s.substring(0, s.length - x) that can be replaced with s.dropLast(x).
Using corresponding functions makes your code simpler.
The quick-fix replaces the substring call with dropLast.
Example:
fun foo(s: String) {
s.substring(0, s.length - 5)
}
After the quick-fix is applied:
fun foo(s: String) {
s.dropLast(5)
}