Flesh out Proxied impls for $Msg$View and $Msg$Mut :: v0.6 SingularMessageField

This CL adds function bodies for: {as_view, into_view, as_mut, into_mut, set_on} [1].

Our prior cl/552609955 didn't have `RawMessage` inside $Msg$Mut, so that's also been rectified in this CL.

[1] Everything in this set is a part of the Proxied trait, except set_on, which belongs to the SettableValue trait.

PiperOrigin-RevId: 553935803

rust/test/shared/accessors_test.rs

@@ -229,11 +229,9 @@ fn test_optional_bytes_accessors() {
 }
 
 #[test]
-#[should_panic = "b/285309454"]
-#[allow(unreachable_code)]
 fn test_singular_msg_field() {
     let msg = TestAllTypes::new();
     // TODO("b/285309454"): fetch the inner integer `bb`
     // call should look like msg.optional_nested_message().bb()
-    match msg.optional_nested_message() {}
+    let _msg: unittest_proto::proto2_unittest::TestAllTypesView = msg.optional_nested_message();
 }

src/google/protobuf/compiler/rust/accessors/singular_message.cc

@@ -50,8 +50,8 @@ class SingularMessage final : public AccessorGenerator {
         },
         R"rs(
           // inMsgImpl
-          pub fn $field$(&self) -> std::convert::Infallible {
-            todo!("b/285309454")
+          pub fn r#$field$(&self) -> $Msg$View {
+            $Msg$View { msg: self.msg, _phantom: std::marker::PhantomData }
           }
         )rs");
   }

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

@@ -301,8 +301,9 @@ void MessageGenerator::GenerateRs(Context<Descriptor> msg) {
         }
 
         #[derive(Debug, Copy, Clone)]
+        #[allow(dead_code)]
         pub struct $Msg$View<'a> {
-          _msg: $pbi$::RawMessage,
+          msg: $pbi$::RawMessage,
           _phantom: $Phantom$<&'a ()>,
         }
 
@@ -310,35 +311,43 @@ void MessageGenerator::GenerateRs(Context<Descriptor> msg) {
           type Proxied = $Msg$;
 
           fn as_view(&self) -> $pb$::View<'a, $Msg$> {
-            todo!("b/285309454")
+            *self
+          }
+          fn into_view<'shorter>(self) -> $pb$::View<'shorter, $Msg$> where 'a: 'shorter {
+            self
           }
-          fn into_view<'shorter>(self) -> $pb$::View<'shorter, $Msg$> where 'a: 'shorter { todo!("b/285309454") }
         }
 
         impl<'a> $pb$::SettableValue<$Msg$> for $Msg$View<'a> {
           fn set_on(self, _private: $pb$::__internal::Private, _mutator: $pb$::Mut<$Msg$>) {
-            todo!("b/285309454")
+            todo!()
           }
         }
 
-        #[derive(Debug)]
+        #[derive(Debug, Copy, Clone)]
+        #[allow(dead_code)]
         pub struct $Msg$Mut<'a> {
+          msg: $pbi$::RawMessage,
           _phantom: $Phantom$<&'a mut ()>,
         }
 
+        unsafe impl Sync for $Msg$Mut<'_> {}
+
         impl<'a> $pb$::MutProxy<'a> for $Msg$Mut<'a> {
           fn as_mut(&mut self) -> $pb$::Mut<'_, $Msg$> {
-            todo!("b/285309454")
+            $Msg$Mut { msg: self.msg, _phantom: self._phantom }
           }
-          fn into_mut<'shorter>(self) -> $pb$::Mut<'shorter, $Msg$> where 'a : 'shorter { todo!("b/285309454") }
+          fn into_mut<'shorter>(self) -> $pb$::Mut<'shorter, $Msg$> where 'a : 'shorter { self }
         }
 
         impl<'a> $pb$::ViewProxy<'a> for $Msg$Mut<'a> {
           type Proxied = $Msg$;
           fn as_view(&self) -> $pb$::View<'_, $Msg$> {
-            todo!("b/285309454")
+            $Msg$View { msg: self.msg, _phantom: std::marker::PhantomData }
+          }
+          fn into_view<'shorter>(self) -> $pb$::View<'shorter, $Msg$> where 'a: 'shorter {
+            $Msg$View { msg: self.msg, _phantom: std::marker::PhantomData }
           }
-          fn into_view<'shorter>(self) -> $pb$::View<'shorter, $Msg$> where 'a: 'shorter { todo!("b/285309454") }
         }
 
         impl $Msg$ {