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);
}

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]`.

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]
    }
}

#[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]
    }
}