Clippy lint · Suspicious · checked with Clippy 0.1.99

clippy::mut_range_bound: attempt to mutate range bound within loop

What it meansChecks for loops with a range bound that is a mutable variable. It is in the suspicious group and warns by default.

What you see

warning: attempt to mutate range bound within loop
 --> main.rs:5:5
  |
5 |     x += 1; // x is a range bound that is mutated
  |     ^
  |
  = note: the range of the loop is unchanged
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#mut_range_bound
  = note: `#[warn(clippy::mut_range_bound)]` 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?

One might think that modifying the mutable variable changes the loop bounds. It doesn’t.

Code that is most likely wrong or useless. Warns by default.

How to fix it

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

Flagged

let mut x = 5;
for _ in 0..x {
    x += 1; // x is a range bound that is mutated
    ..; // some other expression
    break; // leaves the loop, so mutation is not an issue
}

Write this instead

let mut foo = 42;
for i in 0..foo {
    foo -= 1;
    println!("{i}"); // prints numbers from 0 to 41, not 0 to 21
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
mut_range_bound = "allow"

One run

cargo clippy -- -A clippy::mut-range-bound

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.

Suspiciousgroup

warndefault level

nofixed by cargo clippy --fix

pre 1.29.0added in Rust

Clippy's own documentation

What it does

Checks for loops with a range bound that is a mutable variable.

Why is this bad?

One might think that modifying the mutable variable changes the loop bounds. It doesn’t.

Known problems

False positive when mutation is followed by a break, but the break is not immediately after the mutation:

let mut x = 5;
for _ in 0..x {
    x += 1; // x is a range bound that is mutated
    ..; // some other expression
    break; // leaves the loop, so mutation is not an issue
}

False positive on nested loops (#6072)

Example

let mut foo = 42;
for i in 0..foo {
    foo -= 1;
    println!("{i}"); // prints numbers from 0 to 41, not 0 to 21
}

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

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