rust - Is there a good way to convert a Vec<T> to an array?

Question

Is there a good way to convert a Vec<T> with size S to an array of type [T; S]? Specifically, I'm using a function that returns a 128-bit hash as a Vec<u8>, which will always have length 16, and I would like to deal with the hash as a [u8, 16].

Is there something built-in akin to the as_slice method which gives me what I want, or should I write my own function which allocates a fixed-size array, iterates through the vector copying each element, and returns the array?

Answer 1

Arrays must be completely initialized, so you quickly run into concerns about what to do when you convert a vector with too many or too few elements into an array. These examples simply panic.

As of Rust 1.51 you can parameterize over an array's length.

use std::convert::TryInto;

fn demo<T, const N: usize>(v: Vec<T>) -> [T; N] {
    v.try_into()
        .unwrap_or_else(|v: Vec<T>| panic!("Expected a Vec of length {} but it was {}", N, v.len()))
}

As of Rust 1.48, each size needs to be a specialized implementation:

use std::convert::TryInto;

fn demo<T>(v: Vec<T>) -> [T; 4] {
    v.try_into()
        .unwrap_or_else(|v: Vec<T>| panic!("Expected a Vec of length {} but it was {}", 4, v.len()))
}

As of Rust 1.43:

use std::convert::TryInto;

fn demo<T>(v: Vec<T>) -> [T; 4] {
    let boxed_slice = v.into_boxed_slice();
    let boxed_array: Box<[T; 4]> = match boxed_slice.try_into() {
        Ok(ba) => ba,
        Err(o) => panic!("Expected a Vec of length {} but it was {}", 4, o.len()),
    };
    *boxed_array
}

See also:

• How to get a slice as an array in Rust?
• How do I get an owned value out of a `Box`?
• Is it possible to control the size of an array using the type parameter of a generic?