Unchecked Uninitialized Memory

One interesting exception to this rule is working with arrays. Safe Rust doesn't permit you to partially initialize an array. When you initialize an array, you can either set every value to the same thing with let x = [val; N], or you can specify each member individually with let x = [val1, val2, val3]. Unfortunately this is pretty rigid, especially if you need to initialize your array in a more incremental or dynamic way.

Unsafe Rust gives us a powerful tool to handle this problem: MaybeUninit. This type can be used to handle memory that has not been fully initialized yet.

With MaybeUninit, we can initialize an array element by element as follows:

#![allow(unused)]
fn main() {
use std::mem::{self, MaybeUninit};

// Size of the array is hard-coded but easy to change (meaning, changing just
// the constant is sufficient). This means we can't use [a, b, c] syntax to
// initialize the array, though, as we would have to keep that in sync
// with `SIZE`!
const SIZE: usize = 10;

let x = {
    // Create an uninitialized array of `MaybeUninit`. The `assume_init` is
    // safe because the type we are claiming to have initialized here is a
    // bunch of `MaybeUninit`s, which do not require initialization.
    let mut x: [MaybeUninit<Box<u32>>; SIZE] = unsafe {
        MaybeUninit::uninit().assume_init()
    };

    // Dropping a `MaybeUninit` does nothing. Thus using raw pointer
    // assignment instead of `ptr::write` does not cause the old
    // uninitialized value to be dropped.
    // Exception safety is not a concern because Box can't panic
    for i in 0..SIZE {
        x[i] = MaybeUninit::new(Box::new(i as u32));
    }

    // Everything is initialized. Transmute the array to the
    // initialized type.
    unsafe { mem::transmute::<_, [Box<u32>; SIZE]>(x) }
};

dbg!(x);
}

This code proceeds in three steps:

Create an array of MaybeUninit<T>. With current stable Rust, we have to use unsafe code for this: we take some uninitialized piece of memory (MaybeUninit::uninit()) and claim we have fully initialized it (assume_init()). This seems ridiculous, because we didn't! The reason this is correct is that the array consists itself entirely of MaybeUninit, which do not actually require initialization. For most other types, doing MaybeUninit::uninit().assume_init() produces an invalid instance of said type, so you got yourself some Undefined Behavior.
Initialize the array. The subtle aspect of this is that usually, when we use = to assign to a value that the Rust type checker considers to already be initialized (like x[i]), the old value stored on the left-hand side gets dropped. This would be a disaster. However, in this case, the type of the left-hand side is MaybeUninit<Box<u32>>, and dropping that does not do anything! See below for some more discussion of this drop issue.
Finally, we have to change the type of our array to remove the MaybeUninit. With current stable Rust, this requires a transmute. This transmute is legal because in memory, MaybeUninit<T> looks the same as T.

However, note that in general, Container<MaybeUninit<T>>> does not look the same as Container<T>! Imagine if Container was Option, and T was bool, then Option<bool> exploits that bool only has two valid values, but Option<MaybeUninit<bool>> cannot do that because the bool does not have to be initialized.

So, it depends on Container whether transmuting away the MaybeUninit is allowed. For arrays, it is (and eventually the standard library will acknowledge that by providing appropriate methods).

It's worth spending a bit more time on the loop in the middle, and in particular the assignment operator and its interaction with drop. If we wrote something like:

*x[i].as_mut_ptr() = Box::new(i as u32); // WRONG!

we would actually overwrite a Box<u32>, leading to drop of uninitialized data, which would cause much sadness and pain.

The correct alternative, if for some reason we cannot use MaybeUninit::new, is to use the ptr module. In particular, it provides three functions that allow us to assign bytes to a location in memory without dropping the old value: write, copy, and copy_nonoverlapping.

ptr::write(ptr, val) takes a val and moves it into the address pointed to by ptr.
ptr::copy(src, dest, count) copies the bits that count T items would occupy from src to dest. (this is equivalent to C's memmove -- note that the argument order is reversed!)
ptr::copy_nonoverlapping(src, dest, count) does what copy does, but a little faster on the assumption that the two ranges of memory don't overlap. (this is equivalent to C's memcpy -- note that the argument order is reversed!)

It should go without saying that these functions, if misused, will cause serious havoc or just straight up Undefined Behavior. The only requirement of these functions themselves is that the locations you want to read and write are allocated and properly aligned. However, the ways writing arbitrary bits to arbitrary locations of memory can break things are basically uncountable!

It's worth noting that you don't need to worry about ptr::write-style shenanigans with types which don't implement Drop or contain Drop types, because Rust knows not to try to drop them. This is what we relied on in the above example.

However when working with uninitialized memory you need to be ever-vigilant for Rust trying to drop values you make like this before they're fully initialized. Every control path through that variable's scope must initialize the value before it ends, if it has a destructor. This includes code panicking. MaybeUninit helps a bit here, because it does not implicitly drop its content - but all this really means in case of a panic is that instead of a double-free of the not yet initialized parts, you end up with a memory leak of the already initialized parts.

Note that, to use the ptr methods, you need to first obtain a raw pointer to the data you want to initialize. It is illegal to construct a reference to uninitialized data, which implies that you have to be careful when obtaining said raw pointer:

For an array of T, you can use base_ptr.add(idx) where base_ptr: *mut T to compute the address of array index idx. This relies on how arrays are laid out in memory.
For a struct, however, in general we do not know how it is laid out, and we also cannot use &mut base_ptr.field as that would be creating a reference. So, you must carefully use the addr_of_mut macro. This creates a raw pointer to the field without creating an intermediate reference:

#![allow(unused)]
fn main() {
use std::{ptr, mem::MaybeUninit};

struct Demo {
    field: bool,
}

let mut uninit = MaybeUninit::<Demo>::uninit();
// `&uninit.as_mut().field` would create a reference to an uninitialized `bool`,
// and thus be Undefined Behavior!
let f1_ptr = unsafe { ptr::addr_of_mut!((*uninit.as_mut_ptr()).field) };
unsafe { f1_ptr.write(true); }

let init = unsafe { uninit.assume_init() };
}

One last remark: when reading old Rust code, you might stumble upon the deprecated mem::uninitialized function. That function used to be the only way to deal with uninitialized memory on the stack, but it turned out to be impossible to properly integrate with the rest of the language. Always use MaybeUninit instead in new code, and port old code over when you get the opportunity.

And that's about it for working with uninitialized memory! Basically nothing anywhere expects to be handed uninitialized memory, so if you're going to pass it around at all, be sure to be really careful.

The Rustonomicon

Unchecked Uninitialized Memory