Fix diagnostic regression for associated const equality

Restore the "associated const equality is incomplete" error message by
gating associated const equality spans with
`sym::associated_const_equality` and handling them specially in
feature_gate.rs to emit the correct diagnostic.

Author: AprilNEA

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -517,6 +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_parse/src/parser/path.rs

@@ -797,7 +797,7 @@ impl<'a> Parser<'a> {
         let term = match arg {
             Some(GenericArg::Type(ty)) => ty.into(),
             Some(GenericArg::Const(c)) => {
-                self.psess.gated_spans.gate(sym::min_generic_const_args, span);
+                self.psess.gated_spans.gate(sym::associated_const_equality, span);
                 c.into()
             }
             Some(GenericArg::Lifetime(lt)) => {

tests/ui/associated-type-bounds/issue-99828.rs

@@ -1,7 +1,7 @@
 fn get_iter(vec: &[i32]) -> impl Iterator<Item = {}> + '_ {
     //~^ ERROR expected type, found constant
     //~| ERROR expected type, found constant
-    //~| ERROR unbraced const blocks as const args are experimental
+    //~| ERROR associated const equality is incomplete
     vec.iter()
 }
 

tests/ui/associated-type-bounds/issue-99828.stderr

@@ -1,4 +1,4 @@
-error[E0658]: unbraced const blocks as const args are experimental
+error[E0658]: associated const equality is incomplete
   --> $DIR/issue-99828.rs:1:43
    |
 LL | fn get_iter(vec: &[i32]) -> impl Iterator<Item = {}> + '_ {

tests/ui/associated-types/associated-types-eq-2.rs

@@ -75,15 +75,15 @@ trait Tr3<const N: i32, T2, T3> {
 // our suggestion spans are kosher in the face of such formatting)
 impl Tr3<N
 //~^ ERROR associated item constraints are not allowed here
-//~| ERROR unbraced const blocks as const args are experimental
+//~| ERROR associated const equality is incomplete
 = 42, T2 = Qux, T3 = usize> for Bar {
 }
 
 // Test for when equality constraint's ident
 // matches the const param's ident but has a different case
 impl Tr3<n = 42, T2 = Qux, T3 = usize> for Qux {
 //~^ ERROR associated item constraints are not allowed here
-//~| ERROR unbraced const blocks as const args are experimental
+//~| ERROR associated const equality is incomplete
 //~| ERROR trait takes 3 generic arguments but 0 generic arguments were supplied
 }
 
@@ -97,15 +97,15 @@ impl Tr3<N = u32, T2 = Qux, T3 = usize> for Bar {
 // matches a type param ident but the constraint is a const arg
 impl Tr3<42, T2 = 42, T3 = usize> for Bar {
 //~^ ERROR associated item constraints are not allowed here
-//~| ERROR unbraced const blocks as const args are experimental
+//~| ERROR associated const equality is incomplete
 //~| ERROR trait takes 3 generic arguments but 1 generic argument was supplied
 }
 
 // Test for when equality constraint's ident
 // matches none of the param idents
 impl Tr3<X = 42, Y = Qux, Z = usize> for Bar {
 //~^ ERROR associated item constraints are not allowed here
-//~| ERROR unbraced const blocks as const args are experimental
+//~| ERROR associated const equality is incomplete
 //~| ERROR trait takes 3 generic arguments but 0 generic arguments were supplied
 }
 

tests/ui/associated-types/associated-types-eq-2.stderr

@@ -1,4 +1,4 @@
-error[E0658]: unbraced const blocks as const args are experimental
+error[E0658]: associated const equality is incomplete
   --> $DIR/associated-types-eq-2.rs:76:10
    |
 LL |   impl Tr3<N
@@ -11,7 +11,7 @@ LL | | = 42, T2 = Qux, T3 = usize> for Bar {
    = help: add `#![feature(min_generic_const_args)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error[E0658]: unbraced const blocks as const args are experimental
+error[E0658]: associated const equality is incomplete
   --> $DIR/associated-types-eq-2.rs:84:10
    |
 LL | impl Tr3<n = 42, T2 = Qux, T3 = usize> for Qux {
@@ -21,7 +21,7 @@ LL | impl Tr3<n = 42, T2 = Qux, T3 = usize> for Qux {
    = help: add `#![feature(min_generic_const_args)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error[E0658]: unbraced const blocks as const args are experimental
+error[E0658]: associated const equality is incomplete
   --> $DIR/associated-types-eq-2.rs:98:14
    |
 LL | impl Tr3<42, T2 = 42, T3 = usize> for Bar {
@@ -31,7 +31,7 @@ LL | impl Tr3<42, T2 = 42, T3 = usize> for Bar {
    = help: add `#![feature(min_generic_const_args)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error[E0658]: unbraced const blocks as const args are experimental
+error[E0658]: associated const equality is incomplete
   --> $DIR/associated-types-eq-2.rs:106:10
    |
 LL | impl Tr3<X = 42, Y = Qux, Z = usize> for Bar {

tests/ui/const-generics/parser-error-recovery/issue-89013-no-kw.rs

@@ -8,7 +8,7 @@ const T: usize = 42;
 
 impl Foo<N = 3> for Bar {
 //~^ ERROR associated item constraints are not allowed here
-//~| ERROR unbraced const blocks as const args are experimental
+//~| ERROR associated const equality is incomplete
     fn do_x(&self) -> [u8; 3] {
         [0u8; 3]
     }

tests/ui/const-generics/parser-error-recovery/issue-89013-no-kw.stderr

@@ -1,4 +1,4 @@
-error[E0658]: unbraced const blocks as const args are experimental
+error[E0658]: associated const equality is incomplete
   --> $DIR/issue-89013-no-kw.rs:9:10
    |
 LL | impl Foo<N = 3> for Bar {

tests/ui/const-generics/parser-error-recovery/issue-89013.rs

@@ -9,7 +9,7 @@ const T: usize = 42;
 impl Foo<N = const 3> for Bar {
 //~^ ERROR expected lifetime, type, or constant, found keyword `const`
 //~| ERROR associated item constraints are not allowed here
-//~| ERROR unbraced const blocks as const args are experimental
+//~| ERROR associated const equality is incomplete
     fn do_x(&self) -> [u8; 3] {
         [0u8; 3]
     }

tests/ui/const-generics/parser-error-recovery/issue-89013.stderr

@@ -10,7 +10,7 @@ LL - impl Foo<N = const 3> for Bar {
 LL + impl Foo<N = 3> for Bar {
    |
 
-error[E0658]: unbraced const blocks as const args are experimental
+error[E0658]: associated const equality is incomplete
   --> $DIR/issue-89013.rs:9:10
    |
 LL | impl Foo<N = const 3> for Bar {