arg.span(),
         E0747,
         "{} provided when a {} was expected",
@@ -171,14 +172,15 @@ fn generic_arg_mismatch_err(
 /// - `inferred_kind`: if no parameter was provided, and inference
 ///   is enabled, then creates a suitable inference variable.
 pub fn lower_generic_args<'tcx: 'a, 'a>(
-    tcx: TyCtxt<'tcx>,
+    cx: &dyn HirTyLowerer<'tcx>,
     def_id: DefId,
     parent_args: &[ty::GenericArg<'tcx>],
     has_self: bool,
     self_ty: Option<Ty<'tcx>>,
     arg_count: &GenericArgCountResult,
     ctx: &mut impl GenericArgsLowerer<'a, 'tcx>,
 ) -> GenericArgsRef<'tcx> {
+    let tcx = cx.tcx();
     // Collect the segments of the path; we need to instantiate arguments
     // for parameters throughout the entire path (wherever there are
     // generic parameters).
@@ -326,7 +328,7 @@ pub fn lower_generic_args<'tcx: 'a, 'a>(
                                 param_types_present.dedup();
 
                                 generic_arg_mismatch_err(
-                                    tcx,
+                                    cx,
                                     arg,
                                     param,
                                     !args_iter.clone().is_sorted_by_key(|arg| arg.to_ord()),
@@ -381,7 +383,7 @@ pub fn lower_generic_args<'tcx: 'a, 'a>(
                         assert_eq!(kind, "lifetime");
                         let (provided_arg, param) =
                             force_infer_lt.expect("lifetimes ought to have been inferred");
-                        generic_arg_mismatch_err(tcx, provided_arg, param, false, None);
+                        generic_arg_mismatch_err(cx, provided_arg, param, false, None);
                     }
 
                     break;
@@ -405,7 +407,7 @@ pub fn lower_generic_args<'tcx: 'a, 'a>(
 /// Checks that the correct number of generic arguments have been provided.
 /// Used specifically for function calls.
 pub fn check_generic_arg_count_for_call(
-    tcx: TyCtxt<'_>,
+    cx: &dyn HirTyLowerer<'_>,
     def_id: DefId,
     generics: &ty::Generics,
     seg: &hir::PathSegment<'_>,
@@ -416,14 +418,14 @@ pub fn check_generic_arg_count_for_call(
         IsMethodCall::No => GenericArgPosition::Value,
     };
     let has_self = generics.parent.is_none() && generics.has_self;
-    check_generic_arg_count(tcx, def_id, seg, generics, gen_pos, has_self)
+    check_generic_arg_count(cx, def_id, seg, generics, gen_pos, has_self)
 }
 
 /// Checks that the correct number of generic arguments have been provided.
 /// This is used both for datatypes and function calls.
-#[instrument(skip(tcx, gen_pos), level = "debug")]
+#[instrument(skip(cx, gen_pos), level = "debug")]
 pub(crate) fn check_generic_arg_count(
-    tcx: TyCtxt<'_>,
+    cx: &dyn HirTyLowerer<'_>,
     def_id: DefId,
     seg: &hir::PathSegment<'_>,
     gen_params: &ty::Generics,
@@ -456,11 +458,11 @@ pub(crate) fn check_generic_arg_count(
     if gen_pos != GenericArgPosition::Type
         && let Some(c) = gen_args.constraints.first()
     {
-        prohibit_assoc_item_constraint(tcx, c, None);
+        prohibit_assoc_item_constraint(cx, c, None);
     }
 
     let explicit_late_bound =
-        prohibit_explicit_late_bound_lifetimes(tcx, gen_params, gen_args, gen_pos);
+        prohibit_explicit_late_bound_lifetimes(cx, gen_params, gen_args, gen_pos);
 
     let mut invalid_args = vec![];
 
@@ -486,8 +488,8 @@ pub(crate) fn check_generic_arg_count(
             GenericArgsInfo::MissingLifetimes { num_missing_args }
         };
 
-        let reported = tcx.dcx().emit_err(WrongNumberOfGenericArgs::new(
-            tcx,
+        let reported = cx.dcx().emit_err(WrongNumberOfGenericArgs::new(
+            cx.tcx(),
             gen_args_info,
             seg,
             gen_params,
@@ -571,9 +573,9 @@ pub(crate) fn check_generic_arg_count(
         debug!(?gen_args_info);
 
         let reported = gen_args.has_err().unwrap_or_else(|| {
-            tcx.dcx()
+            cx.dcx()
                 .create_err(WrongNumberOfGenericArgs::new(
-                    tcx,
+                    cx.tcx(),
                     gen_args_info,
                     seg,
                     gen_params,
@@ -621,7 +623,7 @@ pub(crate) fn check_generic_arg_count(
 /// Prohibits explicit lifetime arguments if late-bound lifetime parameters
 /// are present. This is used both for datatypes and function calls.
 pub(crate) fn prohibit_explicit_late_bound_lifetimes(
-    tcx: TyCtxt<'_>,
+    cx: &dyn HirTyLowerer<'_>,
     def: &ty::Generics,
     args: &hir::GenericArgs<'_>,
     position: GenericArgPosition,
@@ -642,13 +644,13 @@ pub(crate) fn prohibit_explicit_late_bound_lifetimes(
         if position == GenericArgPosition::Value
             && args.num_lifetime_params() != param_counts.lifetimes
         {
-            struct_span_code_err!(tcx.dcx(), span, E0794, "{}", msg)
+            struct_span_code_err!(cx.dcx(), span, E0794, "{}", msg)
                 .with_span_note(span_late, note)
                 .emit();
         } else {
             let mut multispan = MultiSpan::from_span(span);
             multispan.push_span_label(span_late, note);
-            tcx.node_span_lint(
+            cx.tcx().node_span_lint(
                 LATE_BOUND_LIFETIME_ARGUMENTS,
                 args.args[0].hir_id(),
                 multispan,

compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs

@@ -60,7 +60,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             let msg = "trait objects must include the `dyn` keyword";
             let label = "add `dyn` keyword before this trait";
             let mut diag =
-                rustc_errors::struct_span_code_err!(tcx.dcx(), self_ty.span, E0782, "{}", msg);
+                rustc_errors::struct_span_code_err!(self.dcx(), self_ty.span, E0782, "{}", msg);
             if self_ty.span.can_be_used_for_suggestions()
                 && !self.maybe_suggest_impl_trait(self_ty, &mut diag)
             {

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -28,7 +28,8 @@ use crate::require_c_abi_if_c_variadic;
 use rustc_ast::TraitObjectSyntax;
 use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
 use rustc_errors::{
-    codes::*, struct_span_code_err, Applicability, Diag, ErrorGuaranteed, FatalError,
+    codes::*, struct_span_code_err, Applicability, Diag, DiagCtxtHandle, ErrorGuaranteed,
+    FatalError,
 };
 use rustc_hir as hir;
 use rustc_hir::def::{CtorOf, DefKind, Namespace, Res};
@@ -102,6 +103,8 @@ pub enum RegionInferReason<'a> {
 pub trait HirTyLowerer<'tcx> {
     fn tcx(&self) -> TyCtxt<'tcx>;
 
+    fn dcx(&self) -> DiagCtxtHandle<'_>;
+
     /// Returns the [`LocalDefId`] of the overarching item whose constituents get lowered.
     fn item_def_id(&self) -> LocalDefId;
 
@@ -177,12 +180,6 @@ pub trait HirTyLowerer<'tcx> {
     /// The inference context of the lowering context if applicable.
     fn infcx(&self) -> Option<&InferCtxt<'tcx>>;
 
-    /// Taint the context with errors.
-    ///
-    /// Invoke this when you encounter an error from some prior pass like name resolution.
-    /// This is used to help suppress derived errors typeck might otherwise report.
-    fn set_tainted_by_errors(&self, e: ErrorGuaranteed);
-
     /// Convenience method for coercing the lowering context into a trait object type.
     ///
     /// Most lowering routines are defined on the trait object type directly
@@ -323,7 +320,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             ty::BoundConstness::NotConst,
         );
         if let Some(c) = item_segment.args().constraints.first() {
-            prohibit_assoc_item_constraint(self.tcx(), c, Some((def_id, item_segment, span)));
+            prohibit_assoc_item_constraint(self, c, Some((def_id, item_segment, span)));
         }
         args
     }
@@ -394,18 +391,14 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         }
 
         let mut arg_count = check_generic_arg_count(
-            tcx,
+            self,
             def_id,
             segment,
             generics,
             GenericArgPosition::Type,
             self_ty.is_some(),
         );
 
-        if let Err(err) = &arg_count.correct {
-            self.set_tainted_by_errors(err.reported);
-        }
-
         // Skip processing if type has no generic parameters.
         // Traits always have `Self` as a generic parameter, which means they will not return early
         // here and so associated item constraints will be handled regardless of whether there are
@@ -562,11 +555,10 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             && generics.has_self
             && !tcx.is_const_trait(def_id)
         {
-            let reported = tcx.dcx().emit_err(crate::errors::ConstBoundForNonConstTrait {
+            let reported = self.dcx().emit_err(crate::errors::ConstBoundForNonConstTrait {
                 span,
                 modifier: constness.as_str(),
             });
-            self.set_tainted_by_errors(reported);
             arg_count.correct = Err(GenericArgCountMismatch { reported, invalid_args: vec![] });
         }
 
@@ -579,7 +571,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             incorrect_args: &arg_count.correct,
         };
         let args = lower_generic_args(
-            tcx,
+            self,
             def_id,
             parent_args,
             self_ty.is_some(),
@@ -609,7 +601,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             ty::BoundConstness::NotConst,
         );
         if let Some(c) = item_segment.args().constraints.first() {
-            prohibit_assoc_item_constraint(self.tcx(), c, Some((item_def_id, item_segment, span)));
+            prohibit_assoc_item_constraint(self, c, Some((item_def_id, item_segment, span)));
         }
         args
     }
@@ -715,7 +707,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             // would not be well-formed!
             if polarity != ty::PredicatePolarity::Positive {
                 assert!(
-                    self.tcx().dcx().has_errors().is_some(),
+                    self.dcx().has_errors().is_some(),
                     "negative trait bounds should not have assoc item constraints",
                 );
                 continue;
@@ -761,11 +753,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             constness,
         );
         if let Some(c) = trait_segment.args().constraints.first() {
-            prohibit_assoc_item_constraint(
-                self.tcx(),
-                c,
-                Some((trait_def_id, trait_segment, span)),
-            );
+            prohibit_assoc_item_constraint(self, c, Some((trait_def_id, trait_segment, span)));
         }
         ty::TraitRef::new_from_args(self.tcx(), trait_def_id, generic_args)
     }
@@ -877,7 +865,6 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 span,
                 constraint,
             );
-            self.set_tainted_by_errors(reported);
             return Err(reported);
         };
         debug!(?bound);
@@ -887,7 +874,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
             let assoc_kind_str = assoc_kind_str(assoc_kind);
             let ty_param_name = &ty_param_name.to_string();
-            let mut err = tcx.dcx().create_err(crate::errors::AmbiguousAssocItem {
+            let mut err = self.dcx().create_err(crate::errors::AmbiguousAssocItem {
                 span,
                 assoc_kind: assoc_kind_str,
                 assoc_name,
@@ -960,7 +947,6 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 ));
             }
             let reported = err.emit();
-            self.set_tainted_by_errors(reported);
             if !where_bounds.is_empty() {
                 return Err(reported);
             }
@@ -1059,7 +1045,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 // trait reference.
                 let Some(trait_ref) = tcx.impl_trait_ref(impl_def_id) else {
                     // A cycle error occurred, most likely.
-                    tcx.dcx().span_bug(span, "expected cycle error");
+                    self.dcx().span_bug(span, "expected cycle error");
                 };
 
                 self.probe_single_bound_for_assoc_item(
@@ -1089,10 +1075,10 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 let reported = if variant_resolution.is_some() {
                     // Variant in type position
                     let msg = format!("expected type, found variant `{assoc_ident}`");
-                    tcx.dcx().span_err(span, msg)
+                    self.dcx().span_err(span, msg)
                 } else if qself_ty.is_enum() {
                     let mut err = struct_span_code_err!(
-                        tcx.dcx(),
+                        self.dcx(),
                         assoc_ident.span,
                         E0599,
                         "no variant named `{}` found for enum `{}`",
@@ -1133,7 +1119,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 } else if let ty::Alias(ty::Opaque, alias_ty) = qself_ty.kind() {
                     // `<impl Trait as OtherTrait>::Assoc` makes no sense.
                     struct_span_code_err!(
-                        tcx.dcx(),
+                        self.dcx(),
                         tcx.def_span(alias_ty.def_id),
                         E0667,
                         "`impl Trait` is not allowed in path parameters"
@@ -1153,7 +1139,6 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         assoc_ident.name,
                     )
                 };
-                self.set_tainted_by_errors(reported);
                 return Err(reported);
             }
         };
@@ -1404,13 +1389,12 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         let tcx = self.tcx();
 
         if !tcx.visibility(item_def_id).is_accessible_from(scope, tcx) {
-            let reported = tcx.dcx().emit_err(crate::errors::AssocItemIsPrivate {
+            self.dcx().emit_err(crate::errors::AssocItemIsPrivate {
                 span,
                 kind: tcx.def_descr(item_def_id),
                 name: ident,
                 defined_here_label: tcx.def_span(item_def_id),
             });
-            self.set_tainted_by_errors(reported);
         }
 
         tcx.check_stability(item_def_id, Some(block), span, None);
@@ -1564,7 +1548,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         for segment in segments {
             // Only emit the first error to avoid overloading the user with error messages.
             if let Some(c) = segment.args().constraints.first() {
-                return Err(prohibit_assoc_item_constraint(self.tcx(), c, None));
+                return Err(prohibit_assoc_item_constraint(self, c, None));
             }
         }
 
@@ -1824,7 +1808,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 // `AlwaysApplicable` impl needs a `T: ?Sized` bound for
                 // this to compile if we were to normalize here.
                 if forbid_generic && ty.has_param() {
-                    let mut err = tcx.dcx().struct_span_err(
+                    let mut err = self.dcx().struct_span_err(
                         path.span,
                         "generic `Self` types are currently not permitted in anonymous constants",
                     );
@@ -1836,7 +1820,6 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         err.span_note(impl_.self_ty.span, "not a concrete type");
                     }
                     let reported = err.emit();
-                    self.set_tainted_by_errors(reported);
                     Ty::new_error(tcx, reported)
                 } else {
                     ty
@@ -1877,7 +1860,6 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     .tcx()
                     .dcx()
                     .span_delayed_bug(path.span, "path with `Res::Err` but no error emitted");
-                self.set_tainted_by_errors(e);
                 Ty::new_error(self.tcx(), e)
             }
             Res::Def(..) => {
@@ -1888,7 +1870,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 );
                 Ty::new_error(
                     self.tcx(),
-                    self.tcx().dcx().span_delayed_bug(span, "incorrect resolution for `Self`"),
+                    self.dcx().span_delayed_bug(span, "incorrect resolution for `Self`"),
                 )
             }
             _ => span_bug!(span, "unexpected resolution: {:?}", path.res),
@@ -1961,7 +1943,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         let sig_span = self.tcx().def_span(sig_id);
         let mut try_emit = |descr| {
             if emit {
-                self.tcx().dcx().emit_err(crate::errors::NotSupportedDelegation {
+                self.dcx().emit_err(crate::errors::NotSupportedDelegation {
                     span,
                     descr,
                     callee_span: sig_span,
@@ -2011,8 +1993,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     ) -> Ty<'tcx> {
         if self.check_delegation_constraints(sig_id, span, idx == hir::InferDelegationKind::Output)
         {
-            let e = self.tcx().dcx().span_delayed_bug(span, "not supported delegation case");
-            self.set_tainted_by_errors(e);
+            let e = self.dcx().span_delayed_bug(span, "not supported delegation case");
             return Ty::new_error(self.tcx(), e);
         };
         let sig = self.tcx().fn_sig(sig_id);
@@ -2177,7 +2158,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 let ty = self.lower_ty(ty);
                 let pat_ty = match pat.kind {
                     hir::PatKind::Wild => {
-                        let err = tcx.dcx().emit_err(WildPatTy { span: pat.span });
+                        let err = self.dcx().emit_err(WildPatTy { span: pat.span });
                         Ty::new_error(tcx, err)
                     }
                     hir::PatKind::Range(start, end, include_end) => {
@@ -2357,7 +2338,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
         self.validate_late_bound_regions(late_bound_in_args, late_bound_in_ret, |br_name| {
             struct_span_code_err!(
-                tcx.dcx(),
+                self.dcx(),
                 decl.output.span(),
                 E0581,
                 "return type references {}, which is not constrained by the fn input types",
@@ -2408,11 +2389,11 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     }
 
     #[instrument(level = "trace", skip(self, generate_err))]
-    fn validate_late_bound_regions(
-        &self,
+    fn validate_late_bound_regions<'cx>(
+        &'cx self,
         constrained_regions: FxHashSet<ty::BoundRegionKind>,
         referenced_regions: FxHashSet<ty::BoundRegionKind>,
-        generate_err: impl Fn(&str) -> Diag<'tcx>,
+        generate_err: impl Fn(&str) -> Diag<'cx>,
     ) {
         for br in referenced_regions.difference(&constrained_regions) {
             let br_name = match *br {
@@ -2437,7 +2418,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 err.note("consider introducing a named lifetime parameter");
             }
 
-            self.set_tainted_by_errors(err.emit());
+            err.emit();
         }
     }
 
@@ -2478,7 +2459,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         // error.
         let r = derived_region_bounds[0];
         if derived_region_bounds[1..].iter().any(|r1| r != *r1) {
-            self.set_tainted_by_errors(tcx.dcx().emit_err(AmbiguousLifetimeBound { span }));
+            self.dcx().emit_err(AmbiguousLifetimeBound { span });
         }
         Some(r)
     }

compiler/rustc_hir_analysis/src/hir_ty_lowering/object_safety.rs

@@ -236,7 +236,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                             Ty::new_misc_error(tcx).into()
                         } else if arg.walk().any(|arg| arg == dummy_self.into()) {
                             references_self = true;
-                            let guar = tcx.dcx().span_delayed_bug(
+                            let guar = self.dcx().span_delayed_bug(
                                 span,
                                 "trait object trait bounds reference `Self`",
                             );
@@ -262,8 +262,8 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
                 if references_self {
                     let def_id = i.bottom().0.def_id();
-                    let reported = struct_span_code_err!(
-                        tcx.dcx(),
+                    struct_span_code_err!(
+                        self.dcx(),
                         i.bottom().1,
                         E0038,
                         "the {} `{}` cannot be made into an object",
@@ -275,7 +275,6 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                             .error_msg(),
                     )
                     .emit();
-                    self.set_tainted_by_errors(reported);
                 }
 
                 ty::ExistentialTraitRef { def_id: trait_ref.def_id, args }

compiler/rustc_hir_typeck/src/coercion.rs

@@ -59,7 +59,7 @@ use rustc_span::symbol::sym;
 use rustc_span::{BytePos, DesugaringKind, Span, DUMMY_SP};
 use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::error_reporting::traits::suggestions::TypeErrCtxtExt;
-use rustc_trait_selection::error_reporting::traits::TypeErrCtxtExt as _;
+use rustc_trait_selection::error_reporting::traits::TypeErrCtxtSelectionErrExt as _;
 use rustc_trait_selection::infer::InferCtxtExt as _;
 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
 use rustc_trait_selection::traits::{

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -1139,7 +1139,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // parameter's value explicitly, so we have to do some error-
             // checking here.
             let arg_count =
-                check_generic_arg_count_for_call(tcx, def_id, generics, seg, IsMethodCall::No);
+                check_generic_arg_count_for_call(self, def_id, generics, seg, IsMethodCall::No);
 
             if let ExplicitLateBound::Yes = arg_count.explicit_late_bound {
                 explicit_late_bound = ExplicitLateBound::Yes;
@@ -1375,7 +1375,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         let args_raw = self_ctor_args.unwrap_or_else(|| {
             lower_generic_args(
-                tcx,
+                self,
                 def_id,
                 &[],
                 has_self,

compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs

@@ -5,7 +5,7 @@ mod checks;
 mod inspect_obligations;
 mod suggestions;
 
-use rustc_errors::{DiagCtxtHandle, ErrorGuaranteed};
+use rustc_errors::DiagCtxtHandle;
 
 use crate::coercion::DynamicCoerceMany;
 use crate::fallback::DivergingFallbackBehavior;
@@ -217,6 +217,10 @@ impl<'tcx> HirTyLowerer<'tcx> for FnCtxt<'_, 'tcx> {
         self.tcx
     }
 
+    fn dcx(&self) -> DiagCtxtHandle<'_> {
+        self.root_ctxt.dcx()
+    }
+
     fn item_def_id(&self) -> LocalDefId {
         self.body_id
     }
@@ -338,10 +342,6 @@ impl<'tcx> HirTyLowerer<'tcx> for FnCtxt<'_, 'tcx> {
         Some(&self.infcx)
     }
 
-    fn set_tainted_by_errors(&self, e: ErrorGuaranteed) {
-        self.infcx.set_tainted_by_errors(e)
-    }
-
     fn lower_fn_sig(
         &self,
         decl: &rustc_hir::FnDecl<'tcx>,

compiler/rustc_hir_typeck/src/method/confirm.rs

@@ -354,7 +354,7 @@ impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
         let generics = self.tcx.generics_of(pick.item.def_id);
 
         let arg_count_correct = check_generic_arg_count_for_call(
-            self.tcx,
+            self.fcx,
             pick.item.def_id,
             generics,
             seg,
@@ -425,7 +425,7 @@ impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
         }
 
         let args = lower_generic_args(
-            self.tcx,
+            self.fcx,
             pick.item.def_id,
             parent_args,
             false,

compiler/rustc_lint/src/types.rs

@@ -563,7 +563,8 @@ fn lint_literal<'tcx>(
         ty::Float(t) => {
             let is_infinite = match lit.node {
                 ast::LitKind::Float(v, _) => match t {
-                    // FIXME(f16_f128): add this check once we have library support
+                    // FIXME(f16_f128): add this check once `is_infinite` is reliable (ABI
+                    // issues resolved).
                     ty::FloatTy::F16 => Ok(false),
                     ty::FloatTy::F32 => v.as_str().parse().map(f32::is_infinite),
                     ty::FloatTy::F64 => v.as_str().parse().map(f64::is_infinite),

compiler/rustc_next_trait_solver/src/solve/mod.rs

@@ -48,12 +48,20 @@ enum GoalEvaluationKind {
     Nested,
 }
 
+// FIXME(trait-system-refactor-initiative#117): we don't detect whether a response
+// ended up pulling down any universes.
 fn has_no_inference_or_external_constraints<I: Interner>(
     response: ty::Canonical<I, Response<I>>,
 ) -> bool {
-    response.value.external_constraints.region_constraints.is_empty()
-        && response.value.var_values.is_identity()
-        && response.value.external_constraints.opaque_types.is_empty()
+    let ExternalConstraintsData {
+        ref region_constraints,
+        ref opaque_types,
+        ref normalization_nested_goals,
+    } = *response.value.external_constraints;
+    response.value.var_values.is_identity()
+        && region_constraints.is_empty()
+        && opaque_types.is_empty()
+        && normalization_nested_goals.is_empty()
 }
 
 impl<'a, D, I> EvalCtxt<'a, D>

compiler/rustc_next_trait_solver/src/solve/search_graph.rs

@@ -71,7 +71,7 @@ struct StackEntry<I: Interner> {
     /// C :- D
     /// D :- C
     /// ```
-    cycle_participants: HashSet<CanonicalInput<I>>,
+    nested_goals: HashSet<CanonicalInput<I>>,
     /// Starts out as `None` and gets set when rerunning this
     /// goal in case we encounter a cycle.
     provisional_result: Option<QueryResult<I>>,
@@ -139,18 +139,11 @@ impl<I: Interner> SearchGraph<I> {
         self.mode
     }
 
-    /// Pops the highest goal from the stack, lazily updating the
-    /// the next goal in the stack.
-    ///
-    /// Directly popping from the stack instead of using this method
-    /// would cause us to not track overflow and recursion depth correctly.
-    fn pop_stack(&mut self) -> StackEntry<I> {
-        let elem = self.stack.pop().unwrap();
-        if let Some(last) = self.stack.raw.last_mut() {
-            last.reached_depth = last.reached_depth.max(elem.reached_depth);
-            last.encountered_overflow |= elem.encountered_overflow;
+    fn update_parent_goal(&mut self, reached_depth: StackDepth, encountered_overflow: bool) {
+        if let Some(parent) = self.stack.raw.last_mut() {
+            parent.reached_depth = parent.reached_depth.max(reached_depth);
+            parent.encountered_overflow |= encountered_overflow;
         }
-        elem
     }
 
     pub(super) fn is_empty(&self) -> bool {
@@ -222,8 +215,8 @@ impl<I: Interner> SearchGraph<I> {
         let current_cycle_root = &mut stack[current_root.as_usize()];
         for entry in cycle_participants {
             entry.non_root_cycle_participant = entry.non_root_cycle_participant.max(Some(head));
-            current_cycle_root.cycle_participants.insert(entry.input);
-            current_cycle_root.cycle_participants.extend(mem::take(&mut entry.cycle_participants));
+            current_cycle_root.nested_goals.insert(entry.input);
+            current_cycle_root.nested_goals.extend(mem::take(&mut entry.nested_goals));
         }
     }
 
@@ -342,7 +335,7 @@ impl<I: Interner> SearchGraph<I> {
                 non_root_cycle_participant: None,
                 encountered_overflow: false,
                 has_been_used: HasBeenUsed::empty(),
-                cycle_participants: Default::default(),
+                nested_goals: Default::default(),
                 provisional_result: None,
             };
             assert_eq!(self.stack.push(entry), depth);
@@ -364,14 +357,16 @@ impl<I: Interner> SearchGraph<I> {
             }
 
             debug!("canonical cycle overflow");
-            let current_entry = self.pop_stack();
+            let current_entry = self.stack.pop().unwrap();
             debug_assert!(current_entry.has_been_used.is_empty());
             let result = Self::response_no_constraints(cx, input, Certainty::overflow(false));
             (current_entry, result)
         });
 
         let proof_tree = inspect.finalize_canonical_goal_evaluation(cx);
 
+        self.update_parent_goal(final_entry.reached_depth, final_entry.encountered_overflow);
+
         // We're now done with this goal. In case this goal is involved in a larger cycle
         // do not remove it from the provisional cache and update its provisional result.
         // We only add the root of cycles to the global cache.
@@ -394,15 +389,15 @@ impl<I: Interner> SearchGraph<I> {
             //
             // We must not use the global cache entry of a root goal if a cycle
             // participant is on the stack. This is necessary to prevent unstable
-            // results. See the comment of `StackEntry::cycle_participants` for
+            // results. See the comment of `StackEntry::nested_goals` for
             // more details.
             self.global_cache(cx).insert(
                 cx,
                 input,
                 proof_tree,
                 reached_depth,
                 final_entry.encountered_overflow,
-                final_entry.cycle_participants,
+                final_entry.nested_goals,
                 dep_node,
                 result,
             )
@@ -441,14 +436,9 @@ impl<I: Interner> SearchGraph<I> {
             }
         }
 
-        // Update the reached depth of the current goal to make sure
-        // its state is the same regardless of whether we've used the
-        // global cache or not.
+        // Adjust the parent goal as if we actually computed this goal.
         let reached_depth = self.stack.next_index().plus(additional_depth);
-        if let Some(last) = self.stack.raw.last_mut() {
-            last.reached_depth = last.reached_depth.max(reached_depth);
-            last.encountered_overflow |= encountered_overflow;
-        }
+        self.update_parent_goal(reached_depth, encountered_overflow);
 
         Some(result)
     }
@@ -477,7 +467,7 @@ impl<I: Interner> SearchGraph<I> {
         F: FnMut(&mut Self, &mut ProofTreeBuilder<D>) -> QueryResult<I>,
     {
         let result = prove_goal(self, inspect);
-        let stack_entry = self.pop_stack();
+        let stack_entry = self.stack.pop().unwrap();
         debug_assert_eq!(stack_entry.input, input);
 
         // If the current goal is not the root of a cycle, we are done.
@@ -554,27 +544,27 @@ impl<I: Interner> SearchGraph<I> {
                 non_root_cycle_participant,
                 encountered_overflow: _,
                 has_been_used,
-                ref cycle_participants,
+                ref nested_goals,
                 provisional_result,
             } = *entry;
             let cache_entry = provisional_cache.get(&entry.input).unwrap();
             assert_eq!(cache_entry.stack_depth, Some(depth));
             if let Some(head) = non_root_cycle_participant {
                 assert!(head < depth);
-                assert!(cycle_participants.is_empty());
+                assert!(nested_goals.is_empty());
                 assert_ne!(stack[head].has_been_used, HasBeenUsed::empty());
 
                 let mut current_root = head;
                 while let Some(parent) = stack[current_root].non_root_cycle_participant {
                     current_root = parent;
                 }
-                assert!(stack[current_root].cycle_participants.contains(&input));
+                assert!(stack[current_root].nested_goals.contains(&input));
             }
 
-            if !cycle_participants.is_empty() {
+            if !nested_goals.is_empty() {
                 assert!(provisional_result.is_some() || !has_been_used.is_empty());
                 for entry in stack.iter().take(depth.as_usize()) {
-                    assert_eq!(cycle_participants.get(&entry.input), None);
+                    assert_eq!(nested_goals.get(&entry.input), None);
                 }
             }
         }

compiler/rustc_parse/src/parser/item.rs

@@ -796,7 +796,7 @@ impl<'a> Parser<'a> {
                         self.dcx().struct_span_err(non_item_span, "non-item in item list");
                     self.consume_block(Delimiter::Brace, ConsumeClosingDelim::Yes);
                     if is_let {
-                        err.span_suggestion(
+                        err.span_suggestion_verbose(
                             non_item_span,
                             "consider using `const` instead of `let` for associated const",
                             "const",

compiler/rustc_resolve/src/diagnostics.rs

@@ -819,7 +819,12 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             ResolutionError::CannotCaptureDynamicEnvironmentInFnItem => {
                 self.dcx().create_err(errs::CannotCaptureDynamicEnvironmentInFnItem { span })
             }
-            ResolutionError::AttemptToUseNonConstantValueInConstant(ident, suggestion, current) => {
+            ResolutionError::AttemptToUseNonConstantValueInConstant {
+                ident,
+                suggestion,
+                current,
+                type_span,
+            } => {
                 // let foo =...
                 //     ^^^ given this Span
                 // ------- get this Span to have an applicable suggestion
@@ -836,13 +841,15 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
 
                 let ((with, with_label), without) = match sp {
                     Some(sp) if !self.tcx.sess.source_map().is_multiline(sp) => {
-                        let sp = sp.with_lo(BytePos(sp.lo().0 - (current.len() as u32)));
+                        let sp = sp
+                            .with_lo(BytePos(sp.lo().0 - (current.len() as u32)))
+                            .until(ident.span);
                         (
                         (Some(errs::AttemptToUseNonConstantValueInConstantWithSuggestion {
                                 span: sp,
-                                ident,
                                 suggestion,
                                 current,
+                                type_span,
                             }), Some(errs::AttemptToUseNonConstantValueInConstantLabelWithSuggestion {span})),
                             None,
                         )

compiler/rustc_resolve/src/errors.rs

@@ -240,16 +240,18 @@ pub(crate) struct AttemptToUseNonConstantValueInConstant<'a> {
 }
 
 #[derive(Subdiagnostic)]
-#[suggestion(
+#[multipart_suggestion(
     resolve_attempt_to_use_non_constant_value_in_constant_with_suggestion,
-    code = "{suggestion} {ident}",
-    applicability = "maybe-incorrect"
+    style = "verbose",
+    applicability = "has-placeholders"
 )]
 pub(crate) struct AttemptToUseNonConstantValueInConstantWithSuggestion<'a> {
-    #[primary_span]
+    // #[primary_span]
+    #[suggestion_part(code = "{suggestion} ")]
     pub(crate) span: Span,
-    pub(crate) ident: Ident,
     pub(crate) suggestion: &'a str,
+    #[suggestion_part(code = ": /* Type */")]
+    pub(crate) type_span: Option<Span>,
     pub(crate) current: &'a str,
 }
 

compiler/rustc_resolve/src/ident.rs

@@ -1178,21 +1178,41 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                             if let Some(span) = finalize {
                                 let (span, resolution_error) = match item {
                                     None if rib_ident.as_str() == "self" => (span, LowercaseSelf),
-                                    None => (
-                                        rib_ident.span,
-                                        AttemptToUseNonConstantValueInConstant(
-                                            original_rib_ident_def,
-                                            "const",
-                                            "let",
-                                        ),
-                                    ),
+                                    None => {
+                                        // If we have a `let name = expr;`, we have the span for
+                                        // `name` and use that to see if it is followed by a type
+                                        // specifier. If not, then we know we need to suggest
+                                        // `const name: Ty = expr;`. This is a heuristic, it will
+                                        // break down in the presence of macros.
+                                        let sm = self.tcx.sess.source_map();
+                                        let type_span = match sm.span_look_ahead(
+                                            original_rib_ident_def.span,
+                                            ":",
+                                            None,
+                                        ) {
+                                            None => {
+                                                Some(original_rib_ident_def.span.shrink_to_hi())
+                                            }
+                                            Some(_) => None,
+                                        };
+                                        (
+                                            rib_ident.span,
+                                            AttemptToUseNonConstantValueInConstant {
+                                                ident: original_rib_ident_def,
+                                                suggestion: "const",
+                                                current: "let",
+                                                type_span,
+                                            },
+                                        )
+                                    }
                                     Some((ident, kind)) => (
                                         span,
-                                        AttemptToUseNonConstantValueInConstant(
+                                        AttemptToUseNonConstantValueInConstant {
                                             ident,
-                                            "let",
-                                            kind.as_str(),
-                                        ),
+                                            suggestion: "let",
+                                            current: kind.as_str(),
+                                            type_span: None,
+                                        },
                                     ),
                                 };
                                 self.report_error(span, resolution_error);

compiler/rustc_resolve/src/lib.rs

@@ -236,11 +236,12 @@ enum ResolutionError<'a> {
     /// Error E0434: can't capture dynamic environment in a fn item.
     CannotCaptureDynamicEnvironmentInFnItem,
     /// Error E0435: attempt to use a non-constant value in a constant.
-    AttemptToUseNonConstantValueInConstant(
-        Ident,
-        /* suggestion */ &'static str,
-        /* current */ &'static str,
-    ),
+    AttemptToUseNonConstantValueInConstant {
+        ident: Ident,
+        suggestion: &'static str,
+        current: &'static str,
+        type_span: Option<Span>,
+    },
     /// Error E0530: `X` bindings cannot shadow `Y`s.
     BindingShadowsSomethingUnacceptable {
         shadowing_binding: PatternSource,

compiler/rustc_trait_selection/src/error_reporting/traits/infer_ctxt_ext.rs

@@ -1,3 +1,5 @@
+// FIXME(error_reporting): This should be made into private methods on `TypeErrCtxt`.
+
 use crate::infer::InferCtxt;
 use crate::traits::{Obligation, ObligationCause, ObligationCtxt};
 use rustc_errors::{codes::*, pluralize, struct_span_code_err, Applicability, Diag};
@@ -9,8 +11,6 @@ use rustc_span::{Span, DUMMY_SP};
 
 use super::ArgKind;
 
-pub use rustc_infer::traits::error_reporting::*;
-
 #[extension(pub trait InferCtxtExt<'tcx>)]
 impl<'tcx> InferCtxt<'tcx> {
     /// Given some node representing a fn-like thing in the HIR map,

compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs

@@ -1,23 +1,34 @@
-// ignore-tidy-filelength :(
-
 pub mod ambiguity;
+mod fulfillment_errors;
 mod infer_ctxt_ext;
 pub mod on_unimplemented;
+mod overflow;
 pub mod suggestions;
-mod type_err_ctxt_ext;
 
-use rustc_data_structures::fx::FxIndexSet;
-use rustc_hir as hir;
+use std::iter;
+
+use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
 use rustc_hir::def_id::DefId;
 use rustc_hir::intravisit::Visitor;
-use rustc_infer::traits::{Obligation, ObligationCause, ObligationCauseCode, PredicateObligation};
+use rustc_hir::{self as hir, LangItem};
+use rustc_infer::infer::error_reporting::TypeErrCtxt;
+use rustc_infer::traits::{
+    Obligation, ObligationCause, ObligationCauseCode, PredicateObligation, SelectionError,
+};
+use rustc_macros::extension;
 use rustc_middle::ty::print::PrintTraitRefExt as _;
 use rustc_middle::ty::{self, Ty, TyCtxt};
-use rustc_span::Span;
-use std::ops::ControlFlow;
+use rustc_span::{ErrorGuaranteed, ExpnKind, Span};
+
+use ambiguity::TypeErrCtxtAmbiguityExt as _;
+use fulfillment_errors::TypeErrCtxtExt as _;
+use suggestions::TypeErrCtxtExt as _;
+
+use crate::traits::{FulfillmentError, FulfillmentErrorCode};
 
+pub use self::fulfillment_errors::*;
 pub use self::infer_ctxt_ext::*;
-pub use self::type_err_ctxt_ext::*;
+pub use self::overflow::*;
 
 // When outputting impl candidates, prefer showing those that are more similar.
 //
@@ -89,49 +100,6 @@ impl<'v> Visitor<'v> for FindExprBySpan<'v> {
     }
 }
 
-/// Look for type `param` in an ADT being used only through a reference to confirm that suggesting
-/// `param: ?Sized` would be a valid constraint.
-struct FindTypeParam {
-    param: rustc_span::Symbol,
-    invalid_spans: Vec<Span>,
-    nested: bool,
-}
-
-impl<'v> Visitor<'v> for FindTypeParam {
-    fn visit_where_predicate(&mut self, _: &'v hir::WherePredicate<'v>) {
-        // Skip where-clauses, to avoid suggesting indirection for type parameters found there.
-    }
-
-    fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
-        // We collect the spans of all uses of the "bare" type param, like in `field: T` or
-        // `field: (T, T)` where we could make `T: ?Sized` while skipping cases that are known to be
-        // valid like `field: &'a T` or `field: *mut T` and cases that *might* have further `Sized`
-        // obligations like `Box<T>` and `Vec<T>`, but we perform no extra analysis for those cases
-        // and suggest `T: ?Sized` regardless of their obligations. This is fine because the errors
-        // in that case should make what happened clear enough.
-        match ty.kind {
-            hir::TyKind::Ptr(_) | hir::TyKind::Ref(..) | hir::TyKind::TraitObject(..) => {}
-            hir::TyKind::Path(hir::QPath::Resolved(None, path))
-                if path.segments.len() == 1 && path.segments[0].ident.name == self.param =>
-            {
-                if !self.nested {
-                    debug!(?ty, "FindTypeParam::visit_ty");
-                    self.invalid_spans.push(ty.span);
-                }
-            }
-            hir::TyKind::Path(_) => {
-                let prev = self.nested;
-                self.nested = true;
-                hir::intravisit::walk_ty(self, ty);
-                self.nested = prev;
-            }
-            _ => {
-                hir::intravisit::walk_ty(self, ty);
-            }
-        }
-    }
-}
-
 /// Summarizes information
 #[derive(Clone)]
 pub enum ArgKind {
@@ -163,24 +131,199 @@ impl ArgKind {
     }
 }
 
-struct HasNumericInferVisitor;
+#[derive(Copy, Clone)]
+pub enum DefIdOrName {
+    DefId(DefId),
+    Name(&'static str),
+}
 
-impl<'tcx> ty::TypeVisitor<TyCtxt<'tcx>> for HasNumericInferVisitor {
-    type Result = ControlFlow<()>;
+#[extension(pub trait TypeErrCtxtExt<'a, 'tcx>)]
+impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
+    fn report_fulfillment_errors(
+        &self,
+        mut errors: Vec<FulfillmentError<'tcx>>,
+    ) -> ErrorGuaranteed {
+        self.sub_relations
+            .borrow_mut()
+            .add_constraints(self, errors.iter().map(|e| e.obligation.predicate));
+
+        #[derive(Debug)]
+        struct ErrorDescriptor<'tcx> {
+            predicate: ty::Predicate<'tcx>,
+            index: Option<usize>, // None if this is an old error
+        }
 
-    fn visit_ty(&mut self, ty: Ty<'tcx>) -> Self::Result {
-        if matches!(ty.kind(), ty::Infer(ty::FloatVar(_) | ty::IntVar(_))) {
-            ControlFlow::Break(())
-        } else {
-            ControlFlow::Continue(())
+        let mut error_map: FxIndexMap<_, Vec<_>> = self
+            .reported_trait_errors
+            .borrow()
+            .iter()
+            .map(|(&span, predicates)| {
+                (
+                    span,
+                    predicates
+                        .0
+                        .iter()
+                        .map(|&predicate| ErrorDescriptor { predicate, index: None })
+                        .collect(),
+                )
+            })
+            .collect();
+
+        // Ensure `T: Sized` and `T: WF` obligations come last. This lets us display diagnostics
+        // with more relevant type information and hide redundant E0282 errors.
+        errors.sort_by_key(|e| match e.obligation.predicate.kind().skip_binder() {
+            ty::PredicateKind::Clause(ty::ClauseKind::Trait(pred))
+                if self.tcx.is_lang_item(pred.def_id(), LangItem::Sized) =>
+            {
+                1
+            }
+            ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(_)) => 3,
+            ty::PredicateKind::Coerce(_) => 2,
+            _ => 0,
+        });
+
+        for (index, error) in errors.iter().enumerate() {
+            // We want to ignore desugarings here: spans are equivalent even
+            // if one is the result of a desugaring and the other is not.
+            let mut span = error.obligation.cause.span;
+            let expn_data = span.ctxt().outer_expn_data();
+            if let ExpnKind::Desugaring(_) = expn_data.kind {
+                span = expn_data.call_site;
+            }
+
+            error_map.entry(span).or_default().push(ErrorDescriptor {
+                predicate: error.obligation.predicate,
+                index: Some(index),
+            });
         }
+
+        // We do this in 2 passes because we want to display errors in order, though
+        // maybe it *is* better to sort errors by span or something.
+        let mut is_suppressed = vec![false; errors.len()];
+        for (_, error_set) in error_map.iter() {
+            // We want to suppress "duplicate" errors with the same span.
+            for error in error_set {
+                if let Some(index) = error.index {
+                    // Suppress errors that are either:
+                    // 1) strictly implied by another error.
+                    // 2) implied by an error with a smaller index.
+                    for error2 in error_set {
+                        if error2.index.is_some_and(|index2| is_suppressed[index2]) {
+                            // Avoid errors being suppressed by already-suppressed
+                            // errors, to prevent all errors from being suppressed
+                            // at once.
+                            continue;
+                        }
+
+                        if self.error_implies(error2.predicate, error.predicate)
+                            && !(error2.index >= error.index
+                                && self.error_implies(error.predicate, error2.predicate))
+                        {
+                            info!("skipping {:?} (implied by {:?})", error, error2);
+                            is_suppressed[index] = true;
+                            break;
+                        }
+                    }
+                }
+            }
+        }
+
+        let mut reported = None;
+
+        for from_expansion in [false, true] {
+            for (error, suppressed) in iter::zip(&errors, &is_suppressed) {
+                if !suppressed && error.obligation.cause.span.from_expansion() == from_expansion {
+                    let guar = self.report_fulfillment_error(error);
+                    self.infcx.set_tainted_by_errors(guar);
+                    reported = Some(guar);
+                    // We want to ignore desugarings here: spans are equivalent even
+                    // if one is the result of a desugaring and the other is not.
+                    let mut span = error.obligation.cause.span;
+                    let expn_data = span.ctxt().outer_expn_data();
+                    if let ExpnKind::Desugaring(_) = expn_data.kind {
+                        span = expn_data.call_site;
+                    }
+                    self.reported_trait_errors
+                        .borrow_mut()
+                        .entry(span)
+                        .or_insert_with(|| (vec![], guar))
+                        .0
+                        .push(error.obligation.predicate);
+                }
+            }
+        }
+
+        // It could be that we don't report an error because we have seen an `ErrorReported` from
+        // another source. We should probably be able to fix most of these, but some are delayed
+        // bugs that get a proper error after this function.
+        reported.unwrap_or_else(|| self.dcx().delayed_bug("failed to report fulfillment errors"))
     }
-}
 
-#[derive(Copy, Clone)]
-pub enum DefIdOrName {
-    DefId(DefId),
-    Name(&'static str),
+    #[instrument(skip(self), level = "debug")]
+    fn report_fulfillment_error(&self, error: &FulfillmentError<'tcx>) -> ErrorGuaranteed {
+        let mut error = FulfillmentError {
+            obligation: error.obligation.clone(),
+            code: error.code.clone(),
+            root_obligation: error.root_obligation.clone(),
+        };
+        if matches!(
+            error.code,
+            FulfillmentErrorCode::Select(crate::traits::SelectionError::Unimplemented)
+                | FulfillmentErrorCode::Project(_)
+        ) && self.apply_do_not_recommend(&mut error.obligation)
+        {
+            error.code = FulfillmentErrorCode::Select(SelectionError::Unimplemented);
+        }
+
+        match error.code {
+            FulfillmentErrorCode::Select(ref selection_error) => self.report_selection_error(
+                error.obligation.clone(),
+                &error.root_obligation,
+                selection_error,
+            ),
+            FulfillmentErrorCode::Project(ref e) => {
+                self.report_projection_error(&error.obligation, e)
+            }
+            FulfillmentErrorCode::Ambiguity { overflow: None } => {
+                self.maybe_report_ambiguity(&error.obligation)
+            }
+            FulfillmentErrorCode::Ambiguity { overflow: Some(suggest_increasing_limit) } => {
+                self.report_overflow_no_abort(error.obligation.clone(), suggest_increasing_limit)
+            }
+            FulfillmentErrorCode::Subtype(ref expected_found, ref err) => self
+                .report_mismatched_types(
+                    &error.obligation.cause,
+                    expected_found.expected,
+                    expected_found.found,
+                    *err,
+                )
+                .emit(),
+            FulfillmentErrorCode::ConstEquate(ref expected_found, ref err) => {
+                let mut diag = self.report_mismatched_consts(
+                    &error.obligation.cause,
+                    expected_found.expected,
+                    expected_found.found,
+                    *err,
+                );
+                let code = error.obligation.cause.code().peel_derives().peel_match_impls();
+                if let ObligationCauseCode::WhereClause(..)
+                | ObligationCauseCode::WhereClauseInExpr(..) = code
+                {
+                    self.note_obligation_cause_code(
+                        error.obligation.cause.body_id,
+                        &mut diag,
+                        error.obligation.predicate,
+                        error.obligation.param_env,
+                        code,
+                        &mut vec![],
+                        &mut Default::default(),
+                    );
+                }
+                diag.emit()
+            }
+            FulfillmentErrorCode::Cycle(ref cycle) => self.report_overflow_obligation_cycle(cycle),
+        }
+    }
 }
 
 /// Recovers the "impl X for Y" signature from `impl_def_id` and returns it as a

compiler/rustc_trait_selection/src/error_reporting/traits/on_unimplemented.rs

@@ -1,5 +1,5 @@
 use super::{ObligationCauseCode, PredicateObligation};
-use crate::error_reporting::traits::type_err_ctxt_ext::InferCtxtPrivExt;
+use crate::error_reporting::traits::fulfillment_errors::InferCtxtPrivExt;
 use crate::errors::{
     EmptyOnClauseInOnUnimplemented, InvalidOnClauseInOnUnimplemented, NoValueInOnUnimplemented,
 };

compiler/rustc_trait_selection/src/error_reporting/traits/overflow.rs

@@ -0,0 +1,197 @@
+use std::fmt;
+
+use rustc_errors::{
+    struct_span_code_err, Diag, EmissionGuarantee, ErrorGuaranteed, FatalError, E0275,
+};
+use rustc_hir::def::Namespace;
+use rustc_hir::def_id::LOCAL_CRATE;
+use rustc_infer::infer::error_reporting::TypeErrCtxt;
+use rustc_infer::traits::{Obligation, PredicateObligation};
+use rustc_macros::extension;
+use rustc_middle::ty::print::{FmtPrinter, Print};
+use rustc_middle::ty::{self, TyCtxt};
+use rustc_session::Limit;
+use rustc_span::Span;
+use rustc_type_ir::Upcast;
+
+use super::InferCtxtPrivExt;
+use crate::error_reporting::traits::suggestions::TypeErrCtxtExt;
+
+pub enum OverflowCause<'tcx> {
+    DeeplyNormalize(ty::AliasTerm<'tcx>),
+    TraitSolver(ty::Predicate<'tcx>),
+}
+
+pub fn suggest_new_overflow_limit<'tcx, G: EmissionGuarantee>(
+    tcx: TyCtxt<'tcx>,
+    err: &mut Diag<'_, G>,
+) {
+    let suggested_limit = match tcx.recursion_limit() {
+        Limit(0) => Limit(2),
+        limit => limit * 2,
+    };
+    err.help(format!(
+        "consider increasing the recursion limit by adding a \
+         `#![recursion_limit = \"{}\"]` attribute to your crate (`{}`)",
+        suggested_limit,
+        tcx.crate_name(LOCAL_CRATE),
+    ));
+}
+
+#[extension(pub trait TypeErrCtxtOverflowExt<'a, 'tcx>)]
+impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
+    /// Reports that an overflow has occurred and halts compilation. We
+    /// halt compilation unconditionally because it is important that
+    /// overflows never be masked -- they basically represent computations
+    /// whose result could not be truly determined and thus we can't say
+    /// if the program type checks or not -- and they are unusual
+    /// occurrences in any case.
+    fn report_overflow_error(
+        &self,
+        cause: OverflowCause<'tcx>,
+        span: Span,
+        suggest_increasing_limit: bool,
+        mutate: impl FnOnce(&mut Diag<'_>),
+    ) -> ! {
+        let mut err = self.build_overflow_error(cause, span, suggest_increasing_limit);
+        mutate(&mut err);
+        err.emit();
+        FatalError.raise();
+    }
+
+    fn build_overflow_error(
+        &self,
+        cause: OverflowCause<'tcx>,
+        span: Span,
+        suggest_increasing_limit: bool,
+    ) -> Diag<'a> {
+        fn with_short_path<'tcx, T>(tcx: TyCtxt<'tcx>, value: T) -> String
+        where
+            T: fmt::Display + Print<'tcx, FmtPrinter<'tcx, 'tcx>>,
+        {
+            let s = value.to_string();
+            if s.len() > 50 {
+                // We don't need to save the type to a file, we will be talking about this type already
+                // in a separate note when we explain the obligation, so it will be available that way.
+                let mut cx: FmtPrinter<'_, '_> =
+                    FmtPrinter::new_with_limit(tcx, Namespace::TypeNS, rustc_session::Limit(6));
+                value.print(&mut cx).unwrap();
+                cx.into_buffer()
+            } else {
+                s
+            }
+        }
+
+        let mut err = match cause {
+            OverflowCause::DeeplyNormalize(alias_term) => {
+                let alias_term = self.resolve_vars_if_possible(alias_term);
+                let kind = alias_term.kind(self.tcx).descr();
+                let alias_str = with_short_path(self.tcx, alias_term);
+                struct_span_code_err!(
+                    self.dcx(),
+                    span,
+                    E0275,
+                    "overflow normalizing the {kind} `{alias_str}`",
+                )
+            }
+            OverflowCause::TraitSolver(predicate) => {
+                let predicate = self.resolve_vars_if_possible(predicate);
+                match predicate.kind().skip_binder() {
+                    ty::PredicateKind::Subtype(ty::SubtypePredicate { a, b, a_is_expected: _ })
+                    | ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => {
+                        struct_span_code_err!(
+                            self.dcx(),
+                            span,
+                            E0275,
+                            "overflow assigning `{a}` to `{b}`",
+                        )
+                    }
+                    _ => {
+                        let pred_str = with_short_path(self.tcx, predicate);
+                        struct_span_code_err!(
+                            self.dcx(),
+                            span,
+                            E0275,
+                            "overflow evaluating the requirement `{pred_str}`",
+                        )
+                    }
+                }
+            }
+        };
+
+        if suggest_increasing_limit {
+            suggest_new_overflow_limit(self.tcx, &mut err);
+        }
+
+        err
+    }
+
+    /// Reports that an overflow has occurred and halts compilation. We
+    /// halt compilation unconditionally because it is important that
+    /// overflows never be masked -- they basically represent computations
+    /// whose result could not be truly determined and thus we can't say
+    /// if the program type checks or not -- and they are unusual
+    /// occurrences in any case.
+    fn report_overflow_obligation<T>(
+        &self,
+        obligation: &Obligation<'tcx, T>,
+        suggest_increasing_limit: bool,
+    ) -> !
+    where
+        T: Upcast<TyCtxt<'tcx>, ty::Predicate<'tcx>> + Clone,
+    {
+        let predicate = obligation.predicate.clone().upcast(self.tcx);
+        let predicate = self.resolve_vars_if_possible(predicate);
+        self.report_overflow_error(
+            OverflowCause::TraitSolver(predicate),
+            obligation.cause.span,
+            suggest_increasing_limit,
+            |err| {
+                self.note_obligation_cause_code(
+                    obligation.cause.body_id,
+                    err,
+                    predicate,
+                    obligation.param_env,
+                    obligation.cause.code(),
+                    &mut vec![],
+                    &mut Default::default(),
+                );
+            },
+        );
+    }
+
+    /// Reports that a cycle was detected which led to overflow and halts
+    /// compilation. This is equivalent to `report_overflow_obligation` except
+    /// that we can give a more helpful error message (and, in particular,
+    /// we do not suggest increasing the overflow limit, which is not
+    /// going to help).
+    fn report_overflow_obligation_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
+        let cycle = self.resolve_vars_if_possible(cycle.to_owned());
+        assert!(!cycle.is_empty());
+
+        debug!(?cycle, "report_overflow_error_cycle");
+
+        // The 'deepest' obligation is most likely to have a useful
+        // cause 'backtrace'
+        self.report_overflow_obligation(
+            cycle.iter().max_by_key(|p| p.recursion_depth).unwrap(),
+            false,
+        );
+    }
+
+    fn report_overflow_no_abort(
+        &self,
+        obligation: PredicateObligation<'tcx>,
+        suggest_increasing_limit: bool,
+    ) -> ErrorGuaranteed {
+        let obligation = self.resolve_vars_if_possible(obligation);
+        let mut err = self.build_overflow_error(
+            OverflowCause::TraitSolver(obligation.predicate),
+            obligation.cause.span,
+            suggest_increasing_limit,
+        );
+        self.note_obligation_cause(&mut err, &obligation);
+        self.point_at_returns_when_relevant(&mut err, &obligation);
+        err.emit()
+    }
+}

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -44,7 +44,7 @@ use std::assert_matches::debug_assert_matches;
 use std::borrow::Cow;
 use std::iter;
 
-use crate::error_reporting::traits::type_err_ctxt_ext::InferCtxtPrivExt;
+use crate::error_reporting::traits::fulfillment_errors::InferCtxtPrivExt;
 use crate::infer::InferCtxtExt as _;
 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
 use rustc_middle::ty::print::{
@@ -4623,6 +4623,132 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             );
         }
     }
+
+    #[instrument(level = "debug", skip_all)]
+    fn suggest_unsized_bound_if_applicable(
+        &self,
+        err: &mut Diag<'_>,
+        obligation: &PredicateObligation<'tcx>,
+    ) {
+        let ty::PredicateKind::Clause(ty::ClauseKind::Trait(pred)) =
+            obligation.predicate.kind().skip_binder()
+        else {
+            return;
+        };
+        let (ObligationCauseCode::WhereClause(item_def_id, span)
+        | ObligationCauseCode::WhereClauseInExpr(item_def_id, span, ..)) =
+            *obligation.cause.code().peel_derives()
+        else {
+            return;
+        };
+        if span.is_dummy() {
+            return;
+        }
+        debug!(?pred, ?item_def_id, ?span);
+
+        let (Some(node), true) = (
+            self.tcx.hir().get_if_local(item_def_id),
+            self.tcx.is_lang_item(pred.def_id(), LangItem::Sized),
+        ) else {
+            return;
+        };
+
+        let Some(generics) = node.generics() else {
+            return;
+        };
+        let sized_trait = self.tcx.lang_items().sized_trait();
+        debug!(?generics.params);
+        debug!(?generics.predicates);
+        let Some(param) = generics.params.iter().find(|param| param.span == span) else {
+            return;
+        };
+        // Check that none of the explicit trait bounds is `Sized`. Assume that an explicit
+        // `Sized` bound is there intentionally and we don't need to suggest relaxing it.
+        let explicitly_sized = generics
+            .bounds_for_param(param.def_id)
+            .flat_map(|bp| bp.bounds)
+            .any(|bound| bound.trait_ref().and_then(|tr| tr.trait_def_id()) == sized_trait);
+        if explicitly_sized {
+            return;
+        }
+        debug!(?param);
+        match node {
+            hir::Node::Item(
+                item @ hir::Item {
+                    // Only suggest indirection for uses of type parameters in ADTs.
+                    kind:
+                        hir::ItemKind::Enum(..) | hir::ItemKind::Struct(..) | hir::ItemKind::Union(..),
+                    ..
+                },
+            ) => {
+                if self.suggest_indirection_for_unsized(err, item, param) {
+                    return;
+                }
+            }
+            _ => {}
+        };
+
+        // Didn't add an indirection suggestion, so add a general suggestion to relax `Sized`.
+        let (span, separator, open_paren_sp) =
+            if let Some((s, open_paren_sp)) = generics.bounds_span_for_suggestions(param.def_id) {
+                (s, " +", open_paren_sp)
+            } else {
+                (param.name.ident().span.shrink_to_hi(), ":", None)
+            };
+
+        let mut suggs = vec![];
+        let suggestion = format!("{separator} ?Sized");
+
+        if let Some(open_paren_sp) = open_paren_sp {
+            suggs.push((open_paren_sp, "(".to_string()));
+            suggs.push((span, format!("){suggestion}")));
+        } else {
+            suggs.push((span, suggestion));
+        }
+
+        err.multipart_suggestion_verbose(
+            "consider relaxing the implicit `Sized` restriction",
+            suggs,
+            Applicability::MachineApplicable,
+        );
+    }
+
+    fn suggest_indirection_for_unsized(
+        &self,
+        err: &mut Diag<'_>,
+        item: &hir::Item<'tcx>,
+        param: &hir::GenericParam<'tcx>,
+    ) -> bool {
+        // Suggesting `T: ?Sized` is only valid in an ADT if `T` is only used in a
+        // borrow. `struct S<'a, T: ?Sized>(&'a T);` is valid, `struct S<T: ?Sized>(T);`
+        // is not. Look for invalid "bare" parameter uses, and suggest using indirection.
+        let mut visitor =
+            FindTypeParam { param: param.name.ident().name, invalid_spans: vec![], nested: false };
+        visitor.visit_item(item);
+        if visitor.invalid_spans.is_empty() {
+            return false;
+        }
+        let mut multispan: MultiSpan = param.span.into();
+        multispan.push_span_label(
+            param.span,
+            format!("this could be changed to `{}: ?Sized`...", param.name.ident()),
+        );
+        for sp in visitor.invalid_spans {
+            multispan.push_span_label(
+                sp,
+                format!("...if indirection were used here: `Box<{}>`", param.name.ident()),
+            );
+        }
+        err.span_help(
+            multispan,
+            format!(
+                "you could relax the implicit `Sized` bound on `{T}` if it were \
+                used through indirection like `&{T}` or `Box<{T}>`",
+                T = param.name.ident(),
+            ),
+        );
+        true
+    }
 }
 
 /// Add a hint to add a missing borrow or remove an unnecessary one.
@@ -5126,3 +5252,46 @@ fn get_deref_type_and_refs(mut ty: Ty<'_>) -> (Ty<'_>, Vec<hir::Mutability>) {
 
     (ty, refs)
 }
+
+/// Look for type `param` in an ADT being used only through a reference to confirm that suggesting
+/// `param: ?Sized` would be a valid constraint.
+struct FindTypeParam {
+    param: rustc_span::Symbol,
+    invalid_spans: Vec<Span>,
+    nested: bool,
+}
+
+impl<'v> Visitor<'v> for FindTypeParam {
+    fn visit_where_predicate(&mut self, _: &'v hir::WherePredicate<'v>) {
+        // Skip where-clauses, to avoid suggesting indirection for type parameters found there.
+    }
+
+    fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
+        // We collect the spans of all uses of the "bare" type param, like in `field: T` or
+        // `field: (T, T)` where we could make `T: ?Sized` while skipping cases that are known to be
+        // valid like `field: &'a T` or `field: *mut T` and cases that *might* have further `Sized`
+        // obligations like `Box<T>` and `Vec<T>`, but we perform no extra analysis for those cases
+        // and suggest `T: ?Sized` regardless of their obligations. This is fine because the errors
+        // in that case should make what happened clear enough.
+        match ty.kind {
+            hir::TyKind::Ptr(_) | hir::TyKind::Ref(..) | hir::TyKind::TraitObject(..) => {}
+            hir::TyKind::Path(hir::QPath::Resolved(None, path))
+                if path.segments.len() == 1 && path.segments[0].ident.name == self.param =>
+            {
+                if !self.nested {
+                    debug!(?ty, "FindTypeParam::visit_ty");
+                    self.invalid_spans.push(ty.span);
+                }
+            }
+            hir::TyKind::Path(_) => {
+                let prev = self.nested;
+                self.nested = true;
+                hir::intravisit::walk_ty(self, ty);
+                self.nested = prev;
+            }
+            _ => {
+                hir::intravisit::walk_ty(self, ty);
+            }
+        }
+    }
+}

compiler/rustc_trait_selection/src/solve/normalize.rs

@@ -1,7 +1,7 @@
 use std::fmt::Debug;
 use std::marker::PhantomData;
 
-use crate::error_reporting::traits::{OverflowCause, TypeErrCtxtExt};
+use crate::error_reporting::traits::{OverflowCause, TypeErrCtxtOverflowExt};
 use crate::traits::query::evaluate_obligation::InferCtxtExt;
 use crate::traits::{BoundVarReplacer, PlaceholderReplacer, ScrubbedTraitError};
 use rustc_data_structures::stack::ensure_sufficient_stack;

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -1,4 +1,4 @@
-use crate::error_reporting::traits::TypeErrCtxtExt;
+use crate::error_reporting::traits::TypeErrCtxtOverflowExt;
 use crate::infer::{InferCtxt, TyOrConstInferVar};
 use crate::traits::normalize::normalize_with_depth_to;
 use rustc_data_structures::captures::Captures;

compiler/rustc_trait_selection/src/traits/normalize.rs

@@ -3,7 +3,7 @@
 use super::SelectionContext;
 use super::{project, with_replaced_escaping_bound_vars, BoundVarReplacer, PlaceholderReplacer};
 use crate::error_reporting::traits::OverflowCause;
-use crate::error_reporting::traits::TypeErrCtxtExt;
+use crate::error_reporting::traits::TypeErrCtxtOverflowExt;
 use crate::solve::NextSolverError;
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_infer::infer::at::At;

compiler/rustc_trait_selection/src/traits/query/normalize.rs

@@ -3,7 +3,7 @@
 //! `normalize_canonicalized_projection_ty` query when it encounters projections.
 
 use crate::error_reporting::traits::OverflowCause;
-use crate::error_reporting::traits::TypeErrCtxtExt;
+use crate::error_reporting::traits::TypeErrCtxtOverflowExt;
 use crate::infer::at::At;
 use crate::infer::canonical::OriginalQueryValues;
 use crate::infer::{InferCtxt, InferOk};

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -18,7 +18,7 @@ use super::{
     TraitQueryMode,
 };
 
-use crate::error_reporting::traits::TypeErrCtxtExt;
+use crate::error_reporting::traits::TypeErrCtxtOverflowExt;
 use crate::infer::{InferCtxt, InferCtxtExt, InferOk, TypeFreshener};
 use crate::solve::InferCtxtSelectExt as _;
 use crate::traits::normalize::normalize_with_depth;

compiler/rustc_traits/src/codegen.rs

@@ -7,7 +7,7 @@ use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::bug;
 use rustc_middle::traits::CodegenObligationError;
 use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt};
-use rustc_trait_selection::error_reporting::traits::TypeErrCtxtExt;
+use rustc_trait_selection::error_reporting::traits::TypeErrCtxtOverflowExt;
 use rustc_trait_selection::traits::{
     ImplSource, Obligation, ObligationCause, ObligationCtxt, ScrubbedTraitError, SelectionContext,
     Unimplemented,

compiler/rustc_traits/src/normalize_projection_ty.rs

@@ -2,7 +2,7 @@ use rustc_infer::infer::canonical::{Canonical, QueryResponse};
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::query::Providers;
 use rustc_middle::ty::{ParamEnvAnd, TyCtxt};
-use rustc_trait_selection::error_reporting::traits::TypeErrCtxtExt;
+use rustc_trait_selection::error_reporting::traits::TypeErrCtxtOverflowExt;
 use rustc_trait_selection::infer::InferCtxtBuilderExt;
 use rustc_trait_selection::traits::query::{
     normalize::NormalizationResult, CanonicalAliasGoal, NoSolution,

library/core/src/error.rs

@@ -1008,8 +1008,15 @@ impl<'a> Iterator for Source<'a> {
         self.current = self.current.and_then(Error::source);
         current
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        if self.current.is_some() { (1, None) } else { (0, Some(0)) }
+    }
 }
 
+#[unstable(feature = "error_iter", issue = "58520")]
+impl<'a> crate::iter::FusedIterator for Source<'a> {}
+
 #[stable(feature = "error_by_ref", since = "1.51.0")]
 impl<'a, T: Error + ?Sized> Error for &'a T {
     #[allow(deprecated, deprecated_in_future)]

src/doc/rustdoc/src/advanced-features.md

@@ -80,7 +80,8 @@ pub struct BigX;
 Then, when looking for it through the `rustdoc` search, if you enter "x" or
 "big", search will show the `BigX` struct first.
 
-There are some limitations on the doc alias names though: you can't use `"` or whitespace.
+There are some limitations on the doc alias names though: they cannot contain quotes (`'`, `"`)
+or most whitespace. ASCII space is allowed if it does not start or end the alias.
 
 You can add multiple aliases at the same time by using a list:
 

tests/ui/asm/parse-error.stderr

@@ -371,47 +371,57 @@ LL | global_asm!("{}", label {});
 error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/parse-error.rs:39:37
    |
-LL |     let mut foo = 0;
-   |     ----------- help: consider using `const` instead of `let`: `const foo`
-...
 LL |         asm!("{}", options(), const foo);
    |                                     ^^^ non-constant value
+   |
+help: consider using `const` instead of `let`
+   |
+LL |     const foo: /* Type */ = 0;
+   |     ~~~~~    ++++++++++++
 
 error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/parse-error.rs:71:44
    |
-LL |     let mut foo = 0;
-   |     ----------- help: consider using `const` instead of `let`: `const foo`
-...
 LL |         asm!("{}", clobber_abi("C"), const foo);
    |                                            ^^^ non-constant value
+   |
+help: consider using `const` instead of `let`
+   |
+LL |     const foo: /* Type */ = 0;
+   |     ~~~~~    ++++++++++++
 
 error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/parse-error.rs:74:55
    |
-LL |     let mut foo = 0;
-   |     ----------- help: consider using `const` instead of `let`: `const foo`
-...
 LL |         asm!("{}", options(), clobber_abi("C"), const foo);
    |                                                       ^^^ non-constant value
+   |
+help: consider using `const` instead of `let`
+   |
+LL |     const foo: /* Type */ = 0;
+   |     ~~~~~    ++++++++++++
 
 error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/parse-error.rs:76:31
    |
-LL |     let mut foo = 0;
-   |     ----------- help: consider using `const` instead of `let`: `const foo`
-...
 LL |         asm!("{a}", a = const foo, a = const bar);
    |                               ^^^ non-constant value
+   |
+help: consider using `const` instead of `let`
+   |
+LL |     const foo: /* Type */ = 0;
+   |     ~~~~~    ++++++++++++
 
 error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/parse-error.rs:76:46
    |
-LL |     let mut bar = 0;
-   |     ----------- help: consider using `const` instead of `let`: `const bar`
-...
 LL |         asm!("{a}", a = const foo, a = const bar);
    |                                              ^^^ non-constant value
+   |
+help: consider using `const` instead of `let`
+   |
+LL |     const bar: /* Type */ = 0;
+   |     ~~~~~    ++++++++++++
 
 error: aborting due to 64 previous errors
 

tests/ui/asm/type-check-1.stderr

@@ -1,29 +1,35 @@
 error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/type-check-1.rs:41:26
    |
-LL |         let x = 0;
-   |         ----- help: consider using `const` instead of `let`: `const x`
-...
 LL |         asm!("{}", const x);
    |                          ^ non-constant value
+   |
+help: consider using `const` instead of `let`
+   |
+LL |         const x: /* Type */ = 0;
+   |         ~~~~~  ++++++++++++
 
 error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/type-check-1.rs:44:36
    |
-LL |         let x = 0;
-   |         ----- help: consider using `const` instead of `let`: `const x`
-...
 LL |         asm!("{}", const const_foo(x));
    |                                    ^ non-constant value
+   |
+help: consider using `const` instead of `let`
+   |
+LL |         const x: /* Type */ = 0;
+   |         ~~~~~  ++++++++++++
 
 error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/type-check-1.rs:47:36
    |
-LL |         let x = 0;
-   |         ----- help: consider using `const` instead of `let`: `const x`
-...
 LL |         asm!("{}", const const_bar(x));
    |                                    ^ non-constant value
+   |
+help: consider using `const` instead of `let`
+   |
+LL |         const x: /* Type */ = 0;
+   |         ~~~~~  ++++++++++++
 
 error: invalid `sym` operand
   --> $DIR/type-check-1.rs:49:24

tests/ui/asm/x86_64/x86_64_parse_error.stderr

@@ -15,29 +15,35 @@ LL |         asm!("{1}", in("eax") foo, const bar);
 error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/x86_64_parse_error.rs:13:46
    |
-LL |     let mut bar = 0;
-   |     ----------- help: consider using `const` instead of `let`: `const bar`
-...
 LL |         asm!("{a}", in("eax") foo, a = const bar);
    |                                              ^^^ non-constant value
+   |
+help: consider using `const` instead of `let`
+   |
+LL |     const bar: /* Type */ = 0;
+   |     ~~~~~    ++++++++++++
 
 error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/x86_64_parse_error.rs:15:46
    |
-LL |     let mut bar = 0;
-   |     ----------- help: consider using `const` instead of `let`: `const bar`
-...
 LL |         asm!("{a}", in("eax") foo, a = const bar);
    |                                              ^^^ non-constant value
+   |
+help: consider using `const` instead of `let`
+   |
+LL |     const bar: /* Type */ = 0;
+   |     ~~~~~    ++++++++++++
 
 error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/x86_64_parse_error.rs:17:42
    |
-LL |     let mut bar = 0;
-   |     ----------- help: consider using `const` instead of `let`: `const bar`
-...
 LL |         asm!("{1}", in("eax") foo, const bar);
    |                                          ^^^ non-constant value
+   |
+help: consider using `const` instead of `let`
+   |
+LL |     const bar: /* Type */ = 0;
+   |     ~~~~~    ++++++++++++
 
 error: aborting due to 5 previous errors
 

tests/ui/associated-types/associated-types-eq-expr-path.rs

@@ -7,11 +7,12 @@ trait Foo {
 
 impl Foo for isize {
     type A = usize;
-    fn bar() -> isize { 42 }
+    fn bar() -> isize {
+        42
+    }
 }
 
 pub fn main() {
     let x: isize = Foo::<A = usize>::bar();
     //~^ ERROR associated item constraints are not allowed here
-    //~| ERROR cannot call
 }

tests/ui/associated-types/associated-types-eq-expr-path.stderr

@@ -1,25 +1,9 @@
 error[E0229]: associated item constraints are not allowed here
-  --> $DIR/associated-types-eq-expr-path.rs:14:26
+  --> $DIR/associated-types-eq-expr-path.rs:16:26
    |
 LL |     let x: isize = Foo::<A = usize>::bar();
    |                          ^^^^^^^^^ associated item constraint not allowed here
 
-error[E0790]: cannot call associated function on trait without specifying the corresponding `impl` type
-  --> $DIR/associated-types-eq-expr-path.rs:14:20
-   |
-LL |     fn bar() -> isize;
-   |     ------------------ `Foo::bar` defined here
-...
-LL |     let x: isize = Foo::<A = usize>::bar();
-   |                    ^^^^^^^^^^^^^^^^^^^^^^^ cannot call associated function of trait
-   |
-help: use the fully-qualified path to the only available implementation
-   |
-LL -     let x: isize = Foo::<A = usize>::bar();
-LL +     let x: isize = <isize as Foo<A = usize>>::bar();
-   |
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
-Some errors have detailed explanations: E0229, E0790.
-For more information about an error, try `rustc --explain E0229`.
+For more information about this error, try `rustc --explain E0229`.

tests/ui/const-generics/assoc_const_as_type_argument.rs

@@ -8,7 +8,6 @@ fn foo<T: Trait>() {
     bar::<<T as Trait>::ASSOC>();
     //~^ ERROR: expected associated type, found associated constant `Trait::ASSOC`
     //~| ERROR: unresolved item provided when a constant was expected
-    //~| ERROR type annotations needed
 }
 
 fn main() {}

tests/ui/const-generics/assoc_const_as_type_argument.stderr

@@ -15,19 +15,7 @@ help: if this generic argument was intended as a const parameter, surround it wi
 LL |     bar::<{ <T as Trait>::ASSOC }>();
    |           +                     +
 
-error[E0284]: type annotations needed
-  --> $DIR/assoc_const_as_type_argument.rs:8:5
-   |
-LL |     bar::<<T as Trait>::ASSOC>();
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer the value of the const parameter `N` declared on the function `bar`
-   |
-note: required by a const generic parameter in `bar`
-  --> $DIR/assoc_const_as_type_argument.rs:5:8
-   |
-LL | fn bar<const N: usize>() {}
-   |        ^^^^^^^^^^^^^^ required by this const generic parameter in `bar`
-
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0284, E0575, E0747.
-For more information about an error, try `rustc --explain E0284`.
+Some errors have detailed explanations: E0575, E0747.
+For more information about an error, try `rustc --explain E0575`.