Support enums in DefaultValue

PiperOrigin-RevId: 596769895

src/google/protobuf/compiler/rust/BUILD.bazel

@@ -204,6 +204,7 @@ cc_library(
     strip_include_prefix = "/src",
     deps = [
         ":context",
+        ":naming",
         "//src/google/protobuf",
         "//src/google/protobuf/io:tokenizer",
         "@com_google_absl//absl/log:absl_check",

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

@@ -15,6 +15,8 @@
 #include "absl/strings/escaping.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_format.h"
+#include "google/protobuf/compiler/rust/context.h"
+#include "google/protobuf/compiler/rust/naming.h"
 #include "google/protobuf/descriptor.h"
 #include "google/protobuf/io/strtod.h"
 
@@ -23,7 +25,7 @@ namespace protobuf {
 namespace compiler {
 namespace rust {
 
-std::string DefaultValue(const FieldDescriptor& field) {
+std::string DefaultValue(Context& ctx, const FieldDescriptor& field) {
   switch (field.type()) {
     case FieldDescriptor::TYPE_DOUBLE:
       if (std::isfinite(field.default_value_double())) {
@@ -74,9 +76,20 @@ std::string DefaultValue(const FieldDescriptor& field) {
     case FieldDescriptor::TYPE_BYTES:
       return absl::StrFormat("b\"%s\"",
                              absl::CHexEscape(field.default_value_string()));
+    case FieldDescriptor::TYPE_ENUM:
+      // `$EnumName$::default()` might seem like the right choice here, but
+      // it is not. The default value for the enum type isn't the same as the
+      // field, since in `syntax = "proto2"`, an enum field can have a default
+      // value other than the first listed in the enum.
+      //
+      // Even in cases where there is no custom field default, `default()` can't
+      // be used. This is because the vtables for field mutators store the
+      // default value. They are `static`s which are constructed with a `const`
+      // expression. Trait methods in a `const` context aren't currently stable.
+      return absl::StrCat(RsTypePath(ctx, field),
+                          "::", EnumValueRsName(*field.default_value_enum()));
     case FieldDescriptor::TYPE_GROUP:
     case FieldDescriptor::TYPE_MESSAGE:
-    case FieldDescriptor::TYPE_ENUM:
       ABSL_LOG(FATAL) << "Unsupported field type: " << field.type_name();
   }
   ABSL_LOG(FATAL) << "unreachable";

src/google/protobuf/compiler/rust/accessors/helpers.h

@@ -10,6 +10,7 @@
 
 #include <string>
 
+#include "google/protobuf/compiler/rust/context.h"
 #include "google/protobuf/descriptor.h"
 
 namespace google {
@@ -22,7 +23,7 @@ namespace rust {
 // Both strings and bytes are represented as a byte string literal, i.e. in the
 // format `b"default value here"`. It is the caller's responsibility to convert
 // the byte literal to an actual string, if needed.
-std::string DefaultValue(const FieldDescriptor& field);
+std::string DefaultValue(Context& ctx, const FieldDescriptor& field);
 
 }  // namespace rust
 }  // namespace compiler

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

@@ -25,7 +25,7 @@ void SingularScalar::InMsgImpl(Context& ctx,
           {"field", field.name()},
           {"Scalar", RsTypePath(ctx, field)},
           {"hazzer_thunk", ThunkName(ctx, field, "has")},
-          {"default_value", DefaultValue(field)},
+          {"default_value", DefaultValue(ctx, field)},
           {"getter",
            [&] {
              ctx.Emit({}, R"rs(

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

@@ -68,7 +68,7 @@ void SingularString::InMsgImpl(Context& ctx,
                    {
                        {"field", field.name()},
                        {"proxied_type", proxied_type},
-                       {"default_val", DefaultValue(field)},
+                       {"default_val", DefaultValue(ctx, field)},
                        {"view_type", proxied_type},
                        {"transform_field_entry",
                         [&] {