Type-casting arrays/vectors in Rust

Question

What would be the idiomatic way of converting arrays or vectors of one type to another in Rust? The desired effect is

let x = ~[0 as int, 1 as int, 2 as int];
let y = vec::map(x, |&e| { e as uint });

but I'm not sure if the same could be achieved in a more concise fashion, similar to scalar type-casts.

I seem to fail at finding clues in the Rust manual or reference. TIA.

Lopez

Answer 1

In general, the best you are going to get is similar to what you have (this allocates a new vector):

let x = vec![0, 1, 2];
let y = x.iter().map(|&e| e as u32).collect();

Although, if you know the bit patterns of the things you are casting between are the same (e.g. a newtype struct to the type it wraps, or casting between uint and int), you can do an in-place cast, that will not allocate a new vector (although it means that the old x can not be accessed):

let x = [0, 1, 2];
let y: [u32; 3] = unsafe { cast::transmute(x) };

(Note that this is unsafe, and can cause Bad Things to happen.)

Counterespionage

Thank you. Going the usual functional way and using map will be good enough. – Lopez

Looks like this answer needs an update to account for newer versions of Rust (e.g. there's no more do keyword, and I think you need .iter() and .collect() in there somewhere) – Urey

Not to mention that we now have proper Box instead of whatever that squiggly meant before. – Orthorhombic

@Counterespionage can this be used to cast ro a vector of a larger int type? – Insignificant

Your example transmutes an array, which is fine, but your words imply you can transmute a vector. Which is definitely disallowed. The only valid way to do that is from_raw_parts(). But, better you use something like bytemuck's cast_vec(). Also note that transmuting an array will copy (unless the compiler optimizes that out), and transmuting a slice, while not strictly forbidden, should be replaced by pointer casts. – Zitvaa

Answer 2

Starting with Rust 1.55, arrays can be converted between types concisely and safely using the map() method:

let x = [1, 2, 3];
let x_i64 = x.map(i64::from);

This is specific to arrays, and won't work on Vecs.

Photomural