Clippy lints / Pedantic
Clippy lint · Pedantic · checked with Clippy 0.1.99
clippy::unused_async: unused `async` for function with no await statements
What it meansChecks for functions that are declared async but have no .awaits inside of them. It is in the pedantic group and off by default.
What you see
warning: unused `async` for function with no await statements --> main.rs:3:1 | 3 | / async fn get_random_number() -> i64 { 4 | | 4 // Chosen by fair dice roll. Guaranteed to be random. 5 | | } | |_^ | = help: consider removing the `async` from this function = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#unused_async = note: `-W clippy::unused-async` implied by `-W clippy::pedantic`
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?
Async functions with no async code create overhead, both mentally and computationally. Callers of async methods either need to be calling from an async function themselves or run it on an executor, both of which causes runtime overhead and hassle for the caller.
Stricter lints with some false positives. Off by default; opt in with clippy::pedantic.
How to fix it
Clippy does not fix this one automatically. Change the code by hand:
Flagged
async fn get_random_number() -> i64 {
4 // Chosen by fair dice roll. Guaranteed to be random.
}
let number_future = get_random_number();
Write this instead
fn get_random_number_improved() -> i64 {
4 // Chosen by fair dice roll. Guaranteed to be random.
}
let number_future = async { get_random_number_improved() };
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]
unused_async = "warn"In code
#![warn(clippy::unused_async)]One run
cargo clippy -- -W clippy::unused-asyncThe [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 functions that are declared async but have no .awaits inside of them.
Why is this bad?
Async functions with no async code create overhead, both mentally and computationally. Callers of async methods either need to be calling from an async function themselves or run it on an executor, both of which causes runtime overhead and hassle for the caller.
Example
async fn get_random_number() -> i64 {
4 // Chosen by fair dice roll. Guaranteed to be random.
}
let number_future = get_random_number();
Use instead:
fn get_random_number_improved() -> i64 {
4 // Chosen by fair dice roll. Guaranteed to be random.
}
let number_future = async { get_random_number_improved() };
From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.
Related lints
- too_many_linesChecks for functions with a large amount of lines
- missing_errors_docChecks the doc comments of publicly visible functions that return a Result type and warns if there is no # Errors section
- must_use_candidateChecks for public functions that have no #[must_use] attribute, but return something not already marked must-use, have no mutable arg and mutate no statics
- unnested_or_patternsunnested or-patterns
- unreadable_literallong literal lacking separators
- unsafe_derive_deserializeChecks for deriving serde::Deserialize on a type that has methods using unsafe
- unused_async_trait_implunused `async` for async trait impl function with no `.await` statements
- unused_selfunused `self` argument
- used_underscore_bindingused underscore-prefixed binding
All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
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