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ref-cast

★ 224 Open GitHub ↗

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

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RefCast
=======

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

```toml
[dependencies]
ref-cast = "1.0"
```

## Basic example

```rust
use ref_cast::RefCast;

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

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

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

Note that `#[repr(transparent)]` is required in order for the conversion to be
sound. The derive macro will refuse to compile if that is not present.

## 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(transparent)]
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);
 Strided::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 <a href="LICENSE-APACHE">Apache License, Version
2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.

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.

ref-cast

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