Clippy lint · Correctness · checked with Clippy 0.1.99

clippy::uninit_assumed_init: this call for this type may be undefined behavior

What it meansChecks for MaybeUninit::uninit().assume_init(). It is in the correctness group and denied by default (the build fails).

What you see

error: this call for this type may be undefined behavior
 --> main.rs:6:25
  |
6 | let _: usize = unsafe { MaybeUninit::uninit().assume_init() };
  |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#uninit_assumed_init
  = note: `#[deny(clippy::uninit_assumed_init)]` on by default

Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.

Does it matter?

For most types, this is undefined behavior.

Code that is almost certainly wrong. Denied by default: the build fails until you fix it or allow it.

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

// Beware the UB
use std::mem::MaybeUninit;

let _: usize = unsafe { MaybeUninit::uninit().assume_init() };

Write this instead

use std::mem::MaybeUninit;

let _: [MaybeUninit<bool>; 5] = unsafe {
    MaybeUninit::uninit().assume_init()
};

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

On one item

#[allow(clippy::uninit_assumed_init)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
uninit_assumed_init = "allow"

One run

cargo clippy -- -A clippy::uninit-assumed-init

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.

Correctnessgroup

denydefault level

nofixed by cargo clippy --fix

1.39.0added in Rust

Clippy's own documentation

What it does

Checks for MaybeUninit::uninit().assume_init().

Why is this bad?

For most types, this is undefined behavior.

Known problems

For now, we accept empty tuples and tuples / arrays of MaybeUninit. There may be other types that allow uninitialized data, but those are not yet rigorously defined.

Example

// Beware the UB
use std::mem::MaybeUninit;

let _: usize = unsafe { MaybeUninit::uninit().assume_init() };

Note that the following is OK:

use std::mem::MaybeUninit;

let _: [MaybeUninit<bool>; 5] = unsafe {
    MaybeUninit::uninit().assume_init()
};

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

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