Clippy lint · Correctness · checked with Clippy 0.1.99

clippy::mem_replace_with_uninit: Checks for mem::replace(&mut _, mem::uninitialized()) and mem::replace(&mut _, mem::zeroed())

What it meansChecks for mem::replace(&mut _, mem::uninitialized()) and mem::replace(&mut _, mem::zeroed()). It is in the correctness group and denied by default (the build fails).

Does it matter?

This will lead to undefined behavior even if the value is overwritten later, because the uninitialized value may be observed in the case of a panic.

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

How to fix it

Clippy can fix this one for you: run cargo clippy --fix (add --allow-dirty if you have uncommitted changes). Review the diff before you commit.

Flagged

use std::mem;

#[allow(deprecated, invalid_value)]
fn myfunc (v: &mut Vec<i32>) {
    let taken_v = unsafe { mem::replace(v, mem::uninitialized()) };
    let new_v = may_panic(taken_v); // undefined behavior on panic
    mem::forget(mem::replace(v, new_v));
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
mem_replace_with_uninit = "allow"

One run

cargo clippy -- -A clippy::mem-replace-with-uninit

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

yesfixed by cargo clippy --fix

1.39.0added in Rust

Clippy's own documentation

What it does

Checks for mem::replace(&mut _, mem::uninitialized()) and mem::replace(&mut _, mem::zeroed()).

Why is this bad?

This will lead to undefined behavior even if the value is overwritten later, because the uninitialized value may be observed in the case of a panic.

Example

use std::mem;

#[allow(deprecated, invalid_value)]
fn myfunc (v: &mut Vec<i32>) {
    let taken_v = unsafe { mem::replace(v, mem::uninitialized()) };
    let new_v = may_panic(taken_v); // undefined behavior on panic
    mem::forget(mem::replace(v, new_v));
}

The take_mut crate offers a sound solution, at the cost of either lazily creating a replacement value or aborting on panic, to ensure that the uninitialized value cannot be observed.

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

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