Rust: make generated views implement `std::default::Default`

The immediate motivation for this is that it will facilitate writing a blanket
implementation of `ProxiedInMapValue` for C++-backed messages. The default
instance gives us access to the message vtable in cases where we don't already
have a message to work with.

However, it also seems generally useful just to have an implementation of
`Default`, so I implemented it for both C++ and upb-backed message views.

PiperOrigin-RevId: 677808048

rust/codegen_traits.rs

@@ -37,7 +37,7 @@ pub trait Message: SealedInternal
 pub trait MessageView<'msg>: SealedInternal
     + ViewProxy<'msg, Proxied = Self::Message>
     // Read traits:
-    + Debug + Serialize
+    + Debug + Serialize + Default
     // Thread safety:
     + Send + Sync
     // Copy/Clone:

rust/test/shared/serialization_test.rs

@@ -7,6 +7,7 @@
 
 use googletest::prelude::*;
 use protobuf::prelude::*;
+use protobuf::View;
 
 use edition_unittest_rust_proto::TestAllTypes as TestAllTypesEditions;
 use paste::paste;
@@ -31,6 +32,12 @@ macro_rules! generate_parameterized_serialization_test {
                 assert_that!(serialized.len(), eq(0));
             }
 
+            #[gtest]
+            fn [< serialize_default_view $name_ext>]() {
+                let default = View::<[< $type >]>::default();
+                assert_that!(default.serialize().unwrap().len(), eq(0));
+            }
+
             #[gtest]
             fn [< serialize_deserialize_message_ $name_ext>]() {
                 let mut msg = [< $type >]::new();

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

@@ -195,6 +195,7 @@ void CppMessageExterns(Context& ctx, const Descriptor& msg) {
   ABSL_CHECK(ctx.is_cpp());
   ctx.Emit(
       {{"new_thunk", ThunkName(ctx, msg, "new")},
+       {"default_instance_thunk", ThunkName(ctx, msg, "default_instance")},
        {"repeated_new_thunk", ThunkName(ctx, msg, "repeated_new")},
        {"repeated_free_thunk", ThunkName(ctx, msg, "repeated_free")},
        {"repeated_len_thunk", ThunkName(ctx, msg, "repeated_len")},
@@ -207,6 +208,7 @@ void CppMessageExterns(Context& ctx, const Descriptor& msg) {
        {"map_size_info_thunk", ThunkName(ctx, msg, "size_info")}},
       R"rs(
       fn $new_thunk$() -> $pbr$::RawMessage;
+      fn $default_instance_thunk$() -> $pbr$::RawMessage;
       fn $repeated_new_thunk$() -> $pbr$::RawRepeatedField;
       fn $repeated_free_thunk$(raw: $pbr$::RawRepeatedField);
       fn $repeated_len_thunk$(raw: $pbr$::RawRepeatedField) -> usize;
@@ -733,6 +735,16 @@ void MessageProxiedInMapValue(Context& ctx, const Descriptor& msg) {
   }
 }
 
+void GenerateDefaultInstanceImpl(Context& ctx, const Descriptor& msg) {
+  if (ctx.is_upb()) {
+    ctx.Emit("$pbr$::ScratchSpace::zeroed_block()");
+  } else {
+    ctx.Emit(
+        {{"default_instance_thunk", ThunkName(ctx, msg, "default_instance")}},
+        "unsafe { $default_instance_thunk$() }");
+  }
+}
+
 }  // namespace
 
 void GenerateRs(Context& ctx, const Descriptor& msg) {
@@ -749,6 +761,8 @@ void GenerateRs(Context& ctx, const Descriptor& msg) {
        {"Msg::drop", [&] { MessageDrop(ctx, msg); }},
        {"Msg::debug", [&] { MessageDebug(ctx, msg); }},
        {"MsgMut::merge_from", [&] { MessageMutMergeFrom(ctx, msg); }},
+       {"default_instance_impl",
+        [&] { GenerateDefaultInstanceImpl(ctx, msg); }},
        {"accessor_fns",
         [&] {
           for (int i = 0; i < msg.field_count(); ++i) {
@@ -948,6 +962,12 @@ void GenerateRs(Context& ctx, const Descriptor& msg) {
           }
         }
 
+        impl $std$::default::Default for $Msg$View<'_> {
+          fn default() -> $Msg$View<'static> {
+            $Msg$View::new($pbi$::Private, $default_instance_impl$)
+          }
+        }
+
         #[allow(dead_code)]
         impl<'msg> $Msg$View<'msg> {
           #[doc(hidden)]
@@ -1367,6 +1387,7 @@ void GenerateThunksCc(Context& ctx, const Descriptor& msg) {
        {"Msg", RsSafeName(msg.name())},
        {"QualifiedMsg", cpp::QualifiedClassName(&msg)},
        {"new_thunk", ThunkName(ctx, msg, "new")},
+       {"default_instance_thunk", ThunkName(ctx, msg, "default_instance")},
        {"repeated_new_thunk", ThunkName(ctx, msg, "repeated_new")},
        {"repeated_free_thunk", ThunkName(ctx, msg, "repeated_free")},
        {"repeated_len_thunk", ThunkName(ctx, msg, "repeated_len")},
@@ -1403,6 +1424,10 @@ void GenerateThunksCc(Context& ctx, const Descriptor& msg) {
         extern $abi$ {
         void* $new_thunk$() { return new $QualifiedMsg$(); }
 
+        const google::protobuf::MessageLite* $default_instance_thunk$() {
+          return &$QualifiedMsg$::default_instance();
+        }
+
         void* $repeated_new_thunk$() {
           return new google::protobuf::RepeatedPtrField<$QualifiedMsg$>();
         }