sync with go1.19beta1's gofmt

The changes since go1.18 roughly include:

* new diff package, which is much faster

* error on empty Go files

* use SkipObjectResolution to avoid unnecessary work

format/simplify.go

@@ -53,22 +53,26 @@ func (s simplifier) Visit(node ast.Node) ast.Visitor {
 		// can be simplified to: s[a:]
 		// if s is "simple enough" (for now we only accept identifiers)
 		//
-		// Note: This may not be correct because len may have been redeclared in another
-		//       file belonging to the same package. However, this is extremely unlikely
-		//       and so far (April 2016, after years of supporting this rewrite feature)
+		// Note: This may not be correct because len may have been redeclared in
+		//       the same package. However, this is extremely unlikely and so far
+		//       (April 2022, after years of supporting this rewrite feature)
 		//       has never come up, so let's keep it working as is (see also #15153).
+		//
+		// Also note that this code used to use go/ast's object tracking,
+		// which was removed in exchange for go/parser.Mode.SkipObjectResolution.
+		// False positives are extremely unlikely as described above,
+		// and go/ast's object tracking is incomplete in any case.
 		if n.Max != nil {
 			// - 3-index slices always require the 2nd and 3rd index
 			break
 		}
-		if s, _ := n.X.(*ast.Ident); s != nil && s.Obj != nil {
-			// the array/slice object is a single, resolved identifier
+		if s, _ := n.X.(*ast.Ident); s != nil {
+			// the array/slice object is a single identifier
 			if call, _ := n.High.(*ast.CallExpr); call != nil && len(call.Args) == 1 && !call.Ellipsis.IsValid() {
 				// the high expression is a function call with a single argument
-				if fun, _ := call.Fun.(*ast.Ident); fun != nil && fun.Name == "len" && fun.Obj == nil {
-					// the function called is "len" and it is not locally defined; and
-					// because we don't have dot imports, it must be the predefined len()
-					if arg, _ := call.Args[0].(*ast.Ident); arg != nil && arg.Obj == s.Obj {
+				if fun, _ := call.Fun.(*ast.Ident); fun != nil && fun.Name == "len" {
+					// the function called is "len"
+					if arg, _ := call.Args[0].(*ast.Ident); arg != nil && arg.Name == s.Name {
 						// the len argument is the array/slice object
 						n.High = nil
 					}

gofmt.go

@@ -25,6 +25,8 @@ import (
 	"strings"
 	"sync"
 
+	// TODO: we can soon use os/exec thanks to
+	// https://go.dev/issue/43724
 	"golang.org/x/sync/semaphore"
 	exec "golang.org/x/sys/execabs"
 
@@ -100,7 +102,7 @@ func usage() {
 }
 
 func initParserMode() {
-	parserMode = parser.ParseComments
+	parserMode = parser.ParseComments | parser.SkipObjectResolution
 	if *allErrors {
 		parserMode |= parser.AllErrors
 	}
@@ -348,12 +350,9 @@ func processFile(filename string, info fs.FileInfo, in io.Reader, r *reporter, e
 			}
 		}
 		if *doDiff {
-			data, err := diffWithReplaceTempFile(src, res, filename)
-			if err != nil {
-				return fmt.Errorf("computing diff: %s", err)
-			}
-			fmt.Fprintf(r, "diff -u %s %s\n", filepath.ToSlash(filename+".orig"), filepath.ToSlash(filename))
-			r.Write(data)
+			newName := filepath.ToSlash(filename)
+			oldName := newName + ".orig"
+			r.Write(diff.Diff(oldName, src, newName, res))
 		}
 	}
 
@@ -412,7 +411,12 @@ func readFile(filename string, info fs.FileInfo, in io.Reader) ([]byte, error) {
 	// stop to avoid corrupting it.)
 	src := make([]byte, size+1)
 	n, err := io.ReadFull(in, src)
-	if err != nil && err != io.ErrUnexpectedEOF {
+	switch err {
+	case nil, io.EOF, io.ErrUnexpectedEOF:
+		// io.ReadFull returns io.EOF (for an empty file) or io.ErrUnexpectedEOF
+		// (for a non-empty file) if the file was changed unexpectedly. Continue
+		// with comparing file sizes in those cases.
+	default:
 		return nil, err
 	}
 	if n < size {
@@ -589,43 +593,6 @@ func fileWeight(path string, info fs.FileInfo) int64 {
 	return info.Size()
 }
 
-func diffWithReplaceTempFile(b1, b2 []byte, filename string) ([]byte, error) {
-	data, err := diff.Diff("gofumpt", b1, b2)
-	if len(data) > 0 {
-		return replaceTempFilename(data, filename)
-	}
-	return data, err
-}
-
-// replaceTempFilename replaces temporary filenames in diff with actual one.
-//
-// --- /tmp/gofumpt316145376	2017-02-03 19:13:00.280468375 -0500
-// +++ /tmp/gofumpt617882815	2017-02-03 19:13:00.280468375 -0500
-// ...
-// ->
-// --- path/to/file.go.orig	2017-02-03 19:13:00.280468375 -0500
-// +++ path/to/file.go	2017-02-03 19:13:00.280468375 -0500
-// ...
-func replaceTempFilename(diff []byte, filename string) ([]byte, error) {
-	bs := bytes.SplitN(diff, []byte{'\n'}, 3)
-	if len(bs) < 3 {
-		return nil, fmt.Errorf("got unexpected diff for %s", filename)
-	}
-	// Preserve timestamps.
-	var t0, t1 []byte
-	if i := bytes.LastIndexByte(bs[0], '\t'); i != -1 {
-		t0 = bs[0][i:]
-	}
-	if i := bytes.LastIndexByte(bs[1], '\t'); i != -1 {
-		t1 = bs[1][i:]
-	}
-	// Always print filepath with slash separator.
-	f := filepath.ToSlash(filename)
-	bs[0] = []byte(fmt.Sprintf("--- %s%s", f+".orig", t0))
-	bs[1] = []byte(fmt.Sprintf("+++ %s%s", f, t1))
-	return bytes.Join(bs, []byte{'\n'}), nil
-}
-
 const chmodSupported = runtime.GOOS != "windows"
 
 // backupFile writes data to a new file named filename<number> with permissions perm,