Allow an underscore as the identifier in const items

Author: ecstatic-morse

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 {}

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);

crates/ra_syntax/test_data/parser/inline/err/0013_static_underscore.rs

@@ -0,0 +1 @@
+static _: i32 = 5;