Add Comments data structure (#4641)

crates/ruff_python_formatter/src/comments/map.rs

@@ -209,6 +209,12 @@ impl<K: std::hash::Hash + Eq, V> MultiMap<K, V> {
         }
     }
 
+    pub fn keys(&self) -> Keys<'_, K> {
+        Keys {
+            inner: self.index.keys(),
+        }
+    }
+
     /// Returns the *leading* parts of `key` in insertion-order.
     pub fn leading(&self, key: &K) -> &[V] {
         match self.index.get(key) {
@@ -759,6 +765,26 @@ impl PartIndex {
     }
 }
 
+/// Iterator over the keys of a comments multi map
+pub struct Keys<'a, K> {
+    inner: std::collections::hash_map::Keys<'a, K, Entry>,
+}
+
+impl<'a, K> Iterator for Keys<'a, K> {
+    type Item = &'a K;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.inner.next()
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
+}
+
+impl<K> ExactSizeIterator for Keys<'_, K> {}
+impl<K> FusedIterator for Keys<'_, K> {}
+
 #[cfg(test)]
 mod tests {
     use crate::comments::map::MultiMap;