Re-enable ConstArgKind::Path lowering by default

...and remove the `const_arg_path` feature gate as a result. It was only
a stopgap measure to fix the regression that the new lowering introduced
(which should now be fixed by this PR).

Author: camelid

compiler/rustc_ast_lowering/src/asm.rs

@@ -220,9 +220,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             let parent_def_id = self.current_def_id_parent;
                             let node_id = self.next_node_id();
                             // HACK(min_generic_const_args): see lower_anon_const
-                            if !self.tcx.features().const_arg_path
-                                || !expr.is_potential_trivial_const_arg()
-                            {
+                            if !expr.is_potential_trivial_const_arg() {
                                 self.create_def(
                                     parent_def_id,
                                     node_id,

compiler/rustc_ast_lowering/src/expr.rs

@@ -387,7 +387,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let node_id = self.next_node_id();
 
                 // HACK(min_generic_const_args): see lower_anon_const
-                if !self.tcx.features().const_arg_path || !arg.is_potential_trivial_const_arg() {
+                if !arg.is_potential_trivial_const_arg() {
                     // Add a definition for the in-band const def.
                     self.create_def(parent_def_id, node_id, kw::Empty, DefKind::AnonConst, f.span);
                 }

compiler/rustc_ast_lowering/src/lib.rs

@@ -2335,7 +2335,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         span: Span,
     ) -> &'hir hir::ConstArg<'hir> {
         let ct_kind = match res {
-            Res::Def(DefKind::ConstParam, _) if self.tcx.features().const_arg_path => {
+            Res::Def(DefKind::ConstParam, _) => {
                 let qpath = self.lower_qpath(
                     ty_id,
                     &None,
@@ -2410,8 +2410,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             self.resolver.get_partial_res(expr.id).and_then(|partial_res| partial_res.full_res());
         debug!("res={:?}", maybe_res);
         // FIXME(min_generic_const_args): for now we only lower params to ConstArgKind::Path
-        if self.tcx.features().const_arg_path
-            && let Some(res) = maybe_res
+        if let Some(res) = maybe_res
             && let Res::Def(DefKind::ConstParam, _) = res
             && let ExprKind::Path(qself, path) = &expr.kind
         {
@@ -2442,7 +2441,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     /// See [`hir::ConstArg`] for when to use this function vs
     /// [`Self::lower_anon_const_to_const_arg`].
     fn lower_anon_const_to_anon_const(&mut self, c: &AnonConst) -> &'hir hir::AnonConst {
-        if self.tcx.features().const_arg_path && c.value.is_potential_trivial_const_arg() {
+        if c.value.is_potential_trivial_const_arg() {
             // HACK(min_generic_const_args): see DefCollector::visit_anon_const
             // Over there, we guess if this is a bare param and only create a def if
             // we think it's not. However we may can guess wrong (see there for example)

compiler/rustc_feature/src/unstable.rs

@@ -193,8 +193,6 @@ declare_features! (
     (unstable, anonymous_lifetime_in_impl_trait, "1.63.0", None),
     /// Allows identifying the `compiler_builtins` crate.
     (internal, compiler_builtins, "1.13.0", None),
-    /// Gating for a new desugaring of const arguments of usages of const parameters
-    (internal, const_arg_path, "1.81.0", None),
     /// Allows writing custom MIR
     (internal, custom_mir, "1.65.0", None),
     /// Outputs useful `assert!` messages

compiler/rustc_middle/src/ty/consts.rs

@@ -295,20 +295,12 @@ impl<'tcx> Const<'tcx> {
             _ => expr,
         };
 
-        if let hir::ExprKind::Path(
-            qpath @ hir::QPath::Resolved(
-                _,
-                &hir::Path { res: Res::Def(DefKind::ConstParam, _), .. },
-            ),
-        ) = expr.kind
+        if let hir::ExprKind::Path(hir::QPath::Resolved(
+            _,
+            &hir::Path { res: Res::Def(DefKind::ConstParam, _), .. },
+        )) = expr.kind
         {
-            if tcx.features().const_arg_path {
-                span_bug!(
-                    expr.span,
-                    "try_from_lit: received const param which shouldn't be possible"
-                );
-            }
-            return Some(Const::from_param(tcx, qpath, expr.hir_id));
+            span_bug!(expr.span, "try_from_lit: received const param which shouldn't be possible");
         };
 
         let lit_input = match expr.kind {

compiler/rustc_resolve/src/def_collector.rs

@@ -347,22 +347,20 @@ impl<'a, 'b, 'tcx> visit::Visitor<'a> for DefCollector<'a, 'b, 'tcx> {
     }
 
     fn visit_anon_const(&mut self, constant: &'a AnonConst) {
-        if self.resolver.tcx.features().const_arg_path {
-            // HACK(min_generic_const_args): don't create defs for anon consts if we think they will
-            // later be turned into ConstArgKind::Path's. because this is before resolve is done, we
-            // may accidentally identify a construction of a unit struct as a param and not create a
-            // def. we'll then create a def later in ast lowering in this case. the parent of nested
-            // items will be messed up, but that's ok because there can't be any if we're just looking
-            // for bare idents.
-
-            if matches!(constant.value.maybe_unwrap_block().kind, ExprKind::MacCall(..)) {
-                // See self.pending_anon_const_info for explanation
-                self.pending_anon_const_info =
-                    Some(PendingAnonConstInfo { id: constant.id, span: constant.value.span });
-                return visit::walk_anon_const(self, constant);
-            } else if constant.value.is_potential_trivial_const_arg() {
-                return visit::walk_anon_const(self, constant);
-            }
+        // HACK(min_generic_const_args): don't create defs for anon consts if we think they will
+        // later be turned into ConstArgKind::Path's. because this is before resolve is done, we
+        // may accidentally identify a construction of a unit struct as a param and not create a
+        // def. we'll then create a def later in ast lowering in this case. the parent of nested
+        // items will be messed up, but that's ok because there can't be any if we're just looking
+        // for bare idents.
+
+        if matches!(constant.value.maybe_unwrap_block().kind, ExprKind::MacCall(..)) {
+            // See self.pending_anon_const_info for explanation
+            self.pending_anon_const_info =
+                Some(PendingAnonConstInfo { id: constant.id, span: constant.value.span });
+            return visit::walk_anon_const(self, constant);
+        } else if constant.value.is_potential_trivial_const_arg() {
+            return visit::walk_anon_const(self, constant);
         }
 
         let def = self.create_def(constant.id, kw::Empty, DefKind::AnonConst, constant.value.span);

compiler/rustc_span/src/symbol.rs

@@ -599,7 +599,6 @@ symbols! {
         conservative_impl_trait,
         console,
         const_allocate,
-        const_arg_path,
         const_async_blocks,
         const_closures,
         const_compare_raw_pointers,

tests/ui-fulldeps/stable-mir/check_instance.rs

@@ -35,7 +35,7 @@ fn test_stable_mir() -> ControlFlow<()> {
     // Get all items and split generic vs monomorphic items.
     let (generic, mono): (Vec<_>, Vec<_>) =
         items.into_iter().partition(|item| item.requires_monomorphization());
-    assert_eq!(mono.len(), 4, "Expected 3 mono functions");
+    assert_eq!(mono.len(), 3, "Expected 3 mono functions");
     assert_eq!(generic.len(), 2, "Expected 2 generic functions");
 
     // For all monomorphic items, get the correspondent instances.

tests/ui/coherence/negative-coherence/generic_const_type_mismatch.rs

@@ -5,9 +5,7 @@
 #![feature(with_negative_coherence)]
 trait Trait {}
 impl<const N: u8> Trait for [(); N] {}
-//~^ ERROR: mismatched types
 impl<const N: i8> Trait for [(); N] {}
 //~^ ERROR: conflicting implementations of trait `Trait`
-//~| ERROR: mismatched types
 
 fn main() {}

tests/ui/coherence/negative-coherence/generic_const_type_mismatch.stderr

@@ -1,25 +1,11 @@
 error[E0119]: conflicting implementations of trait `Trait` for type `[(); _]`
-  --> $DIR/generic_const_type_mismatch.rs:9:1
+  --> $DIR/generic_const_type_mismatch.rs:8:1
    |
 LL | impl<const N: u8> Trait for [(); N] {}
    | ----------------------------------- first implementation here
-LL |
 LL | impl<const N: i8> Trait for [(); N] {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `[(); _]`
 
-error[E0308]: mismatched types
-  --> $DIR/generic_const_type_mismatch.rs:7:34
-   |
-LL | impl<const N: u8> Trait for [(); N] {}
-   |                                  ^ expected `usize`, found `u8`
-
-error[E0308]: mismatched types
-  --> $DIR/generic_const_type_mismatch.rs:9:34
-   |
-LL | impl<const N: i8> Trait for [(); N] {}
-   |                                  ^ expected `usize`, found `i8`
-
-error: aborting due to 3 previous errors
+error: aborting due to 1 previous error
 
-Some errors have detailed explanations: E0119, E0308.
-For more information about an error, try `rustc --explain E0119`.
+For more information about this error, try `rustc --explain E0119`.