build: modernize with "go fix -slicesbackward": use slices.Backward

fs/operations/operations.go

@@ -17,6 +17,7 @@ import (
 	"path"
 	"path/filepath"
 	"runtime"
+	"slices"
 	"sort"
 	"strconv"
 	"strings"
@@ -1569,8 +1570,7 @@ func Rmdirs(ctx context.Context, f fs.Fs, dir string, leaveRoot bool) error {
 
 	errCount := errcount.New()
 	// Delete all directories at the same level in parallel
-	for level := len(toDelete) - 1; level >= 0; level-- {
-		dirs := toDelete[level]
+	for level, dirs := range slices.Backward(toDelete) {
 		if len(dirs) == 0 {
 			continue
 		}
@@ -2511,8 +2511,8 @@ func DirMove(ctx context.Context, f fs.Fs, srcRemote, dstRemote string) (err err
 	}
 
 	// Remove the source directories in reverse order
-	for i := len(dirs) - 1; i >= 0; i-- {
-		err := f.Rmdir(ctx, dirs[i])
+	for _, dir := range slices.Backward(dirs) {
+		err := f.Rmdir(ctx, dir)
 		if err != nil {
 			return fmt.Errorf("RenameDir rmdir: %w", err)
 		}

fs/sync/sync.go

@@ -684,8 +684,7 @@ func (s *syncCopyMove) deleteEmptyDirectories(ctx context.Context, f fs.Fs, entr
 	sort.Sort(entries)
 	var errorCount int
 	var okCount int
-	for i := len(entries) - 1; i >= 0; i-- {
-		entry := entries[i]
+	for _, entry := range slices.Backward(entries) {
 		dir, ok := entry.(fs.Directory)
 		if ok {
 			// TryRmdir only deletes empty directories

fs/sync/sync_test.go

@@ -11,6 +11,7 @@ import (
 	"os"
 	"os/exec"
 	"runtime"
+	"slices"
 	"sort"
 	"strings"
 	"testing"
@@ -564,8 +565,7 @@ func TestSyncSetDelayedModTimes(t *testing.T) {
 
 	// Timestamp the directories in reverse order
 	ts := t1
-	for i := len(dirs) - 1; i >= 0; i-- {
-		dir := dirs[i]
+	for _, dir := range slices.Backward(dirs) {
 		_, err := operations.SetDirModTime(ctx, r.Flocal, nil, dir, ts)
 		require.NoError(t, err)
 		ts = ts.Add(time.Minute)