Map Box

Map the value in a Box, re-using the allocation when possible.

For example, this code will not re-allocate.

use map_box::Map;

let b = Box::new(42u64);
let b = b.map_box(|v| v as i64);

The signature of map is:

impl<T1> Box<T1> {
    fn map_box<T2>(self, f: impl FnMut(T1) -> T2) -> Box<T2>;
}

Limitations

If the alignment requirements of the type changes, even if the alignment becomes lower, the Box needs to be reallocated. This is because:

1. alloc::dealloc requires the layout to be identical to the layout that the allocation was made with
2. alloc::realloc only takes a new size, it cannot change the layout of the allocation downwards