Tidy up iter_header a bit

Author: jieyouxu

src/tools/tidy/src/iter_header.rs

@@ -13,19 +13,20 @@ pub(crate) struct HeaderLine<'ln> {
 pub(crate) fn iter_header<'ln>(contents: &'ln str, it: &mut dyn FnMut(HeaderLine<'ln>)) {
     for ln in contents.lines() {
         let ln = ln.trim();
-        if ln.starts_with(COMMENT) && ln[COMMENT.len()..].trim_start().starts_with('[') {
-            if let Some(close_brace) = ln.find(']') {
-                let open_brace = ln.find('[').unwrap();
-                let revision = &ln[open_brace + 1..close_brace];
-                it(HeaderLine {
-                    revision: Some(revision),
-                    directive: ln[(close_brace + 1)..].trim_start(),
-                });
-            } else {
-                panic!("malformed condition directive: expected `//@[foo]`, found `{ln}`")
-            }
-        } else if ln.starts_with(COMMENT) {
-            it(HeaderLine { revision: None, directive: ln[COMMENT.len()..].trim_start() });
+
+        // We're left with potentially `[rev]name: value`.
+        let Some(remainder) = ln.strip_prefix(COMMENT) else {
+            continue;
+        };
+
+        if let Some(remainder) = remainder.trim_start().strip_prefix('[') {
+            let Some((revision, remainder)) = remainder.split_once(']') else {
+                panic!("malformed revision directive: expected `//@[rev]`, found `{ln}`");
+            };
+            // We trimmed off the `[rev]` portion, left with `name: value`.
+            it(HeaderLine { revision: Some(revision), directive: remainder.trim() });
+        } else {
+            it(HeaderLine { revision: None, directive: remainder.trim() });
         }
     }
 }