# Map Box

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

For example, this code will not re-allocate.

```rust
use map_box::Map;

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

The signature of `map` is:
```rust
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](https://doc.rust-lang.org/stable/std/alloc/trait.GlobalAlloc.html#tymethod.dealloc) requires the layout to be identical to the layout that the allocation was made with
2. [alloc::realloc](https://doc.rust-lang.org/stable/std/alloc/trait.GlobalAlloc.html#method.realloc) only takes a new size, it cannot change the layout of the allocation downwards