Clippy lints / Style
Clippy lint · Style · checked with Clippy 0.1.99
clippy::manual_async_fn: this function can be simplified using the `async fn` syntax
What it meansIt checks for manual implementations of async functions. It is in the style group and warns by default.
What you see
warning: this function can be simplified using the `async fn` syntax --> main.rs:5:1 | 5 | fn foo() -> impl Future<Output = i32> { async { 42 } } | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#manual_async_fn = note: `#[warn(clippy::manual_async_fn)]` on by default help: make the function `async` and return the output of the future directly | 5 - fn foo() -> impl Future<Output = i32> { async { 42 } } 5 + async fn foo() -> i32 { 42 } |
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?
It’s more idiomatic to use the dedicated syntax.
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
use std::future::Future;
fn foo() -> impl Future<Output = i32> { async { 42 } }
Write this instead
async fn foo() -> i32 { 42 }
Turn it off, or on
Allow it where it does not fit your code, as narrowly as you can:
On one item
#[allow(clippy::manual_async_fn)]
fn my_function() { /* ... */ }Whole crate (Cargo.toml)
[lints.clippy]
manual_async_fn = "allow"One run
cargo clippy -- -A clippy::manual-async-fnThe [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
It checks for manual implementations of async functions.
Why is this bad?
It’s more idiomatic to use the dedicated syntax.
Example
use std::future::Future;
fn foo() -> impl Future<Output = i32> { async { 42 } }
Use instead:
async fn foo() -> i32 { 42 }
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
- let_and_returnreturning the result of a `let` binding from a block
- let_unit_valuethis let-binding has unit value
- main_recursionrecursing into entrypoint `main`
- manual_bitsusage of `size_of::<T>()` to obtain the size of `T` in bits
- manual_dangling_ptrmanual creation of a dangling pointer
- manual_is_ascii_checkmanual check for common ascii range
All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
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