strings, bytes: add CutPrefix and CutSuffix

Fixes #42537

Change-Id: Ie03c2614ffee30ebe707acad6b9f6c28fb134a45
Reviewed-on: https://go-review.googlesource.com/c/go/+/407176
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Changkun Ou <mail@changkun.de>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/strings/strings.go

@@ -1187,3 +1187,25 @@ func Cut(s, sep string) (before, after string, found bool) {
 	}
 	return s, "", false
 }
+
+// CutPrefix returns s without the provided leading prefix string
+// and reports whether it found the prefix.
+// If s doesn't start with prefix, CutPrefix returns s, false.
+// If prefix is the empty string, CutPrefix returns s, true.
+func CutPrefix(s, prefix string) (after string, found bool) {
+	if !HasPrefix(s, prefix) {
+		return s, false
+	}
+	return s[len(prefix):], true
+}
+
+// CutSuffix returns s without the provided ending suffix string
+// and reports whether it found the suffix.
+// If s doesn't end with suffix, CutSuffix returns s, false.
+// If suffix is the empty string, CutSuffix returns s, true.
+func CutSuffix(s, suffix string) (after string, found bool) {
+	if !HasSuffix(s, suffix) {
+		return s, false
+	}
+	return s[:len(s)-len(suffix)], true
+}