Improve handling of legacy style directives

Author: jieyouxu

src/tools/compiletest/src/header.rs

@@ -834,16 +834,31 @@ fn iter_header_extra(
 
             for candidate in DIAGNOSTICS_DIRECTIVE_NAMES.iter() {
                 if rest.starts_with(candidate) {
-                    // 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:{:#?}",
-                        testfile.display(),
-                        line_number,
-                        line_directive("//", ln),
-                    );
-                    return;
+                    let Some(prefix_removed) = rest.strip_prefix(candidate) else {
+                        // 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:{:#?}",
+                            testfile.display(),
+                            line_number,
+                            line_directive("//", ln),
+                        );
+                        return;
+                    };
+
+                    if prefix_removed.starts_with([' ', ':']) {
+                        // 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:{:#?}",
+                            testfile.display(),
+                            line_number,
+                            line_directive("//", ln),
+                        );
+                        return;
+                    }
                 }
             }
         }