Partial Initialization

use std::mem::MaybeUninit;

fn main() {
    // let mut buf = [0u8; 2048];
    let mut buf = [const { MaybeUninit::<u8>::uninit() }; 2048];

    let external_data = b"Hello, Rust!";
    let len = external_data.len();

    for (dest, src) in buf.iter_mut().zip(external_data) {
        dest.write(*src);
    }

    // SAFETY: We initialized exactly 'len' bytes of `buf` with UTF-8 text
    let text: &str = unsafe {
        let ptr: *const u8 = buf.as_ptr().cast::<u8>();
        let init: &[u8] = std::slice::from_raw_parts(ptr, len);
        std::str::from_utf8_unchecked(init)
    };

    println!("{text}");
}

This code simulates receiving data from some external source.

When reading bytes from an external source into a buffer, you typically don’t know how many bytes you’ll receive. Using MaybeUninit<T> lets you allocate the buffer once without paying for a redundant initialization pass.

If we were to create the array with the standard syntax (buf = [0u8; 2048]), the whole buffer would be flushed with zeroes. MaybeUninit<T> tells the compiler to reserve space, but not to touch the memory yet.

Q: Which part of the code snippet is performing a similar role to .assume_init()? A: The pointer cast and the implicit read.

We cannot call assume_init() on the whole array. That would be unsound because most elements remain uninitialized. Instead, we cast the pointer from *const MaybeUninit<u8> to *const u8 and build a slice covering only the initialised portion.

Comprehensive Rust 🦀

Partial Initialization