Rust: ensure we can handle name collision between local module and cr…

…ate dependency

This change updates the generated code to always prefix the names of crates
with `::` so that they can't conflict with local modules. I'm not aware of this
being an issue for anyone yet, but figured we might as well fix it proactively.

PiperOrigin-RevId: 703122266

rust/test/fields_with_imported_types.proto

@@ -20,3 +20,9 @@ message MsgWithFieldsWithImportedTypes {
     imported_types.ImportedEnum imported_enum_oneof = 4;
   }
 }
+
+// This will produce a module called imported_types_proto. Let's verify that we
+// can do this without conflicting with the crate of the same name.
+message ImportedTypesProto {
+  message Nested {}
+}

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

@@ -39,7 +39,7 @@ namespace compiler {
 namespace rust {
 
 std::string GetCrateName(Context& ctx, const FileDescriptor& dep) {
-  return RsSafeName(ctx.ImportPathToCrateName(dep.name()));
+  return absl::StrCat("::", RsSafeName(ctx.ImportPathToCrateName(dep.name())));
 }
 
 std::string GetRsFile(Context& ctx, const FileDescriptor& file) {