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MaybeUninit::zeroed()

// Copyright 2026 Google LLC
// SPDX-License-Identifier: Apache-2.0

use std::mem::{MaybeUninit, transmute};

fn main() {
    let mut x = [const { MaybeUninit::<u32>::zeroed() }; 10];

    x[6].write(7);

    // SAFETY: All values of `x` have been written to
    let x: [u32; 10] = unsafe { transmute(x) };
    println!("{x:?}")
}

“MaybeUninit::zeroed() is an alternative constructor to MaybeUninit::uninit(). It instructs the compiler to fill the bits of T with zeros.”

Q: “Although the memory has been written to, the type remains MaybeUninit<T>. Can anyone think of why?”

A: Some types require their values to be non-zero or non-null. The classic case is references, but this applies to many other types as well. Consider the NonZeroUsize integer type and others in its family.