#![allow(unused_variables)]
#![feature(allocator_api, box_vec_non_null)]
fn main() {
    use std::alloc::System;
    use std::ptr::NonNull;
    use std::mem;
    let mut v = Vec::with_capacity_in(3, System);
    v.push(1);
    v.push(2);
    v.push(3);
    let mut v = mem::ManuallyDrop::new(v);
    let p = unsafe { NonNull::new_unchecked(v.as_mut_ptr()) };
    let len = v.len();
    let cap = v.capacity();
    let alloc = v.allocator();
    unsafe {
        for i in 0..len {
            p.add(i).write(4 + i);
        }
        let rebuilt = Vec::from_parts_in(p, len, cap, alloc.clone());
        assert_eq!(rebuilt, [4, 5, 6]);
    }
}