Rust for C-Programmers

25.10 Example: Splitting a Mutable Slice (split_at_mut)

A well-known example in the standard library is the split_at_mut function, which splits a mutable slice into two non-overlapping mutable slices. Safe Rust does not permit creating two mutable slices of the same data because it cannot prove the slices do not overlap. The example below uses unsafe functions (like std::slice::from_raw_parts_mut) and pointer arithmetic to implement this functionality:

fn my_split_at_mut(slice: &mut [u8], mid: usize) -> (&mut [u8], &mut [u8]) {
    let len = slice.len();
    assert!(mid <= len);
    let ptr = slice.as_mut_ptr();
    unsafe {
        (
            std::slice::from_raw_parts_mut(ptr, mid),
            std::slice::from_raw_parts_mut(ptr.add(mid), len - mid),
        )
    }
}

fn main() {
    let mut data = [1, 2, 3, 4, 5];
    let (left, right) = my_split_at_mut(&mut data, 2);
    left[0] = 42;
    right[0] = 99;
    println!("{:?}", data); // Outputs: [42, 2, 99, 4, 5]
}

By carefully ensuring that the two returned slices do not overlap, the function safely exposes low-level pointer arithmetic in a high-level, safe API.