Clippy lint · Suspicious · checked with Clippy 0.1.99

clippy::let_underscore_future: non-binding `let` on a future

What it meansChecks for let _ = <expr> where the resulting type of expr implements Future It is in the suspicious group and warns by default.

What you see

warning: non-binding `let` on a future
 --> main.rs:6:1
  |
6 | let _ = foo();
  | ^^^^^^^^^^^^^^
  |
  = help: consider awaiting the future or dropping explicitly with `std::mem::drop`
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#let_underscore_future
  = note: `#[warn(clippy::let_underscore_future)]` 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?

Futures must be polled for work to be done. The original intention was most likely to await the future and ignore the resulting value.

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

async fn foo() -> Result<(), ()> {
    Ok(())
}
let _ = foo();

Write this instead

async fn foo() -> Result<(), ()> {
    Ok(())
}
let _ = foo().await;

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
let_underscore_future = "allow"

One run

cargo clippy -- -A clippy::let-underscore-future

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

1.67.0added in Rust

Clippy's own documentation

What it does

Checks for let _ = <expr> where the resulting type of expr implements Future

Why is this bad?

Futures must be polled for work to be done. The original intention was most likely to await the future and ignore the resulting value.

Example

async fn foo() -> Result<(), ()> {
    Ok(())
}
let _ = foo();

Use instead:

async fn foo() -> Result<(), ()> {
    Ok(())
}
let _ = foo().await;

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

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