Crate ref_cast

  

This crate provides a derive macro for generating safe conversions from &T to &U where the struct U contains a single field of type T.

Basic example

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.

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.

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.

#[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]
    }
}

Traits

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

Attribute Macros

• ref_cast_custom
  Create a function for a RefCast-style reference cast. Call site gets control of the visibility, function name, argument name, constness, unsafety, and documentation.

Derive Macros

• RefCast
  Derive the RefCast trait.
• RefCastCustom
  Derive that makes the ref_cast_custom attribute able to generate freestanding reference casting functions for a type.