Fix header parsing somewhat

previously it just errored on any comment

It still errors on `//[thing]` style comments

Author: oli-obk

src/tools/compiletest/src/header.rs

@@ -697,14 +697,15 @@ fn iter_header_extra(
         // First try to accept `ui_test` style comments
         } else if let Some((lncfg, ln)) = line_directive(comment, ln) {
             it(lncfg, ln, line_number);
-        } else if mode == Mode::Ui && line_directive("//", ln).is_some() {
+        } else if mode == Mode::Ui && matches!(line_directive("//", ln), Some((Some(_), _))) {
             // We have a comment that's *successfully* parsed as an legacy-style directive.
             // We emit an error here to warn the user.
             *poisoned = true;
             eprintln!(
-                "error: detected legacy-style directives in ui test: {}:{}, please use `ui_test`-style directives `//@` instead",
+                "error: detected legacy-style directives in ui test: {}:{}, please use `ui_test`-style directives `//@` instead:{:#?}",
                 testfile.display(),
                 line_number,
+                line_directive("//", ln),
             );
             return;
         }