s.substring(0, s.indexOf(x)) that can be replaced with s.substringBefore(x).
Using substringBefore() makes your code simpler.
The quick-fix replaces the substring call with substringBefore.
Example:
fun foo(s: String) {
s.substring(0, s.indexOf('x'))
}
After the quick-fix is applied:
fun foo(s: String) {
s.substringBefore('x')
}