Fix unused variable warnings

Removing `_LIBCPP_DISABLE_UNUSED_STRUCT_WARNINGS` from libc++ enables
`[[gnu::__warn_unused__]]` on standard library types (such as
`std::string`), surfacing `-Wunused-variable` warnings in
`//third_party/protobuf`. See [upstream LLVM commit](https://github.com/llvm/llvm-project/commit/ab8b037ef7b398aa864959a69f328fa69b567f74) for reference.

- Remove unreferenced `std::string` declarations (`ext_scope` in
  `gen_extensions.cc` and `default_value` in `extension.cc`) whose
  initializers have no side effects.
- Annotate `class_name`, `mini_table_name`, and `crate_name` with
  `[[maybe_unused]]` where their initializers invoke helper functions.

PiperOrigin-RevId: 999636704

hpb_generator/gen_accessors.cc

diff --git a/hpb_generator/gen_accessors.cc b/hpb_generator/gen_accessors.cc
index 44534f9..0da4d65 100644
--- a/hpb_generator/gen_accessors.cc
+++ b/hpb_generator/gen_accessors.cc
@@ -788,7 +788,6 @@
 void WriteOneofAccessorsInHeader(const google::protobuf::Descriptor* desc, Context& ctx) {
   // Generate const methods.
   auto indent = ctx.printer().WithIndent();
-  std::string class_name = ClassName(desc);
   auto field_names = CreateFieldNameMap(desc);
   for (int i = 0; i < desc->real_oneof_decl_count(); ++i) {
     const google::protobuf::OneofDescriptor* oneof = desc->oneof_decl(i);

hpb_generator/gen_extensions.cc

diff --git a/hpb_generator/gen_extensions.cc b/hpb_generator/gen_extensions.cc
index 2c7bea8..09ffd3f 100644
--- a/hpb_generator/gen_extensions.cc
+++ b/hpb_generator/gen_extensions.cc
@@ -26,7 +26,6 @@
 
 std::string ExtensionIdentifierBase(const google::protobuf::FieldDescriptor* ext) {
   assert(ext->is_extension());
-  std::string ext_scope;
   if (ext->extension_scope()) {
     return upb::generator::CApiMessageType(ext->extension_scope()->full_name());
   } else {
@@ -55,8 +54,7 @@
        {"extension_type", ext_type},
        {"default_value", DefaultValue(ext)},
        {"linkage", linkage},
-       {"mini_table_name",
-        absl::StrCat(ExtensionIdentifierBase(ext), "_", ext->name(), "_ext")},
+       {"mini_table_name", mini_table_name},
        Sub("extension_name", ResolveKeywordConflict(ext->name()))
            .AnnotatedAs(ext),
        {"extension_number", ext->number()}},

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

diff --git a/src/google/protobuf/compiler/rust/generator.cc b/src/google/protobuf/compiler/rust/generator.cc
index 3b83e9d..a9fe1b8 100644
--- a/src/google/protobuf/compiler/rust/generator.cc
+++ b/src/google/protobuf/compiler/rust/generator.cc
@@ -55,7 +55,6 @@
 // messages and enums of a `dep`. This should only be
 // called for 'import public' deps.
 void EmitPublicImportsForDepFile(Context& ctx, const FileDescriptor* dep) {
-  std::string crate_name = GetCrateName(ctx, *dep);
   for (int i = 0; i < dep->message_type_count(); ++i) {
     auto* msg = dep->message_type(i);
     auto path = RsTypePath(ctx, *msg);