add Map::retain method

CHANGELOG.md

@@ -6,6 +6,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+- Add `Map::retain` method ([#460](https://github.com/ron-rs/ron/pull/460))
 - Add CIFuzz GitHub action
 - Fix issues [#277](https://github.com/ron-rs/ron/issues/277) and [#405](https://github.com/ron-rs/ron/issues/405) with `Value::Map` `IntoIter` and extraneous item check for `Value::Seq` ([#406](https://github.com/ron-rs/ron/pull/406))
 - Fix issue [#401](https://github.com/ron-rs/ron/issues/401) with correct raw struct name identifier parsing ([#402](https://github.com/ron-rs/ron/pull/402))

src/value/mod.rs

@@ -80,6 +80,19 @@ impl Map {
     pub fn values_mut(&mut self) -> impl Iterator<Item = &mut Value> + DoubleEndedIterator {
         self.0.values_mut()
     }
+
+    /// Retains only the elements specified by the `keep` predicate.
+    ///
+    /// In other words, remove all pairs `(k, v)` for which `keep(&k, &mut v)`
+    /// returns `false`.
+    ///
+    /// The elements are visited in iteration order.
+    pub fn retain<F>(&mut self, keep: F)
+    where
+        F: FnMut(&Value, &mut Value) -> bool,
+    {
+        self.0.retain(keep);
+    }
 }
 
 impl FromIterator<(Value, Value)> for Map {