Clippy lint · Correctness · checked with Clippy 0.1.99

clippy::while_immutable_condition: variables in the condition are not mutated in the loop body

What it meansChecks whether variables used within while loop condition can be (and are) mutated in the body. It is in the correctness group and denied by default (the build fails).

What you see

error: variables in the condition are not mutated in the loop body
 --> main.rs:4:7
  |
4 | while i > 10 {
  |       ^^^^^^
  |
  = note: this may lead to an infinite or to a never running loop
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#while_immutable_condition
  = note: `#[deny(clippy::while_immutable_condition)]` 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?

If the condition is unchanged, entering the body of the loop will lead to an infinite loop.

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

let i = 0;
while i > 10 {
    println!("let me loop forever!");
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
while_immutable_condition = "allow"

One run

cargo clippy -- -A clippy::while-immutable-condition

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

pre 1.29.0added in Rust

Clippy's own documentation

What it does

Checks whether variables used within while loop condition can be (and are) mutated in the body.

Why is this bad?

If the condition is unchanged, entering the body of the loop will lead to an infinite loop.

Known problems

If the while-loop is in a closure, the check for mutation of the condition variables in the body can cause false negatives. For example when only Upvar a is in the condition and only Upvar b gets mutated in the body, the lint will not trigger.

Example

let i = 0;
while i > 10 {
    println!("let me loop forever!");
}

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

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