Implement Default::default() for owned messages.

PiperOrigin-RevId: 618758568

rust/test/shared/accessors_test.rs

@@ -13,7 +13,7 @@ use unittest_proto::{test_all_types, TestAllTypes};
 
 #[test]
 fn test_default_accessors() {
-    let msg = TestAllTypes::new();
+    let msg: TestAllTypes = Default::default();
     assert_that!(
         msg,
         matches_pattern!(TestAllTypes{

src/google/protobuf/compiler/rust/message.cc

@@ -815,6 +815,12 @@ void GenerateRs(Context& ctx, const Descriptor& msg) {
           }
         }
 
+        impl std::default::Default for $Msg$ {
+          fn default() -> Self {
+            Self::new()
+          }
+        }
+
         // SAFETY:
         // - `$Msg$` is `Sync` because it does not implement interior mutability.
         //    Neither does `$Msg$Mut`.