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

I'm having trouble initializing a fixed length array. My attempts so far all result in the same "use of possibly uninitialized variable: foo_array" error:

#[derive(Debug)]
struct Foo { a: u32, b: u32 }

impl Default for Foo {
    fn default() -> Foo { Foo{a:1, b:2} }
}

pub fn main() {
    let mut foo_array: [Foo; 10];

    // Do something here to in-place initialize foo_array?

    for f in foo_array.iter() {
        println!("{:?}", f);
    }
}

error[E0381]: use of possibly uninitialized variable: foo_array
--> src/main.rs:13:14
|
13 | for f in foo_array.iter() {
| ^^^^^^^^^ use of possibly uninitialized foo_array


I implemented the Default trait, but Rust does not seem to call this by default akin to a C++ constructor.

What is the proper way to initialize a fixed length array? I'd like to do an efficient in-place initialization rather than some sort of copy.

Related: Why is the Copy trait needed for default (struct valued) array initialization?

Related: Is there a way to not have to initialize arrays twice?

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; 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::(array) }
};

for element in array.iter() {
println!("{:?}", element);
}
}


This is recommended by the documentation of MaybeUninit.

Stack Overflow Q#31360993, score: 45