cmd/protoc-gen-go: add test for "import option" directive

See https://protobuf.dev/editions/overview/#import-option for context.

Change-Id: Iea73299fdd828301fe451ca3c6b9234e8dd9acf3
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/699715
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mike Kruskal <mkruskal@google.com>
Reviewed-by: Lasse Folger <lassefolger@google.com>

Author: stapelberg

cmd/protoc-gen-go/importoption_test.go

@@ -0,0 +1,56 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	importoptionpb "google.golang.org/protobuf/cmd/protoc-gen-go/testdata/import_option"
+	"google.golang.org/protobuf/encoding/protowire"
+	"google.golang.org/protobuf/proto"
+	"google.golang.org/protobuf/types/descriptorpb"
+
+	// Ensure the custom option is linked into this test binary.
+	// NB: import_option_unlinked is not linked into this test binary.
+	importoptioncustompb "google.golang.org/protobuf/cmd/protoc-gen-go/testdata/import_option_custom"
+)
+
+func TestImportOption(t *testing.T) {
+	var nilMessage *importoptionpb.TestMessage
+	md := nilMessage.ProtoReflect().Descriptor()
+
+	// Options from import option that are linked in should be available through
+	// the extension API as usual.
+	{
+		fd := md.Fields().ByName("hello")
+		fopts := fd.Options().(*descriptorpb.FieldOptions)
+		if !proto.HasExtension(fopts, importoptioncustompb.E_FieldOption) {
+			t.Errorf("FieldDescriptor(hello) does not have FieldOption extension set")
+		}
+	}
+
+	// Options from import option that are not linked in should be in unknown bytes.
+	{
+		fd := md.Fields().ByName("world")
+		fopts := fd.Options().(*descriptorpb.FieldOptions)
+		unknown := fopts.ProtoReflect().GetUnknown()
+		var fields []protowire.Number
+		b := unknown
+		for len(b) > 0 {
+			num, _, n := protowire.ConsumeField(b)
+			if n < 0 {
+				t.Errorf("FieldDescriptor(world) contains invalid wire format: ConsumeField = %d", n)
+			}
+			fields = append(fields, num)
+			b = b[n:]
+		}
+		want := []protowire.Number{504589222}
+		if diff := cmp.Diff(want, fields); diff != "" {
+			t.Errorf("FieldDescriptor(world) unknown bytes contain unexpected fields: diff (-want +got):\n%s", diff)
+		}
+	}
+
+}

cmd/protoc-gen-go/testdata/gen_test.go

@@ -0,0 +1,18 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+edition = "2024";
+
+package testimportoption;
+
+import "google/protobuf/descriptor.proto";
+import option "cmd/protoc-gen-go/testdata/import_option_custom/import_option_custom.proto";
+import option "cmd/protoc-gen-go/testdata/import_option_unlinked/import_option_unlinked.proto";
+
+option go_package = "google.golang.org/protobuf/cmd/protoc-gen-go/testdata/import_option";
+
+message TestMessage {
+  string hello = 1 [(testimportoption_custom.field_option) = { plain_field: 23 }];
+  string world = 2 [(testimportoption_unlinked.field_option) = { plain_field: 23 }];
+}