Refactor std::marker::PhantomData to $phantom$

PiperOrigin-RevId: 553165873

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

@@ -238,6 +238,7 @@ bool RustGenerator::Generate(const FileDescriptor* file_desc,
       {"pbi", "::__pb::__internal"},
       {"pbr", "::__pb::__runtime"},
       {"NonNull", "::__std::ptr::NonNull"},
+      {"Phantom", "::__std::marker::PhantomData"},
   });
 
   file.Emit({{"kernel", KernelRsName(file.opts().kernel)}}, R"rs(

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

@@ -303,7 +303,7 @@ void MessageGenerator::GenerateRs(Context<Descriptor> msg) {
         #[derive(Debug, Copy, Clone)]
         pub struct $Msg$View<'a> {
           _msg: $pbi$::RawMessage,
-          _phantom: std::marker::PhantomData<&'a ()>,
+          _phantom: $Phantom$<&'a ()>,
         }
 
         impl<'a> $pb$::ViewProxy<'a> for $Msg$View<'a> {
@@ -323,7 +323,7 @@ void MessageGenerator::GenerateRs(Context<Descriptor> msg) {
 
         #[derive(Debug)]
         pub struct $Msg$Mut<'a> {
-          _phantom: std::marker::PhantomData<&'a mut ()>,
+          _phantom: $Phantom$<&'a mut ()>,
         }
 
         impl<'a> $pb$::MutProxy<'a> for $Msg$Mut<'a> {