Fix fn front matter parsing ICE from invalid code.

This commit fixes an "unreachable code" ICE that results from parsing
invalid code where the compiler is expecting the next trait item
declaration in the middle of the previous trait item due to extra
closing braces.

1 files changed, 6 insertions, 1 deletions

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index a5adb37f745..8e6db8a541a 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -6575,7 +6575,12 @@ impl<'a> Parser<'a> {
             };
             (respan(self.prev_span, Constness::NotConst), unsafety, abi)
         };
-        self.expect_keyword(keywords::Fn)?;
+        if !self.eat_keyword(keywords::Fn) {
+            // It is possible for `expect_one_of` to recover given the contents of
+            // `self.expected_tokens`, therefore, do not use `self.unexpected()` which doesn't
+            // account for this.
+            if !self.expect_one_of(&[], &[])? { unreachable!() }
+        }
         Ok((constness, unsafety, asyncness, abi))
     }