Fix fn front matter parsing ICE from invalid code.

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))
     }
 

src/test/ui/issue-60075.rs

@@ -0,0 +1,12 @@
+fn main() {}
+
+trait T {
+    fn qux() -> Option<usize> {
+        let _ = if true {
+        });
+//~^ ERROR expected one of `async`, `const`, `extern`, `fn`, `type`, `unsafe`, or `}`, found `;`
+//~^^ ERROR expected one of `.`, `;`, `?`, `else`, or an operator, found `}`
+//~^^^ ERROR 6:11: 6:12: expected identifier, found `;`
+//~^^^^ ERROR missing `fn`, `type`, or `const` for trait-item declaration
+        Some(4)
+    }

src/test/ui/issue-60075.stderr

@@ -0,0 +1,37 @@
+error: expected one of `.`, `;`, `?`, `else`, or an operator, found `}`
+  --> $DIR/issue-60075.rs:6:10
+   |
+LL |         });
+   |          ^ expected one of `.`, `;`, `?`, `else`, or an operator here
+
+error: expected one of `async`, `const`, `extern`, `fn`, `type`, `unsafe`, or `}`, found `;`
+  --> $DIR/issue-60075.rs:6:11
+   |
+LL |     fn qux() -> Option<usize> {
+   |                               - unclosed delimiter
+LL |         let _ = if true {
+LL |         });
+   |           ^
+   |           |
+   |           help: `}` may belong here
+
+error: expected identifier, found `;`
+  --> $DIR/issue-60075.rs:6:11
+   |
+LL |         });
+   |           ^ expected identifier
+
+error: missing `fn`, `type`, or `const` for trait-item declaration
+  --> $DIR/issue-60075.rs:6:12
+   |
+LL |           });
+   |  ____________^
+LL | |
+LL | |
+LL | |
+LL | |
+LL | |         Some(4)
+   | |________^ missing `fn`, `type`, or `const`
+
+error: aborting due to 4 previous errors
+