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RefCast
=======
[<img alt="github" src="https://img.shields.io/badge/github-dtolnay/ref--cast-8da0cb?style=for-the-badge&labelColor=555555&logo=github" height="20">](https://github.com/dtolnay/ref-cast)
[<img alt="crates.io" src="https://img.shields.io/crates/v/ref-cast.svg?style=for-the-badge&color=fc8d62&logo=rust" height="20">](https://crates.io/crates/ref-cast)
[<img alt="docs.rs" src="https://img.shields.io/badge/docs.rs-ref--cast-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs" height="20">](https://docs.rs/ref-cast)
[<img alt="build status" src="https://img.shields.io/github/actions/workflow/status/dtolnay/ref-cast/ci.yml?branch=master&style=for-the-badge" height="20">](https://github.com/dtolnay/ref-cast/actions?query=branch%3Amaster)
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]
}
}
```
<br>
#### License
<sup>
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.
</sup>
<br>
<sub>
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.
</sub>
|
