Auto merge of #60132 - davidtwco:issue-60075, r=estebank

Fix fn front matter parsing ICE from invalid code.

Fixes #60075.

This PR 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.

r? @estebank (thanks for the minimized test case)

src/libsyntax/parse/parser.rs

@@ -6618,7 +6618,12 @@ 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))
     }