Reject message-set fields used in options

experimental/ir/ir_test.go

@@ -46,6 +46,7 @@ import (
 	"github.com/bufbuild/protocompile/internal/ext/slicesx"
 	compilerpb "github.com/bufbuild/protocompile/internal/gen/buf/compiler/v1alpha1"
 	"github.com/bufbuild/protocompile/internal/golden"
+	"github.com/bufbuild/protocompile/internal/messageset"
 	"github.com/bufbuild/protocompile/internal/prototest"
 )
 
@@ -85,6 +86,12 @@ type Test struct {
 	// Whether to output a symbol table. Useful for tests that build symbol
 	// tables.
 	Symtab bool `yaml:"symtab"`
+
+	// If true, skip this test when the protobuf-go runtime supports message
+	// sets (e.g. when built with the `protolegacy` build tag). Used for tests
+	// that assert on diagnostics that are only emitted when message sets are
+	// unsupported.
+	RequiresNoMessageSetSupport bool `yaml:"requires_no_message_set_support"`
 }
 
 func (t *Test) Unmarshal(path string, text string) error {
@@ -158,6 +165,10 @@ func TestIR(t *testing.T) {
 		test := new(Test)
 		require.NoError(t, test.Unmarshal(path, text))
 
+		if test.RequiresNoMessageSetSupport && messageset.CanSupportMessageSets() {
+			t.Skip("skipping: test requires that the protobuf-go runtime does not support message sets")
+		}
+
 		var files source.Opener = source.NewMap(maps.Collect(iterx.Map1To2(
 			slices.Values(test.Files),
 			func(f File) (string, *source.File) {

experimental/ir/lower_eval.go

@@ -496,6 +496,8 @@ validate:
 		mapFieldHelp(d)
 	}
 
+	checkMessageSetFieldUsage(member, expr.Key(), e.Report)
+
 	return member
 }
 

experimental/ir/lower_options.go

@@ -27,8 +27,30 @@ import (
 	"github.com/bufbuild/protocompile/experimental/token/keyword"
 	"github.com/bufbuild/protocompile/internal/ext/iterx"
 	"github.com/bufbuild/protocompile/internal/intern"
+	"github.com/bufbuild/protocompile/internal/messageset"
 )
 
+// checkMessageSetFieldUsage reports an error if the given field belongs to a
+// message set type. Message set wire format is a legacy proto1 feature that
+// is not supported.
+func checkMessageSetFieldUsage(field Member, span source.Spanner, r *report.Report) {
+	if field.IsZero() || !field.Container().IsMessageSet() {
+		return
+	}
+	if messageset.CanSupportMessageSets() {
+		return
+	}
+	extendee := field.Container()
+	r.Errorf(
+		"field `%s` may not be used in an option: it uses 'message set wire format' legacy proto1 feature which is not supported",
+		field.FullName(),
+	).Apply(
+		report.Snippet(span),
+		report.PageBreak,
+		report.Snippetf(extendee.AST().Stem(), "`%s` declared as message set here", extendee.FullName()),
+	)
+}
+
 // resolveEarlyOptions resolves options whose values must be discovered very
 // early during compilation. This does not create option values, nor does it
 // generate diagnostics; it simply records this information to special fields
@@ -483,6 +505,8 @@ func (r optionRef) resolve() {
 			}
 		}
 
+		checkMessageSetFieldUsage(field, pc, r.Report)
+
 		if pc.IsFirst() {
 			switch field.InternedFullName() {
 			case ids.MapEntry:

experimental/ir/testdata/options/values/message_set.proto

@@ -0,0 +1,47 @@
+// Copyright 2020-2025 Buf Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//% requires_no_message_set_support: true
+syntax = "proto2";
+
+package test;
+
+import "google/protobuf/descriptor.proto";
+
+message MessageSet {
+    option message_set_wire_format = true;
+    extensions 1 to max;
+}
+
+message Foo {
+    extend MessageSet {
+        optional Foo message_set_field = 12345;
+    }
+    optional string name = 1;
+}
+
+extend google.protobuf.FileOptions {
+    optional MessageSet m1 = 10101;
+    optional MessageSet m2 = 10102;
+}
+
+// Using a message-set extension as an option path is rejected.
+option (m1).(Foo.message_set_field).name = "abc";
+
+// Using a message-set extension inside a message literal is also rejected.
+option (m2) = {
+    [test.Foo.message_set_field] {
+        name: "abc"
+    }
+};

experimental/ir/testdata/options/values/message_set.proto.stderr.txt

@@ -0,0 +1,34 @@
+warning: message set types are deprecated
+  --> testdata/options/values/message_set.proto:23:12
+   |
+22 | message MessageSet {
+   | ------------------
+23 |     option message_set_wire_format = true;
+   |            ^^^^^^^^^^^^^^^^^^^^^^^ declared as message set here
+   |
+   = help: message set types are not implemented correctly in most Protobuf
+           implementations
+
+error: field `test.Foo.message_set_field` may not be used in an option: it uses
+       'message set wire format' legacy proto1 feature which is not supported
+  --> testdata/options/values/message_set.proto:40:12
+   |
+40 | option (m1).(Foo.message_set_field).name = "abc";
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^
+  ::: testdata/options/values/message_set.proto:22:1
+   |
+22 | message MessageSet {
+   | ------------------ `test.MessageSet` declared as message set here
+
+error: field `test.Foo.message_set_field` may not be used in an option: it uses
+       'message set wire format' legacy proto1 feature which is not supported
+  --> testdata/options/values/message_set.proto:44:5
+   |
+44 |     [test.Foo.message_set_field] {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  ::: testdata/options/values/message_set.proto:22:1
+   |
+22 | message MessageSet {
+   | ------------------ `test.MessageSet` declared as message set here
+
+encountered 2 errors and 1 warning