Make bare known-bug an error

Author: compiler-errors

src/tools/compiletest/src/header.rs

@@ -426,10 +426,15 @@ impl TestProps {
                         self.known_bug = true;
                     } else {
                         panic!(
-                            "Invalid known-bug value: {known_bug}\nIt requires comma-separated issue references (`#000` or `chalk#000`) or `unknown`."
+                            "Invalid known-bug value: {known_bug}\nIt requires comma-separated issue references (`#000` or `chalk#000`) or `known-bug: unknown`."
                         );
                     }
+                } else if config.parse_name_directive(ln, KNOWN_BUG) {
+                    panic!(
+                        "Invalid known-bug attribute, requires comma-separated issue references (`#000` or `chalk#000`) or `known-bug: unknown`."
+                    );
                 }
+
                 config.set_name_value_directive(ln, MIR_UNIT_TEST, &mut self.mir_unit_test, |s| {
                     s.trim().to_string()
                 });