ref-cast 0.2.5

RefCast
=======

[![Build Status](https://api.travis-ci.org/dtolnay/ref-cast.svg?branch=master)](https://travis-ci.org/dtolnay/ref-cast)
[![Latest Version](https://img.shields.io/crates/v/ref-cast.svg)](https://crates.io/crates/ref-cast)
[![Rust Documentation](https://img.shields.io/badge/api-rustdoc-blue.svg)](https://docs.rs/ref-cast)

Safely cast `&T` to `&U` where the struct `U` contains a single field of
type `T`.

```toml
[dependencies]
ref-cast = "0.2"
```

## Basic example

```rust
#[macro_use]
extern crate ref_cast;
use ref_cast::RefCast;

#[derive(RefCast)]
#[repr(C)]
struct U(String);

fn main() {
    let s = String::new();

    // Safely cast from `&String` to `&U`.
    let u = U::ref_cast(&s);
}
```

## Realistic example

Suppose we have a multidimensional array represented in a flat buffer in
row-major order for performance reasons, but we want to expose an indexing
operation that works in column-major order because it is more intuitive in
the context of our application.

```rust
const MAP_WIDTH: usize = 4;

struct Tile(u8);

struct TileMap {
    storage: Vec<Tile>,
}

// `tilemap[x][y]` should give us `tilemap.storage[y * MAP_WIDTH + x]`.
```

The signature of the [`Index`] trait in Rust is such that the output is
forced to be borrowed from the type being indexed. So something like the
following is not going to work.

[`Index`]: https://doc.rust-lang.org/std/ops/trait.Index.html

```rust
struct Column<'a> {
    tilemap: &'a TileMap,
    x: usize,
}

// Does not work! The output of Index must be a reference that is
// borrowed from self. Here the type Column is not a reference.
impl Index<usize> for TileMap {
    fn index(&self, x: usize) -> Column {
        assert!(x < MAP_WIDTH);
        Column { tilemap: self, x }
    }
}

impl<'a> Index<usize> for Column<'a> {
    fn index(&self, y: usize) -> &Tile {
        &self.tilemap.storage[y * MAP_WIDTH + self.x]
    }
}
```

Here is a working approach using `RefCast`.

```rust
#[derive(RefCast)]
#[repr(C)]
struct Strided([Tile]);

// Implement `tilemap[x][y]` as `tilemap[x..][y * MAP_WIDTH]`.
impl Index<usize> for TileMap {
    type Output = Strided;
    fn index(&self, x: usize) -> &Self::Output {
        assert!(x < MAP_WIDTH);
        RefCast::ref_cast(&self.storage[x..])
    }
}

impl Index<usize> for Strided {
    type Output = Tile;
    fn index(&self, y: usize) -> &Self::Output {
        &self.0[y * MAP_WIDTH]
    }
}
```

## License

Licensed under either of

 * Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
 * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)

at your option.

### Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
be dual licensed as above, without any additional terms or conditions.