pub fn split_off(&mut self, at: usize) -> String {
        assert!(self.is_char_boundary(at));
        let other = self.vec.split_off(at);
        unsafe { String::from_utf8_unchecked(other) }
    }
Removes the specified range in the string,
and replaces it with the given st
benchmarks/haystacks/code/rust-library.rs