print changed lines

common.go

@@ -114,7 +114,7 @@ func (cmd Common) Tidy() {
 	ErrFatalf(err, "unable to change directory: %v\n", err)
 
 	for repeat := 2; repeat > 0; repeat-- {
-		gomod := exec.Command("go", "mod", "tidy", "-v")
+		gomod := exec.Command("go", "mod", "tidy")
 		gomod.Env = append(os.Environ(), "GO111MODULE=on")
 		gomod.Stdout, gomod.Stderr = os.Stderr, os.Stderr
 		err = gomod.Run()

go.mod

@@ -1 +1,3 @@
 module storj.io/gospace
+
+require github.com/kylelemons/godebug v0.0.0-20170820004349-d65d576e9348

go.sum

@@ -0,0 +1,2 @@
+github.com/kylelemons/godebug v0.0.0-20170820004349-d65d576e9348 h1:MtvEpTB6LX3vkb4ax0b5D2DHbNAUsen0Gx5wZoq3lV4=
+github.com/kylelemons/godebug v0.0.0-20170820004349-d65d576e9348/go.mod h1:B69LEHPfb2qLo0BaaOLcbitczOKLWTsrBG9LczfCD4k=

mod.go

@@ -4,10 +4,15 @@
 package main
 
 import (
+	"bytes"
 	"flag"
 	"fmt"
+	"io/ioutil"
 	"os"
 	"path/filepath"
+	"strings"
+
+	"github.com/kylelemons/godebug/diff"
 )
 
 type IsTidy struct {
@@ -24,16 +29,34 @@ func (cmd *IsTidy) Parse(args []string) error {
 func (cmd *IsTidy) Exec() {
 	repodir := cmd.RepoDir()
 
-	before, err := HashFiles(filepath.Join(repodir, "go.mod"))
+	before, err := ioutil.ReadFile(filepath.Join(repodir, "go.mod"))
 	ErrFatal(err)
 
 	cmd.Tidy()
 
-	after, err := HashFiles(filepath.Join(repodir, "go.mod"))
+	after, err := ioutil.ReadFile(filepath.Join(repodir, "go.mod"))
 	ErrFatal(err)
 
-	if before != after {
+	if string(before) != string(after) {
 		fmt.Fprintln(os.Stderr, "go.mod is not tidy")
+		fmt.Fprintln(os.Stderr, difflines(string(before), string(after)))
 		os.Exit(1)
 	}
 }
+
+func difflines(a, b string) string {
+	alines, blines := strings.Split(a, "\n"), strings.Split(b, "\n")
+
+	chunks := diff.DiffChunks(alines, blines)
+
+	buf := new(bytes.Buffer)
+	for _, c := range chunks {
+		for _, line := range c.Added {
+			fmt.Fprintf(buf, "+%s\n", line)
+		}
+		for _, line := range c.Deleted {
+			fmt.Fprintf(buf, "-%s\n", line)
+		}
+	}
+	return strings.TrimRight(buf.String(), "\n")
+}