gencopy: print a ProTip with the .update target when a check fails

When `bazel test //path:foo_proto_compile_test` fails because a checked-in
generated file has drifted from what `protoc` would produce, the existing
error tells you which file pair mismatched and shows the diff, but leaves
the developer to figure out which target regenerates the source-of-truth
copy.

Append a one-line ProTip to the mismatch error message, naming the
`.update` target so it can be copy-pasted into `bazel run`:

  gencopy mismatch "foo.pb.go.gen" vs. "foo.pb.go" (-want +got):
  ...

  ProTip: to regenerate, run:
      bazel run //proto:foo_proto_compile.update

The label is derived from pkg.TargetLabel (the proto_compile rule's full
label, including any external-repo prefix) by replacing the rule-name
component with cfg.UpdateTargetLabelName -- both already populated by the
proto_compile_gencopy_test Starlark rule via gencopy_config().

Author: pcj

cmd/gencopy/gencopy.go

@@ -11,6 +11,7 @@ import (
 	"os"
 	"path/filepath"
 	"strconv"
+	"strings"
 
 	"github.com/google/go-cmp/cmp"
 )
@@ -105,7 +106,7 @@ func readFileAsString(filename string) (string, error) {
 	return string(bytes), nil
 }
 
-func check(_ *Config, pkg *PackageConfig, pairs []*SrcDst) error {
+func check(cfg *Config, pkg *PackageConfig, pairs []*SrcDst) error {
 	for _, pair := range pairs {
 		expected, err := readFileAsString(pair.Src)
 		if err != nil {
@@ -117,7 +118,10 @@ func check(_ *Config, pkg *PackageConfig, pairs []*SrcDst) error {
 		}
 
 		if diff := cmp.Diff(expected, actual); diff != "" {
-			return fmt.Errorf("gencopy mismatch %q vs. %q (-want +got):\n%s", pair.Src, pair.Dst, diff)
+			return fmt.Errorf(
+				"gencopy mismatch %q vs. %q (-want +got):\n%s\n%s",
+				pair.Src, pair.Dst, diff, regenerateProTip(pkg.TargetLabel, cfg.UpdateTargetLabelName),
+			)
 		}
 	}
 
@@ -129,6 +133,22 @@ func check(_ *Config, pkg *PackageConfig, pairs []*SrcDst) error {
 	return nil
 }
 
+// regenerateProTip returns a friendly hint pointing the developer at the
+// `.update` target that regenerates the checked-in copies. targetLabel is the
+// proto_compile rule's label (e.g. "//proto:foo_proto_compile" or
+// "@@repo//proto:foo_proto_compile"); updateName is the .update target's
+// rule name (e.g. "foo_proto_compile.update").
+func regenerateProTip(targetLabel, updateName string) string {
+	if updateName == "" {
+		return ""
+	}
+	updateLabel := updateName
+	if idx := strings.LastIndex(targetLabel, ":"); idx >= 0 {
+		updateLabel = targetLabel[:idx+1] + updateName
+	}
+	return fmt.Sprintf("\nProTip: to regenerate, run:\n    bazel run %s\n", updateLabel)
+}
+
 func update(cfg *Config, pkg *PackageConfig, pairs []*SrcDst) error {
 	for _, pair := range pairs {
 		pair.Dst += cfg.Extension

cmd/gencopy/gencopy_test.go

@@ -144,6 +144,42 @@ func TestRunPkg(t *testing.T) {
 	}
 }
 
+func TestRegenerateProTip(t *testing.T) {
+	for name, tc := range map[string]struct {
+		targetLabel string
+		updateName  string
+		want        string
+	}{
+		"empty update name": {
+			targetLabel: "//proto:foo_proto_compile",
+			updateName:  "",
+			want:        "",
+		},
+		"local target": {
+			targetLabel: "//proto:foo_proto_compile",
+			updateName:  "foo_proto_compile.update",
+			want:        "\nProTip: to regenerate, run:\n    bazel run //proto:foo_proto_compile.update\n",
+		},
+		"external repo target": {
+			targetLabel: "@@some_repo//pkg:foo_proto_compile",
+			updateName:  "foo_proto_compile.update",
+			want:        "\nProTip: to regenerate, run:\n    bazel run @@some_repo//pkg:foo_proto_compile.update\n",
+		},
+		"target with no colon": {
+			targetLabel: "raw_label_no_colon",
+			updateName:  "foo_proto_compile.update",
+			want:        "\nProTip: to regenerate, run:\n    bazel run foo_proto_compile.update\n",
+		},
+	} {
+		t.Run(name, func(t *testing.T) {
+			got := regenerateProTip(tc.targetLabel, tc.updateName)
+			if got != tc.want {
+				t.Errorf("regenerateProTip: got %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
+
 // listFiles - convenience debugging function to log the files under a given dir
 func listFiles(t *testing.T, dir string) error {
 	return filepath.Walk(dir, func(path string, info os.FileInfo, err error) error {