Clippy lint · Suspicious · checked with Clippy 0.1.99

clippy::zero_repeat_side_effects: expression with side effects as the initial value in a zero-sized array initializer

What it meansChecks for array or vec initializations which contain an expression with side effects, but which have a repeat count of zero. It is in the suspicious group and warns by default.

What you see

warning: expression with side effects as the initial value in a zero-sized array initializer
 --> main.rs:7:1
  |
7 | let a = [side_effect(); 0];
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#zero_repeat_side_effects
  = note: `#[warn(clippy::zero_repeat_side_effects)]` on by default
help: consider performing the side effect separately
  |
7 + side_effect();
8 + let a: [i32; 0] = [];
  |

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?

Such an initialization, despite having a repeat length of 0, will still call the inner function. This may not be obvious and as such there may be unintended side effects in code.

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

fn side_effect() -> i32 {
    println!("side effect");
    10
}
let a = [side_effect(); 0];

Write this instead

fn side_effect() -> i32 {
    println!("side effect");
    10
}
side_effect();
let a: [i32; 0] = [];

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
zero_repeat_side_effects = "allow"

One run

cargo clippy -- -A clippy::zero-repeat-side-effects

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

1.79.0added in Rust

Clippy's own documentation

What it does

Checks for array or vec initializations which contain an expression with side effects, but which have a repeat count of zero.

Why is this bad?

Such an initialization, despite having a repeat length of 0, will still call the inner function. This may not be obvious and as such there may be unintended side effects in code.

Example

fn side_effect() -> i32 {
    println!("side effect");
    10
}
let a = [side_effect(); 0];

Use instead:

fn side_effect() -> i32 {
    println!("side effect");
    10
}
side_effect();
let a: [i32; 0] = [];

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

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