Code shared from the Rust Playground

playground.rs

use std::mem::ManuallyDrop;
use std::convert::TryInto;

fn main() {
    let mut a = String::with_capacity(50);
    a.push_str("öäasdkflieojf");
    let (b,c) = unsafe{split(a, 4)};
    println!("{} and {}, capacities: {}, {}", b, c, b.capacity(), c.capacity());
}



/// Safety: at needs to be at a valid utf-8 boundary
unsafe fn split(s: String, at: usize) -> (String, String) {
    
    // Prevent automatically dropping the String's data
    let mut s = ManuallyDrop::new(s);

    let ptr = s.as_mut_ptr();
    let len = s.len();
    let capacity = s.capacity();

    let s1 = String::from_raw_parts(ptr, at, at);
    let s2 = String::from_raw_parts(ptr.offset(at.try_into().unwrap()), len - at, capacity - at);
    (s1,s2)
}


// Doesn't work yet
fn split_whitespace_owned(s: String) -> Vec<String> {

    // Prevent automatically dropping the String's data
    let mut s = ManuallyDrop::new(s);

    let ptr = s.as_mut_ptr();
    let len = s.len();
    let capacity = s.capacity();
    
    let offsets = s.split_whitespace().map(|x| {
                let start =
                    (x.as_ptr() as isize - s.as_ptr() as isize) as usize;
                let end = start + x.len();
                (start, end)
            });
            

    Vec::new()
}