modfile: use slices.Backward

Change-Id: Ie29e68182cceded6488b6cf2f8c9e663f9701902
Reviewed-on: https://go-review.googlesource.com/c/mod/+/779720
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

modfile/read.go

diff --git a/modfile/read.go b/modfile/read.go
index 504a2f1..5b528c7 100644
--- a/modfile/read.go
+++ b/modfile/read.go
@@ -9,6 +9,7 @@
 	"errors"
 	"fmt"
 	"os"
+	"slices"
 	"strconv"
 	"strings"
 	"unicode"
@@ -105,8 +106,7 @@
 	if hint == nil {
 		// If no hint given, add to the last statement of the given type.
 	Loop:
-		for i := len(x.Stmt) - 1; i >= 0; i-- {
-			stmt := x.Stmt[i]
+		for _, stmt := range slices.Backward(x.Stmt) {
 			switch stmt := stmt.(type) {
 			case *Line:
 				if stmt.Token != nil && stmt.Token[0] == tokens[0] {
@@ -718,9 +718,7 @@
 	}
 
 	// Assign suffix comments to syntax immediately before.
-	for i := len(in.post) - 1; i >= 0; i-- {
-		x := in.post[i]
-
+	for _, x := range slices.Backward(in.post) {
 		start, end := x.Span()
 		if debug {
 			fmt.Fprintf(os.Stderr, "post %T :%d:%d #%d :%d:%d #%d\n", x, start.Line, start.LineRune, start.Byte, end.Line, end.LineRune, end.Byte)

modfile/rule.go

diff --git a/modfile/rule.go b/modfile/rule.go
index 65eb72b..9ab203b 100644
--- a/modfile/rule.go
+++ b/modfile/rule.go
@@ -1784,8 +1784,7 @@
 	// Remove duplicate replacements.
 	// Later replacements take priority over earlier ones.
 	haveReplace := make(map[module.Version]bool)
-	for i := len(*replace) - 1; i >= 0; i-- {
-		x := (*replace)[i]
+	for _, x := range slices.Backward(*replace) {
 		if haveReplace[x.Old] {
 			kill[x.Syntax] = true
 			continue