strings: TrimLeft fails in certain cases

[sanketsudake]

What version of Go are you using (go version)?

$ go version
go version go1.20.7 darwin/amd64

Does this issue reproduce with the latest release?

Yes

https://go.dev/play/p/Abefs5j3At0

What operating system and processor architecture are you using (go env)?

go env Output

$ go env

What did you do?

func main() {
	fmt.Println(strings.TrimLeft("user_support", "user_"))
	// Expected: support Actual: pport
	fmt.Println(strings.TrimLeft("user_conversation", "user_"))
	// Expected: conversation Actual: conversation
	fmt.Println(strings.TrimLeft("user_suconversation", "user_"))
	// Expected: suconversation Actual: conversation
}

https://go.dev/play/p/Abefs5j3At0

What did you expect to see?

TrimLeft output not expected in above cases. Output seems bit different when string contains su substring.

What did you see instead?

Please see above snippet.

[merykitty]

// TrimLeft returns a slice of the string s with all leading
// Unicode code points contained in cutset removed.
//
// To remove a prefix, use TrimPrefix instead.

In other word, cutset is interpreted as a character set, and the longest prefix of s whose characters are all in cutset is chosen and trimmed.