Merge associated_const_equality feature gate into MGCA

This removes `associated_const_equality` as a separate feature gate and makes it part of `min_generic_const_args` (mgca).

Key changes:
  - Remove `associated_const_equality` from unstable features, add to removed
  - Update all test files to use `min_generic_const_args` instead
  - Preserve the original "associated const equality is incomplete" error message by specially handling `sym::associated_const_equality` spans in `feature_gate.rs`
  - Rename FIXME(associated_const_equality) to FIXME(mgca)

Author: AprilNEA

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -505,7 +505,6 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
         half_open_range_patterns_in_slices,
         "half-open range patterns in slices are unstable"
     );
-    gate_all!(associated_const_equality, "associated const equality is incomplete");
     gate_all!(yeet_expr, "`do yeet` expression is experimental");
     gate_all!(const_closures, "const closures are experimental");
     gate_all!(builtin_syntax, "`builtin #` syntax is unstable");
@@ -518,6 +517,23 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(postfix_match, "postfix match is experimental");
     gate_all!(mut_ref, "mutable by-reference bindings are experimental");
     gate_all!(min_generic_const_args, "unbraced const blocks as const args are experimental");
+    // associated_const_equality is stabilized as part of min_generic_const_args
+    if let Some(spans) = spans.get(&sym::associated_const_equality) {
+        for span in spans {
+            if !visitor.features.min_generic_const_args()
+                && !span.allows_unstable(sym::min_generic_const_args)
+            {
+                #[allow(rustc::untranslatable_diagnostic)]
+                feature_err(
+                    &visitor.sess,
+                    sym::min_generic_const_args,
+                    *span,
+                    "associated const equality is incomplete",
+                )
+                .emit();
+            }
+        }
+    }
     gate_all!(global_registration, "global registration is experimental");
     gate_all!(return_type_notation, "return type notation is experimental");
     gate_all!(pin_ergonomics, "pinned reference syntax is experimental");

compiler/rustc_codegen_ssa/src/debuginfo/type_names.rs

@@ -260,7 +260,7 @@ fn push_debuginfo_type_name<'tcx>(
                     .map(|bound| {
                         let ExistentialProjection { def_id: item_def_id, term, .. } =
                             tcx.instantiate_bound_regions_with_erased(bound);
-                        // FIXME(associated_const_equality): allow for consts here
+                        // FIXME(mgca): allow for consts here
                         (item_def_id, term.expect_type())
                     })
                     .collect();

compiler/rustc_feature/src/removed.rs

@@ -60,6 +60,9 @@ declare_features! (
     (removed, allocator, "1.0.0", None, None),
     /// Allows a test to fail without failing the whole suite.
     (removed, allow_fail, "1.60.0", Some(46488), Some("removed due to no clear use cases"), 93416),
+    /// Allows users to enforce equality of associated constants `TraitImpl<AssocConst=3>`.
+    (removed, associated_const_equality, "CURRENT_RUSTC_VERSION", Some(92827),
+     Some("merged into `min_generic_const_args`")),
     (removed, await_macro, "1.38.0", Some(50547),
      Some("subsumed by `.await` syntax"), 62293),
     /// Allows using the `box $expr` syntax.

compiler/rustc_feature/src/unstable.rs

@@ -363,8 +363,6 @@ declare_features! (
     (unstable, asm_goto_with_outputs, "1.85.0", Some(119364)),
     /// Allows the `may_unwind` option in inline assembly.
     (unstable, asm_unwind, "1.58.0", Some(93334)),
-    /// Allows users to enforce equality of associated constants `TraitImpl<AssocConst=3>`.
-    (unstable, associated_const_equality, "1.58.0", Some(92827)),
     /// Allows associated type defaults.
     (unstable, associated_type_defaults, "1.2.0", Some(29661)),
     /// Allows implementing `AsyncDrop`.

compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs

@@ -610,9 +610,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                                 AttributeKind::TypeConst(_)
                             )
                         {
-                            if tcx.features().min_generic_const_args()
-                                || tcx.features().associated_const_equality()
-                            {
+                            if tcx.features().min_generic_const_args() {
                                 let mut err = self.dcx().struct_span_err(
                                     constraint.span,
                                     "use of trait associated const without `#[type_const]`",

compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_trait.rs

@@ -239,7 +239,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         // `trait_object_dummy_self`, so check for that.
                         let references_self = match pred.skip_binder().term.kind() {
                             ty::TermKind::Ty(ty) => ty.walk().any(|arg| arg == dummy_self.into()),
-                            // FIXME(associated_const_equality): We should walk the const instead of not doing anything
+                            // FIXME(mgca): We should walk the const instead of not doing anything
                             ty::TermKind::Const(_) => false,
                         };
 

compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

@@ -359,12 +359,12 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             None
         };
 
-        // FIXME(associated_const_equality): This has quite a few false positives and negatives.
+        // FIXME(mgca): This has quite a few false positives and negatives.
         let wrap_in_braces_sugg = if let Some(constraint) = constraint
             && let Some(hir_ty) = constraint.ty()
             && let ty = self.lower_ty(hir_ty)
             && (ty.is_enum() || ty.references_error())
-            && tcx.features().associated_const_equality()
+            && tcx.features().min_generic_const_args()
         {
             Some(errors::AssocKindMismatchWrapInBracesSugg {
                 lo: hir_ty.span.shrink_to_lo(),

compiler/rustc_middle/src/ty/context.rs

@@ -907,10 +907,6 @@ impl<'tcx> rustc_type_ir::inherent::Features<TyCtxt<'tcx>> for &'tcx rustc_featu
         self.coroutine_clone()
     }
 
-    fn associated_const_equality(self) -> bool {
-        self.associated_const_equality()
-    }
-
     fn feature_bound_holds_in_crate(self, symbol: Symbol) -> bool {
         // We don't consider feature bounds to hold in the crate when `staged_api` feature is
         // enabled, even if it is enabled through `#[feature]`.

compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs

@@ -868,7 +868,7 @@ where
             .map(|(pred, _)| pred),
     ));
 
-    // FIXME(associated_const_equality): Also add associated consts to
+    // FIXME(mgca): Also add associated consts to
     // the requirements here.
     for associated_type_def_id in cx.associated_type_def_ids(trait_ref.def_id) {
         // associated types that require `Self: Sized` do not show up in the built-in

compiler/rustc_type_ir/src/inherent.rs

@@ -646,8 +646,6 @@ pub trait Features<I: Interner>: Copy {
 
     fn coroutine_clone(self) -> bool;
 
-    fn associated_const_equality(self) -> bool;
-
     fn feature_bound_holds_in_crate(self, symbol: I::Symbol) -> bool;
 }