wire: add a diff command

go.mod

@@ -54,6 +54,7 @@ require (
 	github.com/mattn/goveralls v0.0.2 // indirect
 	github.com/opencensus-integrations/ocsql v0.1.1
 	github.com/pkg/errors v0.8.0 // indirect
+	github.com/pmezard/go-difflib v1.0.0
 	github.com/prometheus/client_golang v0.9.0 // indirect
 	github.com/prometheus/common v0.0.0-20181015124227-bcb74de08d37 // indirect
 	github.com/prometheus/procfs v0.0.0-20181005140218-185b4288413d // indirect

wire/cmd/wire/main.go

@@ -23,17 +23,19 @@ import (
 	"fmt"
 	"go/token"
 	"go/types"
+	"io/ioutil"
 	"os"
 	"reflect"
 	"sort"
 	"strconv"
 	"strings"
 
 	"github.com/google/go-cloud/wire/internal/wire"
+	"github.com/pmezard/go-difflib/difflib"
 	"golang.org/x/tools/go/types/typeutil"
 )
 
-const usage = "usage: wire [gen] [PKG] | wire show [...] | wire check [...]"
+const usage = "usage: wire [gen|diff|show|check] [...]"
 
 func main() {
 	var err error
@@ -49,6 +51,10 @@ func main() {
 		err = check(".")
 	case len(os.Args) > 2 && os.Args[1] == "check":
 		err = check(os.Args[2:]...)
+	case len(os.Args) == 2 && os.Args[1] == "diff":
+		err = diff(".")
+	case len(os.Args) > 2 && os.Args[1] == "diff":
+		err = diff(os.Args[2:]...)
 	case len(os.Args) == 2 && os.Args[1] == "gen":
 		err = generate(".")
 	case len(os.Args) > 2 && os.Args[1] == "gen":
@@ -108,6 +114,54 @@ func generate(pkgs ...string) error {
 	return nil
 }
 
+// diff runs the diff subcommand.
+//
+// Given one or more packages, diff will generate the content for the
+// wire_gen.go file, and output the diff against the existing file.
+func diff(pkgs ...string) error {
+	wd, err := os.Getwd()
+	if err != nil {
+		return err
+	}
+	outs, errs := wire.Generate(context.Background(), wd, os.Environ(), pkgs)
+	if len(errs) > 0 {
+		logErrors(errs)
+		return errors.New("generate failed")
+	}
+	if len(outs) == 0 {
+		return nil
+	}
+	success := true
+	for _, out := range outs {
+		if len(out.Errs) > 0 {
+			fmt.Fprintf(os.Stderr, "%s: generate failed\n", out.PkgPath)
+			logErrors(out.Errs)
+			success = false
+		}
+		if len(out.Content) == 0 {
+			// No Wire output. Maybe errors, maybe no Wire directives.
+			continue
+		}
+		// Assumes the current file is empty if we can't read it.
+		cur, _ := ioutil.ReadFile(out.OutputPath)
+		if diff, err := difflib.GetUnifiedDiffString(difflib.UnifiedDiff{
+			A: difflib.SplitLines(string(cur)),
+			B: difflib.SplitLines(string(out.Content)),
+		}); err == nil {
+			if diff != "" {
+				fmt.Fprintf(os.Stderr, "%s: diff from %s:\n%s", out.PkgPath, out.OutputPath, diff)
+			}
+		} else {
+			fmt.Fprintf(os.Stderr, "%s: failed to diff %s: %v\n", out.PkgPath, out.OutputPath, err)
+			success = false
+		}
+	}
+	if !success {
+		return errors.New("at least one generate failure")
+	}
+	return nil
+}
+
 // show runs the show subcommand.
 //
 // Given one or more packages, show will find all the provider sets