all: start using slices.Backward

The resulting code is slightly smaller, but what's nice about it
is that it is much easier to understand its purpose.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I89d86426c9cf983e987cbf326d4e81cac87bfe09
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1209067
Reviewed-by: Roger Peppe <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: mvdan

cue/types.go

@@ -21,6 +21,7 @@ import (
 	"io"
 	"math"
 	"math/big"
+	"slices"
 	"strings"
 
 	"github.com/cockroachdb/apd/v3"
@@ -1579,8 +1580,8 @@ func (v Value) FillPath(p Path, x interface{}) Value {
 	default:
 		expr = convert.GoValueToValue(ctx, x, true)
 	}
-	for i := len(p.path) - 1; i >= 0; i-- {
-		switch sel := p.path[i]; sel.Type() {
+	for _, sel := range slices.Backward(p.path) {
+		switch sel.Type() {
 		case StringLabel | PatternConstraint:
 			expr = &adt.StructLit{Decls: []adt.Decl{
 				&adt.BulkOptionalField{

encoding/jsonschema/util.go

@@ -112,9 +112,7 @@ func pathRefSyntax(cuePath cue.Path, root ast.Expr) (ast.Expr, error) {
 // be exposed as a method on [cue.Path], say
 // `SyntaxForDefinition` or something.
 func exprAtPath(path cue.Path, expr ast.Expr) (ast.Expr, error) {
-	sels := path.Selectors()
-	for i := len(sels) - 1; i >= 0; i-- {
-		sel := sels[i]
+	for i, sel := range slices.Backward(path.Selectors()) {
 		label, err := labelForSelector(sel)
 		if err != nil {
 			return nil, err

encoding/protobuf/parse.go

@@ -20,6 +20,7 @@ import (
 	"os"
 	"path"
 	"path/filepath"
+	"slices"
 	"strconv"
 	"strings"
 	"text/scanner"
@@ -260,8 +261,8 @@ func (p *protoConverter) resolve(pos scanner.Position, name string, options []*p
 	if strings.HasPrefix(name, ".") {
 		return p.resolveTopScope(pos, name[1:], options)
 	}
-	for i := len(p.scope) - 1; i > 0; i-- {
-		if m, ok := p.scope[i][name]; ok {
+	for _, scope := range slices.Backward(p.scope) {
+		if m, ok := scope[name]; ok {
 			return m.cue()
 		}
 	}

internal/encoding/yaml/decode.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"regexp"
+	"slices"
 	"strconv"
 	"strings"
 	"sync"
@@ -420,8 +421,8 @@ func (d *decoder) merge(yn *yaml.Node, m *ast.StructLit, multiline bool) error {
 		return d.insertMap(yn.Alias, m, multiline, true)
 	case yaml.SequenceNode:
 		// Step backwards as earlier nodes take precedence.
-		for i := len(yn.Content) - 1; i >= 0; i-- {
-			if err := d.merge(yn.Content[i], m, multiline); err != nil {
+		for _, c := range slices.Backward(yn.Content) {
+			if err := d.merge(c, m, multiline); err != nil {
 				return err
 			}
 		}

internal/mod/mvs/mvs.go

@@ -257,8 +257,7 @@ func Req[V comparable](mainModule V, base []string, reqs Reqs[V]) ([]V, error) {
 		haveBase[path] = true
 	}
 	// Now the reverse postorder to bring in anything else.
-	for i := len(postorder) - 1; i >= 0; i-- {
-		m := postorder[i]
+	for _, m := range slices.Backward(postorder) {
 		if max[reqs.Path(m)] != reqs.Version(m) {
 			// Older version.
 			continue

mod/modcache/fetch.go

@@ -10,6 +10,7 @@ import (
 	"math/rand"
 	"os"
 	"path/filepath"
+	"slices"
 	"strconv"
 	"strings"
 
@@ -305,8 +306,8 @@ func makeDirsReadOnly(dir string) {
 	})
 
 	// Run over list backward to chmod children before parents.
-	for i := len(dirs) - 1; i >= 0; i-- {
-		os.Chmod(dirs[i].path, dirs[i].mode&^0222)
+	for _, dir := range slices.Backward(dirs) {
+		os.Chmod(dir.path, dir.mode&^0222)
 	}
 }
 

mod/modconfig/modconfig.go

@@ -9,6 +9,7 @@ import (
 	"io/fs"
 	"net/http"
 	"os"
+	"slices"
 	"strings"
 	"sync"
 
@@ -359,8 +360,8 @@ func getenvFunc(env []string) func(string) string {
 		return os.Getenv
 	}
 	return func(key string) string {
-		for i := len(env) - 1; i >= 0; i-- {
-			if e := env[i]; len(e) >= len(key)+1 && e[len(key)] == '=' && e[:len(key)] == key {
+		for _, e := range slices.Backward(env) {
+			if len(e) >= len(key)+1 && e[len(key)] == '=' && e[:len(key)] == key {
 				return e[len(key)+1:]
 			}
 		}

tools/flow/run.go

@@ -28,6 +28,7 @@ package flow
 import (
 	"fmt"
 	"os"
+	"slices"
 
 	"cuelang.org/go/cue/errors"
 	"cuelang.org/go/internal/core/adt"
@@ -210,14 +211,13 @@ func (c *Controller) updateTaskResults(t *Task) bool {
 	}
 
 	expr := t.update
-	for i := len(t.labels) - 1; i >= 0; i-- {
-		label := t.labels[i]
+	for _, label := range slices.Backward(t.labels) {
 		switch label.Typ() {
 		case adt.StringLabel, adt.HiddenLabel:
 			expr = &adt.StructLit{
 				Decls: []adt.Decl{
 					&adt.Field{
-						Label: t.labels[i],
+						Label: label,
 						Value: expr,
 					},
 				},