all: use strings.CutPrefix

Updates #42537

Change-Id: Ice23d7d36bbede27551cbc086119694f6a3b5e4a
GitHub-Last-Rev: 0d65208313
GitHub-Pull-Request: golang/go#55347
Reviewed-on: https://go-review.googlesource.com/c/go/+/432895
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/go/build/build.go

@@ -179,10 +179,11 @@ func hasSubdir(root, dir string) (rel string, ok bool) {
 		root += sep
 	}
 	dir = filepath.Clean(dir)
-	if !strings.HasPrefix(dir, root) {
+	after, found := strings.CutPrefix(dir, root)
+	if !found {
 		return "", false
 	}
-	return filepath.ToSlash(dir[len(root):]), true
+	return filepath.ToSlash(after), true
 }
 
 // readDir calls ctxt.ReadDir (if not nil) or else os.ReadDir.

src/go/printer/comment.go

@@ -36,15 +36,16 @@ func formatDocComment(list []*ast.Comment) []*ast.Comment {
 		kind = "//"
 		var b strings.Builder
 		for _, c := range list {
-			if !strings.HasPrefix(c.Text, "//") {
+			after, found := strings.CutPrefix(c.Text, "//")
+			if !found {
 				return list
 			}
 			// Accumulate //go:build etc lines separately.
-			if isDirective(c.Text[2:]) {
+			if isDirective(after) {
 				directives = append(directives, c)
 				continue
 			}
-			b.WriteString(strings.TrimPrefix(c.Text[2:], " "))
+			b.WriteString(strings.TrimPrefix(after, " "))
 			b.WriteString("\n")
 		}
 		text = b.String()