Add retain to Map

build.rs

@@ -31,6 +31,12 @@ fn main() {
     if minor < 45 {
         println!("cargo:rustc-cfg=no_btreemap_remove_entry");
     }
+
+    // BTreeMap::retain
+    // https://blog.rust-lang.org/2021/06/17/Rust-1.53.0.html#stabilized-apis
+    if minor < 53 {
+        println!("cargo:rustc-cfg=no_btreemap_retain");
+    }
 }
 
 fn rustc_minor_version() -> Option<u32> {

src/map.rs

@@ -234,6 +234,19 @@ impl Map<String, Value> {
         }
     }
 
+    #[cfg(not(no_btreemap_retain))]
+    /// Retains only the elements specified by the predicate.
+    ///
+    /// In other words, remove all pairs `(k, v)` such that `f(&k, &mut v)` returns `false`.
+    /// The elements are visited in ascending key order.
+    #[inline]
+    pub fn retain<F>(&mut self, f: F)
+    where
+        F: FnMut(&String, &mut Value) -> bool,
+    {
+        self.map.retain(f);
+    }
+
     /// Gets an iterator over the values of the map.
     #[inline]
     pub fn values(&self) -> Values {

tests/map.rs

@@ -34,3 +34,16 @@ fn test_append() {
     assert_eq!(keys, EXPECTED);
     assert!(val.is_empty());
 }
+
+#[cfg(not(no_btreemap_retain))]
+#[test]
+fn test_retain() {
+    const EXPECTED: &[&str] = &["a", "c"];
+
+    let mut v: Value = from_str(r#"{"b":null,"a":null,"c":null}"#).unwrap();
+    let val = v.as_object_mut().unwrap();
+    val.retain(|k, _| k.as_str() != "b");
+
+    let keys: Vec<_> = val.keys().collect();
+    assert_eq!(keys, EXPECTED);
+}