Clippy lints / Pedantic
Clippy lint · Pedantic · checked with Clippy 0.1.99
clippy::large_futures: large future with a size of 16385 bytes
What it meansIt checks for the size of a Future created by async fn or async {}. It is in the pedantic group and off by default.
What you see
warning: large future with a size of 16385 bytes --> main.rs:6:5 | 6 | large_future([0u8; 16 * 1024]).await; | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider `Box::pin` on it: `Box::pin(large_future([0u8; 16 * 1024]))` | = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#large_futures = note: `-W clippy::large-futures` 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?
Due to the current unideal implementation of Coroutine, large size of a Future may cause stack overflows.
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 large_future(_x: [u8; 16 * 1024]) {}
pub async fn trigger() {
large_future([0u8; 16 * 1024]).await;
}
Write this instead
async fn large_future(_x: [u8; 16 * 1024]) {}
pub async fn trigger() {
Box::pin(large_future([0u8; 16 * 1024])).await;
}
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]
large_futures = "warn"In code
#![warn(clippy::large_futures)]One run
cargo clippy -- -W clippy::large-futuresThe [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.
Settings in clippy.toml
future-size-threshold: The maximum byte size a Future can have, before it triggers the clippy::large_futures lint
Clippy's own documentation
What it does
It checks for the size of a Future created by async fn or async {}.
Why is this bad?
Due to the current unideal implementation of Coroutine,
large size of a Future may cause stack overflows.
Example
async fn large_future(_x: [u8; 16 * 1024]) {}
pub async fn trigger() {
large_future([0u8; 16 * 1024]).await;
}
Box::pin the big future instead.
async fn large_future(_x: [u8; 16 * 1024]) {}
pub async fn trigger() {
Box::pin(large_future([0u8; 16 * 1024])).await;
}
Configuration
-
future-size-threshold: The maximum byte size aFuturecan have, before it triggers theclippy::large_futureslint(default:
16384)
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
- iter_not_returning_iteratorthis method is named `iter` but its return type does not implement `Iterator`
- iter_without_into_iterLooks for iter and iter_mut methods without an associated IntoIterator for (&|&mut) Type implementation
- large_digit_groupsdigit groups should be smaller
- large_stack_arraysallocating a local array larger than 16384 bytes
- large_types_passed_by_valuethis argument (2048 byte) is passed by value, but might be more efficient if passed by reference (limit: 256 byte)
- linkedlistChecks for usage of any LinkedList, suggesting to use a Vec or a VecDeque (formerly called RingBuf)
All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
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