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>;
}If the alignment requirements of the type changes, even if the alignment becomes lower, the Box needs to be reallocated. This is because:
- alloc::dealloc requires the layout to be identical to the layout that the allocation was made with
- alloc::realloc only takes a new size, it cannot change the layout of the allocation downwards