Make rust_crate_mapping flag optional

If the flag is not passed, it means the current proto_library has no deps.

PiperOrigin-RevId: 598846330

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

@@ -67,12 +67,9 @@ absl::StatusOr<Options> Options::Parse(absl::string_view param) {
 
   auto mapping_arg = absl::c_find_if(
       args, [](auto& arg) { return arg.first == "bazel_crate_mapping"; });
-  if (mapping_arg == args.end()) {
-    return absl::InvalidArgumentError(
-        "Mandatory option `mapping` missing, please specify mapping file "
-        "path.");
+  if (mapping_arg != args.end()) {
+    opts.mapping_file_path = mapping_arg->second;
   }
-  opts.mapping_file_path = mapping_arg->second;
 
   return opts;
 }

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

@@ -23,8 +23,9 @@ namespace rust {
 
 absl::StatusOr<absl::flat_hash_map<std::string, std::string>>
 GetImportPathToCrateNameMap(const Options* opts) {
+  absl::flat_hash_map<std::string, std::string> mapping;
   if (opts->mapping_file_path.empty()) {
-    return absl::InvalidArgumentError("Mapping file path is not specified");
+    return mapping;
   }
   std::string mapping_contents;
   absl::Status status =
@@ -33,7 +34,6 @@ GetImportPathToCrateNameMap(const Options* opts) {
     return status;
   }
 
-  absl::flat_hash_map<std::string, std::string> mapping;
   std::vector<absl::string_view> lines =
       absl::StrSplit(mapping_contents, '\n', absl::SkipEmpty());
   size_t len = lines.size();

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

@@ -15,6 +15,7 @@
 
 using google::protobuf::compiler::rust::Options;
 using testing::Eq;
+using testing::IsEmpty;
 using testing::UnorderedElementsAreArray;
 
 namespace google {
@@ -70,14 +71,11 @@ TEST(RustGenerator, GetImportPathToCrateNameMapSimple) {
   EXPECT_THAT(result.value(), UnorderedElementsAreArray(expected));
 }
 
-TEST(RustGenerator,
-     GetImportPathToCrateNameMapErrorsOnNotSpecifiedMappingFile) {
+TEST(RustGenerator, GetImportPathToCrateNameMapForEmptyMappingFile) {
   const Options opts = {Kernel::kUpb};
   auto result = GetImportPathToCrateNameMap(&opts);
-  EXPECT_FALSE(result.ok());
-  EXPECT_THAT(result.status().code(), Eq(absl::StatusCode::kInvalidArgument));
-  EXPECT_THAT(result.status().message(),
-              Eq("Mapping file path is not specified"));
+  EXPECT_TRUE(result.ok());
+  EXPECT_THAT(result.value(), IsEmpty());
 }
 
 TEST(RustGenerator, GetImportPathToCrateNameMapErrorsOnMissingFile) {