What is the proper way to initialize a fixed length array?

The safe but somewhat inefficient solution:

#[derive(Copy, Clone, Debug)]struct Foo {    a: u32,    b: u32,}fn main() {    let mut foo_array = [Foo { a: 10, b: 10 }; 10];}

Since you're specifically asking for a solution without copies:

use std::mem::MaybeUninit;#[derive(Debug)]struct Foo {    a: u32,    b: u32,}// We're just implementing Drop to prove there are no unnecessary copies.impl Drop for Foo {    fn drop(&mut self) {        println!("Destructor running for a Foo");    }}pub fn main() {    let array = {        // Create an array of uninitialized values.        let mut array: [MaybeUninit<Foo>; 10] = unsafe { MaybeUninit::uninit().assume_init() };        for (i, element) in array.iter_mut().enumerate() {            let foo = Foo { a: i as u32, b: 0 };            *element = MaybeUninit::new(foo);        }        unsafe { std::mem::transmute::<_, [Foo; 10]>(array) }    };    for element in array.iter() {        println!("{:?}", element);    }}

This is recommended by the documentation of MaybeUninit.

You can use the arrayvec crate:

Cargo.toml

[package]name = "initialize_array"version = "0.1.0"authors = ["author"]edition = "2018"[dependencies]arrayvec = "0.4.10"

src/main.rs

use arrayvec::ArrayVec; use std::iter;#[derive(Clone)]struct Foo {    a: u32,    b: u32,}fn main() {    let foo_array: [Foo; 10] = iter::repeat(Foo { a: 10, b: 10 })        .collect::<ArrayVec<_>>()        .into_inner()        .unwrap_or_else(|_| unreachable!());}

The easiest way is to derive Copy on your type and initialize the array with that, copying the element N times:

#[derive(Copy)]struct Foo {    a: u32,    b: u32,}let mut foo_array = [Foo { a: 1, b: 2 }; 10];

If you want to avoid copying, there are a couple options. You can use the Default trait:

let mut foo_array: [Foo; 10] = Default::default();

However, this is limited to arrays up to 32 elements. With const generics, it is now possible for the standard library to provide Default for all arrays. However, this would be a backward incompatible change for subtle reasons that are being worked on.

For now, you can take advantage of the fact that const values are also allowed in array repetition expressions:

const FOO: Foo = Foo { a: 1, b: 2 };let mut foo_array = [FOO; 10];

If you're on nightly, you can use array::map:

#![feature(array_map)]let mut foo_array = [(); 10].map(|_| Foo::default())