Auto merge of #61347 - Centril:stabilize-underscore_const_names, r=petrochenkov

Stabilize underscore_const_names in 1.37.0

You are now permitted to write:

```rust
const _: $type_expression = $term_expression;
```

That is, we change the [grammar of items](https://github.com/rust-lang-nursery/wg-grammar/blob/9d1984d7ae8d6576f943566539a31a5800644c57/grammar/item.lyg#L3-L42), as written in [the *`.lyg`* notation](https://github.com/rust-lang/gll/tree/263bf161dad903e67aa65fc591ced3cab18afa2a#grammar), from:

```java
Item = attrs:OuterAttr* vis:Vis? kind:ItemKind;
ItemKind =
  | ...
  | Const:{ "const" name:IDENT ":" ty:Type "=" value:Expr ";" }
  | ...
  ;
```

into:

```java
Item = attrs:OuterAttr* vis:Vis? kind:ItemKind;
ItemKind =
  | ...
  | Const:{ "const" name:IdentOrUnderscore ":" ty:Type "=" value:Expr ";" }
  | ...
  ;

IdentOrUnderscore =
  | Named:IDENT
  | NoName:"_"
  ;
```

r? @petrochenkov

1 files changed, 3 insertions, 11 deletions

diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 1d8f68ec63a..7f80e2099f6 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -25,7 +25,7 @@ use crate::source_map::Spanned;
 use crate::edition::{ALL_EDITIONS, Edition};
 use crate::visit::{self, FnKind, Visitor};
 use crate::parse::{token, ParseSess};
-use crate::symbol::{Symbol, kw, sym};
+use crate::symbol::{Symbol, sym};
 use crate::tokenstream::TokenTree;
 
 use errors::{Applicability, DiagnosticBuilder, Handler};
@@ -526,9 +526,6 @@ declare_features! (
     // Allows `impl Trait` in bindings (`let`, `const`, `static`).
     (active, impl_trait_in_bindings, "1.30.0", Some(34511), None),
 
-    // Allows `const _: TYPE = VALUE`.
-    (active, underscore_const_names, "1.31.0", Some(54912), None),
-
     // Allows using `reason` in lint attributes and the `#[expect(lint)]` lint check.
     (active, lint_reasons, "1.31.0", Some(54503), None),
 
@@ -851,6 +848,8 @@ declare_features! (
     // Allows using `#[repr(align(X))]` on enums with equivalent semantics
     // to wrapping an enum in a wrapper struct with `#[repr(align(X))]`.
     (accepted, repr_align_enum, "1.37.0", Some(57996), None),
+    // Allows `const _: TYPE = VALUE`.
+    (accepted, underscore_const_names, "1.37.0", Some(54912), None),
 
     // -------------------------------------------------------------------------
     // feature-group-end: accepted features
@@ -2000,13 +1999,6 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 
     fn visit_item(&mut self, i: &'a ast::Item) {
         match i.node {
-            ast::ItemKind::Const(_,_) => {
-                if i.ident.name == kw::Underscore {
-                    gate_feature_post!(&self, underscore_const_names, i.span,
-                                        "naming constants with `_` is unstable");
-                }
-            }
-
             ast::ItemKind::ForeignMod(ref foreign_module) => {
                 self.check_abi(foreign_module.abi, i.span);
             }