Clippy lints / Style
Clippy lint · Style · checked with Clippy 0.1.99
clippy::must_use_unit: this unit-returning function has a `#[must_use]` attribute
What it meansChecks for a #[must_use] attribute on unit-returning functions and methods. It is in the style group and warns by default.
What you see
warning: this unit-returning function has a `#[must_use]` attribute --> main.rs:4:1 | 3 | #[must_use] | ----------- help: remove the attribute 4 | fn useless() { } | ^^^^^^^^^^^^ | = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#must_use_unit = note: `#[warn(clippy::must_use_unit)]` 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?
Unit values are useless. The attribute is likely a remnant of a refactoring that removed the return type.
Code that works but is not idiomatic Rust. Warns by default.
How to fix it
Clippy can fix this one for you: run cargo clippy --fix (add --allow-dirty if you have uncommitted changes). Review the diff before you commit.
Flagged
#[must_use]
fn useless() { }
Turn it off, or on
Allow it where it does not fit your code, as narrowly as you can:
On one item
#[allow(clippy::must_use_unit)]
fn my_function() { /* ... */ }Whole crate (Cargo.toml)
[lints.clippy]
must_use_unit = "allow"One run
cargo clippy -- -A clippy::must-use-unitThe [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.
Clippy's own documentation
What it does
Checks for a #[must_use] attribute on
unit-returning functions and methods.
Why is this bad?
Unit values are useless. The attribute is likely a remnant of a refactoring that removed the return type.
Examples
#[must_use]
fn useless() { }
From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.
Related lints
- needless_returnunneeded `return` statement
- new_without_defaultChecks for public types with a pub fn new() -> Self method and no implementation of Default
- len_zeroChecks for getting the length of something via .len() just to compare to zero, and suggests using .is_empty() where applicable
- mixed_case_hex_literalsinconsistent casing in hexadecimal literal
- module_inceptionChecks for modules that have the same name as their parent module
- multiple_bound_locationsbound is defined in more than one place
- mut_mutex_lockcalling `&mut Mutex::lock` unnecessarily locks an exclusive (mutable) reference
- needless_borrowthis expression creates a reference which is immediately dereferenced by the compiler
- needless_borrows_for_generic_argsthe borrowed expression implements the required traits
All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
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