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

Author: bors

src/librustc_data_structures/macros.rs

@@ -1,6 +1,7 @@
 /// A simple static assertion macro.
 #[macro_export]
-#[allow_internal_unstable(type_ascription, underscore_const_names)]
+#[cfg_attr(bootstrap, allow_internal_unstable(type_ascription, underscore_const_names))]
+#[cfg_attr(not(bootstrap), allow_internal_unstable(type_ascription))]
 macro_rules! static_assert {
     ($test:expr) => {
         // Use the bool to access an array such that if the bool is false, the access
@@ -12,7 +13,7 @@ macro_rules! static_assert {
 
 /// Type size assertion. The first argument is a type and the second argument is its expected size.
 #[macro_export]
-#[allow_internal_unstable(underscore_const_names)]
+#[cfg_attr(bootstrap, allow_internal_unstable(underscore_const_names))]
 macro_rules! static_assert_size {
     ($ty:ty, $size:expr) => {
         const _: [(); $size] = [(); ::std::mem::size_of::<$ty>()];

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);
             }

src/test/ui/consts/const_short_circuit.rs

@@ -1,5 +1,3 @@
-#![feature(underscore_const_names)]
-
 const _: bool = false && false;
 const _: bool = true && false;
 const _: bool = {

src/test/ui/consts/const_short_circuit.stderr

@@ -1,23 +1,23 @@
 error: new features like let bindings are not permitted in constants which also use short circuiting operators
-  --> $DIR/const_short_circuit.rs:6:9
+  --> $DIR/const_short_circuit.rs:4:9
    |
 LL |     let mut x = true && false;
    |         ^^^^^
    |
 note: use of `&&` operator here does not actually short circuit due to the const evaluator presently not being able to do control flow. See https://github.com/rust-lang/rust/issues/49146 for more information.
-  --> $DIR/const_short_circuit.rs:6:22
+  --> $DIR/const_short_circuit.rs:4:22
    |
 LL |     let mut x = true && false;
    |                      ^^
 
 error: new features like let bindings are not permitted in constants which also use short circuiting operators
-  --> $DIR/const_short_circuit.rs:11:9
+  --> $DIR/const_short_circuit.rs:9:9
    |
 LL |     let x = true && false;
    |         ^
    |
 note: use of `&&` operator here does not actually short circuit due to the const evaluator presently not being able to do control flow. See https://github.com/rust-lang/rust/issues/49146 for more information.
-  --> $DIR/const_short_circuit.rs:11:18
+  --> $DIR/const_short_circuit.rs:9:18
    |
 LL |     let x = true && false;
    |                  ^^

src/test/ui/underscore_const_names.rs renamed to src/test/ui/consts/underscore_const_names.rs

@@ -1,9 +1,9 @@
 // compile-pass
 
-#![feature(underscore_const_names)]
+#![deny(unused)]
 
 trait Trt {}
-struct Str {}
+pub struct Str {}
 impl Trt for Str {}
 
 macro_rules! check_impl {
@@ -17,7 +17,6 @@ macro_rules! check_impl {
     }
 }
 
-#[deny(unused)]
 const _ : () = ();
 
 const _ : i32 = 42;

src/test/ui/feature-gates/feature-gate-underscore_const_names.rs

@@ -0,0 +1,30 @@
+// Test that various non-const items and associated consts do not permit `_` as a name.
+
+// Associated `const`s:
+
+pub trait A {
+    const _: () = (); //~ ERROR expected identifier, found reserved identifier `_`
+}
+impl A for () {
+    const _: () = (); //~ ERROR expected identifier, found reserved identifier `_`
+}
+impl dyn A {
+    const _: () = (); //~ ERROR expected identifier, found reserved identifier `_`
+}
+
+// Other kinds of items:
+
+static _: () = (); //~ ERROR expected identifier, found reserved identifier `_`
+struct _(); //~ ERROR expected identifier, found reserved identifier `_`
+enum _ {} //~ ERROR expected identifier, found reserved identifier `_`
+fn _() {} //~ ERROR expected identifier, found reserved identifier `_`
+mod _ {} //~ ERROR expected identifier, found reserved identifier `_`
+type _ = (); //~ ERROR expected identifier, found reserved identifier `_`
+use _; //~ ERROR expected identifier, found reserved identifier `_`
+use _ as g; //~ ERROR expected identifier, found reserved identifier `_`
+trait _ {} //~ ERROR expected identifier, found reserved identifier `_`
+trait _ = Copy; //~ ERROR expected identifier, found reserved identifier `_`
+macro_rules! _ { () => {} } //~ ERROR expected identifier, found reserved identifier `_`
+union _ { f: u8 } //~ ERROR expected one of `!` or `::`, found `_`
+
+fn main() {}