Fix Rust Proto cpp-kernel map insertion behavior to better match Rust's std::HashMap and the upb-kernel behavior of "insert replaces the old value"

PiperOrigin-RevId: 616934577

rust/cpp_kernel/cpp_api.h

@@ -86,6 +86,9 @@ struct PtrAndLen {
   bool __rust_proto_thunk__Map_##rust_key_ty##_##rust_value_ty##_insert(       \
       google::protobuf::Map<key_ty, value_ty>* m, ffi_key_ty key, ffi_value_ty value) {  \
     auto iter_and_inserted = m->try_emplace(to_cpp_key, to_cpp_value);         \
+    if (!iter_and_inserted.second) {                                           \
+      iter_and_inserted.first->second = to_cpp_value;                          \
+    }                                                                          \
     return iter_and_inserted.second;                                           \
   }                                                                            \
   bool __rust_proto_thunk__Map_##rust_key_ty##_##rust_value_ty##_get(          \

rust/map.rs

@@ -528,6 +528,20 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_overwrite_insert() {
+        let mut map: Map<i32, ProtoStr> = Map::new();
+        let mut map_mut = map.as_mut();
+        assert!(map_mut.insert(0, "fizz"));
+        // insert should return false when the key is already present
+        assert!(!map_mut.insert(0, "buzz"));
+
+        assert_that!(
+            map.as_mut().iter().collect::<Vec<_>>(),
+            unordered_elements_are![eq((0, ProtoStr::from_str("buzz"))),]
+        );
+    }
+
     #[test]
     fn test_all_maps_can_be_constructed() {
         macro_rules! gen_proto_values {