vendor/compiler_builtins/src/mem/impls.rs

#[inline(always)]
pub unsafe fn copy_forward(dest: *mut u8, src: *const u8, n: usize) {
    let mut i = 0;
    while i < n {
        *dest.add(i) = *src.add(i);
        i += 1;
    }
}

#[inline(always)]
pub unsafe fn copy_backward(dest: *mut u8, src: *const u8, n: usize) {
    // copy from end
    let mut i = n;
    while i != 0 {
        i -= 1;
        *dest.add(i) = *src.add(i);
    }
}

#[inline(always)]
pub unsafe fn set_bytes(s: *mut u8, c: u8, n: usize) {
    let mut i = 0;
    while i < n {
        *s.add(i) = c;
        i += 1;
    }
}