all: implement strip_enum_prefix editions feature

This change required updating editions_defaults.binpb with
--edition_defaults_maximum=2024 so that we can use edition 2024 in our
testdata/enumprefix.proto test file.

For end users, this feature will only be available once:
1. protoc declares support for edition 2024 (expected early 2025).
2. protoc-gen-go declares support for edition 2024.

related to golang/protobuf#513

Change-Id: Ib8daeecae39ef32eff942279707d256c312f2a53
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/618979
Reviewed-by: Lasse Folger <lassefolger@google.com>
Reviewed-by: Mike Kruskal <mkruskal@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>

cmd/protoc-gen-go/enumprefix_test.go

@@ -0,0 +1,29 @@
+// Copyright 2024 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"
+
+	"google.golang.org/protobuf/cmd/protoc-gen-go/testdata/enumprefix"
+)
+
+func TestStripEnumPrefix(t *testing.T) {
+	// The file default for enumprefix.proto is to strip prefixes:
+	_ = enumprefix.Strip_ZERO
+
+	// The enum Both should generate both names:
+	_ = enumprefix.Both_ZERO
+	_ = enumprefix.Both_BOTH_ZERO
+
+	// The enum BothNoPrefix is configured to generate both names, but there is
+	// no prefix to be stripped, so only an unmodified name is generated.
+	_ = enumprefix.BothNoPrefix_ZERO
+
+	// The enum BothButOne is configured to generate both names, except for the
+	// ONE value, where only the prefixed name is generated.
+	_ = enumprefix.BothButOne_ZERO
+	_ = enumprefix.BothButOne_BOTH_BUT_ONE_ONE
+}

cmd/protoc-gen-go/internal_gengo/main.go

@@ -260,6 +260,7 @@ func genEnum(g *protogen.GeneratedFile, f *fileInfo, e *enumInfo) {
 
 	// Enum value constants.
 	g.P("const (")
+	anyOldName := false
 	for _, value := range e.Values {
 		g.AnnotateSymbol(value.GoIdent.GoName, protogen.Annotation{Location: value.Location})
 		leadingComments := appendDeprecationSuffix(value.Comments.Leading,
@@ -268,9 +269,26 @@ func genEnum(g *protogen.GeneratedFile, f *fileInfo, e *enumInfo) {
 		g.P(leadingComments,
 			value.GoIdent, " ", e.GoIdent, " = ", value.Desc.Number(),
 			trailingComment(value.Comments.Trailing))
+
+		if value.PrefixedAlias.GoName != "" &&
+			value.PrefixedAlias.GoName != value.GoIdent.GoName {
+			anyOldName = true
+		}
 	}
 	g.P(")")
 	g.P()
+	if anyOldName {
+		g.P("// Old (prefixed) names for ", e.GoIdent, " enum values.")
+		g.P("const (")
+		for _, value := range e.Values {
+			if value.PrefixedAlias.GoName != "" &&
+				value.PrefixedAlias.GoName != value.GoIdent.GoName {
+				g.P(value.PrefixedAlias, " ", e.GoIdent, " = ", value.GoIdent)
+			}
+		}
+		g.P(")")
+		g.P()
+	}
 
 	// Enum value maps.
 	g.P("// Enum value maps for ", e.GoIdent, ".")

cmd/protoc-gen-go/testdata/enumprefix/enumprefix.proto

@@ -0,0 +1,39 @@
+// Copyright 2024 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 goproto.protoc.enumprefix;
+
+import "google/protobuf/go_features.proto";
+
+option go_package = "google.golang.org/protobuf/cmd/protoc-gen-go/testdata/enumprefix";
+option features.(pb.go).strip_enum_prefix = STRIP_ENUM_PREFIX_STRIP;
+
+enum Strip {
+  STRIP_ZERO = 0;
+  STRIP_ONE = 1;
+}
+
+enum Both {
+  option features.(pb.go).strip_enum_prefix = STRIP_ENUM_PREFIX_GENERATE_BOTH;
+
+  BOTH_ZERO = 0;
+  BOTH_ONE = 1;
+}
+
+enum BothNoPrefix {
+  option features.(pb.go).strip_enum_prefix = STRIP_ENUM_PREFIX_GENERATE_BOTH;
+
+  ZERO = 0;
+  ONE = 1;
+}
+
+enum BothButOne {
+  option features.(pb.go).strip_enum_prefix = STRIP_ENUM_PREFIX_GENERATE_BOTH;
+
+  BOTH_BUT_ONE_ZERO = 0;
+  BOTH_BUT_ONE_ONE = 1
+      [features.(pb.go).strip_enum_prefix = STRIP_ENUM_PREFIX_KEEP];
+}