rewrite gen.go to use panic

Makes the program much simpler. We don't need fancy error handling,
anyway.

gen.go

@@ -6,9 +6,6 @@
 package main
 
 import (
-	"bytes"
-	"flag"
-	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -18,15 +15,6 @@ import (
 )
 
 func main() {
-	if err := run(); err != nil {
-		fmt.Fprintln(os.Stderr, err)
-		os.Exit(1)
-	}
-}
-
-func run() error {
-	flag.Parse()
-
 	cfg := &packages.Config{Mode: packages.NeedName | packages.NeedFiles}
 	pkgs, err := packages.Load(cfg,
 		"cmd/gofmt",
@@ -40,60 +28,64 @@ func run() error {
 		"golang.org/x/tools/internal/semver",
 	)
 	if err != nil {
-		return err
+		panic(err)
 	}
 	for _, pkg := range pkgs {
-		var err error
 		switch pkg.PkgPath {
 		case "cmd/gofmt":
-			err = copyGofmt(pkg.GoFiles)
+			copyGofmt(pkg.GoFiles)
 		case "golang.org/x/tools/cmd/goimports":
-			err = copyGoimports(pkg.GoFiles)
+			copyGoimports(pkg.GoFiles)
 		default:
 			parts := strings.Split(pkg.PkgPath, "/")
 			dir := filepath.Join(append([]string{"gofumports"}, parts[3:]...)...)
-			err = copyInternal(pkg.GoFiles, dir)
-		}
-		if err != nil {
-			return err
+			copyInternal(pkg.GoFiles, dir)
 		}
 	}
-	return nil
 }
 
-func copyGofmt(files []string) error {
+func readFile(path string) string {
+	body, err := ioutil.ReadFile(path)
+	if err != nil {
+		panic(err)
+	}
+	return string(body)
+}
+
+func writeFile(path, body string) {
+	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
+		panic(err)
+	}
+	if err := ioutil.WriteFile(path, []byte(body), 0644); err != nil {
+		panic(err)
+	}
+}
+
+func copyGofmt(files []string) {
 	const extraSrc = `
 		// This is the only gofumpt change on gofmt's codebase, besides changing
 		// the name in the usage text.
 		internal.Gofumpt(fileSet, file)
 		`
-
-	for _, fpath := range files {
-		bodyBytes, err := ioutil.ReadFile(fpath)
-		if err != nil {
-			return err
-		}
-		body := string(bodyBytes) // to simplify operations later
-		name := filepath.Base(fpath)
+	for _, path := range files {
+		body := readFile(path)
+		name := filepath.Base(path)
 		switch name {
 		case "doc.go":
 			continue // we have our own
 		case "gofmt.go":
 			i := strings.Index(body, "res, err := format(")
 			if i < 0 {
-				return fmt.Errorf("could not insert the gofumpt source code")
+				panic("could not insert the gofumpt source code")
 			}
 			body = body[:i] + "\n" + extraSrc + "\n" + body[i:]
 		}
 		body = strings.Replace(body, "gofmt", "gofumpt", -1)
-		if err := ioutil.WriteFile(name, []byte(body), 0644); err != nil {
-			return err
-		}
+		writeFile(name, body)
 	}
-	return nil
 }
 
-func copyGoimports(files []string) error {
+func copyGoimports(files []string) {
 	const extraSrc = `
 		// This is the only gofumpt change on goimports's codebase, besides
 		// changing the name in the usage text.
@@ -102,58 +94,38 @@ func copyGoimports(files []string) error {
 			return err
 		}
 		`
-
-	for _, fpath := range files {
-		bodyBytes, err := ioutil.ReadFile(fpath)
-		if err != nil {
-			return err
-		}
-		bodyBytes = fixImports(bodyBytes)
-		body := string(bodyBytes) // to simplify operations later
-		name := filepath.Base(fpath)
+	for _, path := range files {
+		body := readFile(path)
+		body = fixImports(body)
+		name := filepath.Base(path)
 		switch name {
 		case "doc.go":
 			continue // we have our own
 		case "goimports.go":
 			i := strings.Index(body, "if !bytes.Equal")
 			if i < 0 {
-				return fmt.Errorf("could not insert the gofumports source code")
+				panic("could not insert the gofumports source code")
 			}
 			body = body[:i] + "\n" + extraSrc + "\n" + body[i:]
 		}
 		body = strings.Replace(body, "goimports", "gofumports", -1)
 
-		dst := filepath.Join("gofumports", name)
-		if err := ioutil.WriteFile(dst, []byte(body), 0644); err != nil {
-			return err
-		}
+		writeFile(filepath.Join("gofumports", name), body)
 	}
-	return nil
 }
 
-func copyInternal(files []string, dir string) error {
-	if err := os.MkdirAll(dir, 0755); err != nil {
-		return err
-	}
-	for _, fpath := range files {
-		body, err := ioutil.ReadFile(fpath)
-		if err != nil {
-			return err
-		}
+func copyInternal(files []string, dir string) {
+	for _, path := range files {
+		body := readFile(path)
 		body = fixImports(body)
-
-		name := filepath.Base(fpath)
-		dst := filepath.Join(dir, name)
-		if err := ioutil.WriteFile(dst, []byte(body), 0644); err != nil {
-			return err
-		}
+		name := filepath.Base(path)
+		writeFile(filepath.Join(dir, name), body)
 	}
-	return nil
 }
 
-func fixImports(body []byte) []byte {
-	return bytes.Replace(body,
-		[]byte("golang.org/x/tools/internal/"),
-		[]byte("mvdan.cc/gofumpt/gofumports/internal/"),
+func fixImports(body string) string {
+	return strings.Replace(body,
+		"golang.org/x/tools/internal/",
+		"mvdan.cc/gofumpt/gofumports/internal/",
 		-1)
 }