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, 2 deletions

diff --git a/src/librustc_data_structures/macros.rs b/src/librustc_data_structures/macros.rs
index b851263aaf9..6e7a8e98853 100644
--- a/src/librustc_data_structures/macros.rs
+++ b/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>()];