Clippy lint · Restriction · checked with Clippy 0.1.99

clippy::let_underscore_must_use: non-binding `let` on an expression with `#[must_use]` type

What it meansChecks for let _ = <expr> where expr is #[must_use] It is in the restriction group and off by default.

What you see

warning: non-binding `let` on an expression with `#[must_use]` type
 --> main.rs:7:1
  |
7 | let _ = f();
  | ^^^^^^^^^^^^
  |
  = help: consider explicitly using expression value
note: type is `std::result::Result`
 --> main.rs:7:9
  |
7 | let _ = f();
  |         ^^^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#let_underscore_must_use
  = note: requested on the command line with `-W clippy::let-underscore-must-use`

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?

To ensure that all #[must_use] types are used rather than ignored.

Lints that forbid language features for specific codebases (no unwrap, no indexing...). Off by default; pick them one by one, never the whole group.

How to fix it

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

Flagged

fn f() -> Result<u32, u32> {
    Ok(0)
}

let _ = f();
// is_ok() is marked #[must_use]
let _ = f().is_ok();

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

Whole crate (Cargo.toml)

[lints.clippy]
let_underscore_must_use = "warn"

In code

#![warn(clippy::let_underscore_must_use)]

One run

cargo clippy -- -W clippy::let-underscore-must-use

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.

Restrictiongroup

allowdefault level

nofixed by cargo clippy --fix

1.42.0added in Rust

Clippy's own documentation

What it does

Checks for let _ = <expr> where expr is #[must_use]

Why restrict this?

To ensure that all #[must_use] types are used rather than ignored.

Example

fn f() -> Result<u32, u32> {
    Ok(0)
}

let _ = f();
// is_ok() is marked #[must_use]
let _ = f().is_ok();

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

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