Less greedily parse `[const]` bounds

(cherry picked from commit f5dad62d4cd)

1 files changed, 8 insertions, 5 deletions

diff --git a/compiler/rustc_parse/src/parser/ty.rs b/compiler/rustc_parse/src/parser/ty.rs
index 740dd10ea8b..6827a9f23a4 100644
--- a/compiler/rustc_parse/src/parser/ty.rs
+++ b/compiler/rustc_parse/src/parser/ty.rs
@@ -951,12 +951,18 @@ impl<'a> Parser<'a> {
             || self.check(exp!(Tilde))
             || self.check_keyword(exp!(For))
             || self.check(exp!(OpenParen))
-            || self.check(exp!(OpenBracket))
+            || self.can_begin_maybe_const_bound()
             || self.check_keyword(exp!(Const))
             || self.check_keyword(exp!(Async))
             || self.check_keyword(exp!(Use))
     }
 
+    fn can_begin_maybe_const_bound(&mut self) -> bool {
+        self.check(exp!(OpenBracket))
+            && self.look_ahead(1, |t| t.is_keyword(kw::Const))
+            && self.look_ahead(2, |t| *t == token::CloseBracket)
+    }
+
     /// Parses a bound according to the grammar:
     /// ```ebnf
     /// BOUND = TY_BOUND | LT_BOUND
@@ -1137,10 +1143,7 @@ impl<'a> Parser<'a> {
             let span = tilde.to(self.prev_token.span);
             self.psess.gated_spans.gate(sym::const_trait_impl, span);
             BoundConstness::Maybe(span)
-        } else if self.check(exp!(OpenBracket))
-            && self.look_ahead(1, |t| t.is_keyword(kw::Const))
-            && self.look_ahead(2, |t| *t == token::CloseBracket)
-        {
+        } else if self.can_begin_maybe_const_bound() {
             let start = self.token.span;
             self.bump();
             self.expect_keyword(exp!(Const)).unwrap();