Avoid name conflict of rust Option with message Option

Functions like `map_get` return a rust Option, which
might conflict with a `message Option` defined in the
current proto file. Use `::__std::option::Option` to
disambiguate rust Option and message Option.

PiperOrigin-RevId: 680333636

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

@@ -155,7 +155,7 @@ void Map::InExternC(Context& ctx, const FieldDescriptor& field) const {
              if (ctx.is_upb()) {
                ctx.Emit({}, R"rs(
                 fn $getter_thunk$(raw_msg: $pbr$::RawMessage)
-                  -> Option<$pbr$::RawMap>;
+                  -> $Option$<$pbr$::RawMap>;
                 fn $getter_mut_thunk$(raw_msg: $pbr$::RawMessage,
                   arena: $pbr$::RawArena) -> $pbr$::RawMap;
               )rs");

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

@@ -179,7 +179,7 @@ void RepeatedField::InExternC(Context& ctx,
                     fn $getter_thunk$(
                       raw_msg: $pbr$::RawMessage,
                       size: *const usize,
-                    ) -> Option<$pbr$::RawRepeatedField>;
+                    ) -> $Option$<$pbr$::RawRepeatedField>;
                   )rs");
                } else {
                  ctx.Emit(R"rs(

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

@@ -203,7 +203,7 @@ void SingularMessage::InExternC(Context& ctx,
              } else {
                // upb kernel may return NULL for a submsg, we can detect this
                // in terra rust if the option returned is None
-               ctx.Emit({}, "Option<$pbr$::RawMessage>;");
+               ctx.Emit({}, "$Option$<$pbr$::RawMessage>;");
              }
            }},
       },

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

@@ -98,7 +98,7 @@ void EnumProxiedInMapValue(Context& ctx, const EnumDescriptor& desc) {
             unsafe { $pbr$::$map_insert_thunk$(map.as_raw($pbi$::Private), $to_ffi_key_expr$, value.into_proxied($pbi$::Private).0) }
         }
 
-        fn map_get<'a>(map: $pb$::MapView<'a, $key_t$, Self>, key: $pb$::View<'_, $key_t$>) -> Option<$pb$::View<'a, Self>> {
+        fn map_get<'a>(map: $pb$::MapView<'a, $key_t$, Self>, key: $pb$::View<'_, $key_t$>) -> $Option$<$pb$::View<'a, Self>> {
             let key = $to_ffi_key_expr$;
             let mut value = $std$::mem::MaybeUninit::uninit();
             let found = unsafe { $pbr$::$map_get_thunk$(map.as_raw($pbi$::Private), key, value.as_mut_ptr()) };
@@ -127,7 +127,7 @@ void EnumProxiedInMapValue(Context& ctx, const EnumDescriptor& desc) {
             }
         }
 
-        fn map_iter_next<'a>(iter: &mut $pb$::MapIter<'a, $key_t$, Self>) -> Option<($pb$::View<'a, $key_t$>, $pb$::View<'a, Self>)> {
+        fn map_iter_next<'a>(iter: &mut $pb$::MapIter<'a, $key_t$, Self>) -> $Option$<($pb$::View<'a, $key_t$>, $pb$::View<'a, Self>)> {
             // SAFETY:
             // - The `MapIter` API forbids the backing map from being mutated for 'a,
             //   and guarantees that it's the correct key and value types.

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

@@ -176,6 +176,7 @@ bool RustGenerator::Generate(const FileDescriptor* file,
       {"NonNull", "::__std::ptr::NonNull"},
       {"Phantom", "::__std::marker::PhantomData"},
       {"Result", "::__std::result::Result"},
+      {"Option", "::__std::option::Option"},
   });
 
   ctx.Emit({{"kernel", KernelRsName(ctx.opts().kernel)}}, R"rs(

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

@@ -634,7 +634,7 @@ void MessageProxiedInMapValue(Context& ctx, const Descriptor& msg) {
                     }
                 }
 
-                fn map_get<'a>(map: $pb$::MapView<'a, $key_t$, Self>, key: $pb$::View<'_, $key_t$>) -> Option<$pb$::View<'a, Self>> {
+                fn map_get<'a>(map: $pb$::MapView<'a, $key_t$, Self>, key: $pb$::View<'_, $key_t$>) -> $Option$<$pb$::View<'a, Self>> {
                     use $pbr$::MapNodeSizeInfoIndexForType;
                     let key = $key_expr$;
                     let mut value = $std$::mem::MaybeUninit::uninit();
@@ -675,7 +675,7 @@ void MessageProxiedInMapValue(Context& ctx, const Descriptor& msg) {
                     }
                 }
 
-                fn map_iter_next<'a>(iter: &mut $pb$::MapIter<'a, $key_t$, Self>) -> Option<($pb$::View<'a, $key_t$>, $pb$::View<'a, Self>)> {
+                fn map_iter_next<'a>(iter: &mut $pb$::MapIter<'a, $key_t$, Self>) -> $Option$<($pb$::View<'a, $key_t$>, $pb$::View<'a, Self>)> {
                     use $pbr$::MapNodeSizeInfoIndexForType;
                     // SAFETY:
                     // - The `MapIter` API forbids the backing map from being mutated for 'a,

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

@@ -188,7 +188,7 @@ void GenerateOneofDefinition(Context& ctx, const OneofDescriptor& oneof) {
         //~ This try_from is not a TryFrom impl so that it isn't
         //~ committed to as part of our public api.
         #[allow(dead_code)]
-        pub(crate) fn try_from(v: u32) -> Option<$case_enum_name$> {
+        pub(crate) fn try_from(v: u32) -> $Option$<$case_enum_name$> {
           match v {
             0 => Some($case_enum_name$::not_set),
             $try_from_cases$