Merge #1847

1847: Allow an underscore as the identifier in `const` items r=matklad a=ecstatic-morse

[RFC 2526](https://github.com/rust-lang/rust/issues/54912) was recently stabilized, meaning `const _: i32 = 5;` is now a valid item.

Co-authored-by: Dylan MacKenzie <[email protected]>

2 files changed, 11 insertions, 2 deletions

diff --git a/crates/ra_parser/src/grammar/items.rs b/crates/ra_parser/src/grammar/items.rs
index eff9d67e4..4dd80d443 100644
--- a/crates/ra_parser/src/grammar/items.rs
+++ b/crates/ra_parser/src/grammar/items.rs
@@ -258,7 +258,7 @@ fn items_without_modifiers(p: &mut Parser, m: Marker) -> Result<(), Marker> {
         }
         T![enum] => nominal::enum_def(p, m),
         T![use] => use_item::use_item(p, m),
-        T![const] if (la == IDENT || la == T![mut]) => consts::const_def(p, m),
+        T![const] if (la == IDENT || la == T![_] || la == T![mut]) => consts::const_def(p, m),
         T![static] => consts::static_def(p, m),
         // test extern_block
         // extern {}
diff --git a/crates/ra_parser/src/grammar/items/consts.rs b/crates/ra_parser/src/grammar/items/consts.rs
index e11546333..63e0e6e0c 100644
--- a/crates/ra_parser/src/grammar/items/consts.rs
+++ b/crates/ra_parser/src/grammar/items/consts.rs
@@ -12,7 +12,16 @@ fn const_or_static(p: &mut Parser, m: Marker, kw: SyntaxKind, def: SyntaxKind) {
     assert!(p.at(kw));
     p.bump_any();
     p.eat(T![mut]); // FIXME: validator to forbid const mut
-    name(p);
+
+    // Allow `_` in place of an identifier in a `const`.
+    let is_const_underscore = kw == T![const] && p.eat(T![_]);
+    if !is_const_underscore {
+        name(p);
+    }
+
+    // test_err static_underscore
+    // static _: i32 = 5;
+
     types::ascription(p);
     if p.eat(T![=]) {
         expressions::expr(p);