casting-between-types.md

% Casting Between Types

Rust, with its focus on safety, provides two different ways of casting different types between each other. The first, as, is for safe casts. In contrast, transmute allows for arbitrary casting, and is one of the most dangerous features of Rust!

as

The as keyword does basic casting:

let x: i32 = 5;

let y = x as i64;

It only allows certain kinds of casting, however:

let a = [0u8, 0u8, 0u8, 0u8];

let b = a as u32; // four eights makes 32

This errors with:

error: non-scalar cast: `[u8; 4]` as `u32`
let b = a as u32; // four eights makes 32
        ^~~~~~~~

It’s a ‘non-scalar cast’ because we have multiple values here: the four elements of the array. These kinds of casts are very dangerous, because they make assumptions about the way that multiple underlying structures are implemented. For this, we need something more dangerous.

transmute

The transmute function is provided by a compiler intrinsic, and what it does is very simple, but very scary. It tells Rust to treat a value of one type as though it were another type. It does this regardless of the typechecking system, and just completely trusts you.

In our previous example, we know that an array of four u8s represents a u32 properly, and so we want to do the cast. Using transmute instead of as, Rust lets us:

use std::mem;

unsafe {
    let a = [0u8, 0u8, 0u8, 0u8];

    let b = mem::transmute::<[u8; 4], u32>(a);
}

We have to wrap the operation in an unsafe block for this to compile successfully. Technically, only the mem::transmute call itself needs to be in the block, but it's nice in this case to enclose everything related, so you know where to look. In this case, the details about a are also important, and so they're in the block. You'll see code in either style, sometimes the context is too far away, and wrapping all of the code in unsafe isn't a great idea.

While transmute does very little checking, it will at least make sure that the types are the same size. This errors:

use std::mem;

unsafe {
    let a = [0u8, 0u8, 0u8, 0u8];

    let b = mem::transmute::<[u8; 4], u64>(a);
}

with:

error: transmute called with differently sized types: [u8; 4] (32 bits) to u64
(64 bits)

Other than that, you're on your own!