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

13 files changed, 134 insertions, 80 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>()];
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);
             }
diff --git a/src/test/ui/consts/const_short_circuit.rs b/src/test/ui/consts/const_short_circuit.rs
index 1e7b7ed3193..87b14a11178 100644
--- a/src/test/ui/consts/const_short_circuit.rs
+++ b/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 = {
diff --git a/src/test/ui/consts/const_short_circuit.stderr b/src/test/ui/consts/const_short_circuit.stderr
index a67bb0b1b6d..0a6536c8837 100644
--- a/src/test/ui/consts/const_short_circuit.stderr
+++ b/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;
    |                  ^^
diff --git a/src/test/ui/underscore_const_names.rs b/src/test/ui/consts/underscore_const_names.rs
index 1db022e8862..8d57e5074f1 100644
--- a/src/test/ui/underscore_const_names.rs
+++ b/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;
diff --git a/src/test/ui/feature-gates/feature-gate-underscore_const_names.rs b/src/test/ui/feature-gates/feature-gate-underscore_const_names.rs
deleted file mode 100644
index 6b97c24a47e..00000000000
--- a/src/test/ui/feature-gates/feature-gate-underscore_const_names.rs
+++ /dev/null
@@ -1,14 +0,0 @@
-trait Trt {}
-struct Str {}
-
-impl Trt for Str {}
-
-const _ : () = {
-//~^ ERROR is unstable
-    use std::marker::PhantomData;
-    struct ImplementsTrait<T: Trt>(PhantomData<T>);
-    let _ = ImplementsTrait::<Str>(PhantomData);
-    ()
-};
-
-fn main() {}
diff --git a/src/test/ui/feature-gates/feature-gate-underscore_const_names.stderr b/src/test/ui/feature-gates/feature-gate-underscore_const_names.stderr
deleted file mode 100644
index 8d925424d8c..00000000000
--- a/src/test/ui/feature-gates/feature-gate-underscore_const_names.stderr
+++ /dev/null
@@ -1,18 +0,0 @@
-error[E0658]: naming constants with `_` is unstable
-  --> $DIR/feature-gate-underscore_const_names.rs:6:1
-   |
-LL | / const _ : () = {
-LL | |
-LL | |     use std::marker::PhantomData;
-LL | |     struct ImplementsTrait<T: Trt>(PhantomData<T>);
-LL | |     let _ = ImplementsTrait::<Str>(PhantomData);
-LL | |     ()
-LL | | };
-   | |__^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/54912
-   = help: add #![feature(underscore_const_names)] to the crate attributes to enable
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/feature-gates/underscore_const_names_feature_gate.rs b/src/test/ui/feature-gates/underscore_const_names_feature_gate.rs
deleted file mode 100644
index e50bbf5b649..00000000000
--- a/src/test/ui/feature-gates/underscore_const_names_feature_gate.rs
+++ /dev/null
@@ -1,3 +0,0 @@
-const _: () = (); //~ ERROR is unstable
-
-fn main() {}
diff --git a/src/test/ui/feature-gates/underscore_const_names_feature_gate.stderr b/src/test/ui/feature-gates/underscore_const_names_feature_gate.stderr
deleted file mode 100644
index 0931145a6e2..00000000000
--- a/src/test/ui/feature-gates/underscore_const_names_feature_gate.stderr
+++ /dev/null
@@ -1,12 +0,0 @@
-error[E0658]: naming constants with `_` is unstable
-  --> $DIR/underscore_const_names_feature_gate.rs:1:1
-   |
-LL | const _: () = ();
-   | ^^^^^^^^^^^^^^^^^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/54912
-   = help: add #![feature(underscore_const_names)] to the crate attributes to enable
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/parser/underscore_item_not_const.rs b/src/test/ui/parser/underscore_item_not_const.rs
new file mode 100644
index 00000000000..375bdc3a463
--- /dev/null
+++ b/src/test/ui/parser/underscore_item_not_const.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() {}
diff --git a/src/test/ui/parser/underscore_item_not_const.stderr b/src/test/ui/parser/underscore_item_not_const.stderr
new file mode 100644
index 00000000000..deb4a012e32
--- /dev/null
+++ b/src/test/ui/parser/underscore_item_not_const.stderr
@@ -0,0 +1,92 @@
+error: expected identifier, found reserved identifier `_`
+  --> $DIR/underscore_item_not_const.rs:6:11
+   |
+LL |     const _: () = ();
+   |           ^ expected identifier, found reserved identifier
+
+error: expected identifier, found reserved identifier `_`
+  --> $DIR/underscore_item_not_const.rs:9:11
+   |
+LL |     const _: () = ();
+   |           ^ expected identifier, found reserved identifier
+
+error: expected identifier, found reserved identifier `_`
+  --> $DIR/underscore_item_not_const.rs:12:11
+   |
+LL |     const _: () = ();
+   |           ^ expected identifier, found reserved identifier
+
+error: expected identifier, found reserved identifier `_`
+  --> $DIR/underscore_item_not_const.rs:17:8
+   |
+LL | static _: () = ();
+   |        ^ expected identifier, found reserved identifier
+
+error: expected identifier, found reserved identifier `_`
+  --> $DIR/underscore_item_not_const.rs:18:8
+   |
+LL | struct _();
+   |        ^ expected identifier, found reserved identifier
+
+error: expected identifier, found reserved identifier `_`
+  --> $DIR/underscore_item_not_const.rs:19:6
+   |
+LL | enum _ {}
+   |      ^ expected identifier, found reserved identifier
+
+error: expected identifier, found reserved identifier `_`
+  --> $DIR/underscore_item_not_const.rs:20:4
+   |
+LL | fn _() {}
+   |    ^ expected identifier, found reserved identifier
+
+error: expected identifier, found reserved identifier `_`
+  --> $DIR/underscore_item_not_const.rs:21:5
+   |
+LL | mod _ {}
+   |     ^ expected identifier, found reserved identifier
+
+error: expected identifier, found reserved identifier `_`
+  --> $DIR/underscore_item_not_const.rs:22:6
+   |
+LL | type _ = ();
+   |      ^ expected identifier, found reserved identifier
+
+error: expected identifier, found reserved identifier `_`
+  --> $DIR/underscore_item_not_const.rs:23:5
+   |
+LL | use _;
+   |     ^ expected identifier, found reserved identifier
+
+error: expected identifier, found reserved identifier `_`
+  --> $DIR/underscore_item_not_const.rs:24:5
+   |
+LL | use _ as g;
+   |     ^ expected identifier, found reserved identifier
+
+error: expected identifier, found reserved identifier `_`
+  --> $DIR/underscore_item_not_const.rs:25:7
+   |
+LL | trait _ {}
+   |       ^ expected identifier, found reserved identifier
+
+error: expected identifier, found reserved identifier `_`
+  --> $DIR/underscore_item_not_const.rs:26:7
+   |
+LL | trait _ = Copy;
+   |       ^ expected identifier, found reserved identifier
+
+error: expected identifier, found reserved identifier `_`
+  --> $DIR/underscore_item_not_const.rs:27:14
+   |
+LL | macro_rules! _ { () => {} }
+   |              ^ expected identifier, found reserved identifier
+
+error: expected one of `!` or `::`, found `_`
+  --> $DIR/underscore_item_not_const.rs:28:7
+   |
+LL | union _ { f: u8 }
+   |       ^ expected one of `!` or `::` here
+
+error: aborting due to 15 previous errors
+
diff --git a/src/test/ui/parser/underscore_static.rs b/src/test/ui/parser/underscore_static.rs
deleted file mode 100644
index 21d6a1bc1b3..00000000000
--- a/src/test/ui/parser/underscore_static.rs
+++ /dev/null
@@ -1,3 +0,0 @@
-static _: () = (); //~ ERROR expected identifier, found reserved identifier `_`
-
-fn main() {}
diff --git a/src/test/ui/parser/underscore_static.stderr b/src/test/ui/parser/underscore_static.stderr
deleted file mode 100644
index 4c41afdc3f0..00000000000
--- a/src/test/ui/parser/underscore_static.stderr
+++ /dev/null
@@ -1,8 +0,0 @@
-error: expected identifier, found reserved identifier `_`
-  --> $DIR/underscore_static.rs:1:8
-   |
-LL | static _: () = ();
-   |        ^ expected identifier, found reserved identifier
-
-error: aborting due to previous error
-