How to convert a Vec into an array without copying the elements?

Question

I have a Vec of nontrivial types with a size I am certain of. I need to convert this into fixed size array. Ideally I would like to do this

1. without copying the data over - I would like to consume the Vec
2. without preinitializing the array with zero data because that would be a waste of CPU cycles

Question written as code:

struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let points = vec![
        Point { x: 1, y: 2 },
        Point { x: 3, y: 4 },
        Point { x: 5, y: 6 },
    ];

    // I would like this to be an array of points
    let array: [Point; 3] = ???;
}

This seems like a trivial issue, however I have not been able to find satisfactory solution in Vec docs, slicing sections of Rust Books or by Googling. Only thing that I found is to first initialize the array with zero data and later copy all elements over from Vec, however this does not satisfy my requirements.

Answer 1

Doing this correctly is exceedingly difficult. The problem lies in properly handling a panic when there's a partially uninitialized array. If the type inside the array implements Drop, then it would access uninitialized memory, causing undefined behavior.

The easiest, safest way is to use arrayvec:

extern crate arrayvec;

use arrayvec::ArrayVec;

#[derive(Debug)]
struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let points = vec![
        Point { x: 1, y: 2 },
        Point { x: 3, y: 4 },
        Point { x: 5, y: 6 },
    ];

    let array: ArrayVec<_> = points.into_iter().collect();
    let array: [Point; 3] = array.into_inner().unwrap();
    println!("{:?}", array);
}

Beware this only works for specific sizes of arrays because Rust does not yet have generic integers. into_inner also has a performance warning you should pay attention to.

See also:

• What is the proper way to initialize a fixed length array? (especially this answer)

• Convert vectors to arrays and back, which inadvertently wrote code that worked without copies, but does not explicitly claim it was on purpose

• How to get a slice as an array in Rust?

• Is there a good way to convert a Vec<T> to an array?
• Slice to fixed-size array