Clippy lint · Suspicious · checked with Clippy 0.1.99

clippy::arc_with_non_send_sync: arc with non send sync

What it meansClippy lint arc_with_non_send_sync. It is in the suspicious group and warns by default.

Does it matter?

Arc<T> is a thread-safe Rc<T> and guarantees that updates to the reference counter use atomic operations. To send an Arc<T> across thread boundaries and share ownership between multiple threads, T must be both Send and Sync, so either T should be made Send + Sync or an Rc should be used instead of an Arc.

Code that is most likely wrong or useless. Warns by default.

How to fix it

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

Flagged


fn main() {
    // This is fine, as `i32` implements `Send` and `Sync`.
    let a = Arc::new(42);

    // `RefCell` is `!Sync`, so either the `Arc` should be replaced with an `Rc`
    // or the `RefCell` replaced with something like a `RwLock`
    let b = Arc::new(RefCell::new(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::arc_with_non_send_sync)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
arc_with_non_send_sync = "allow"

One run

cargo clippy -- -A clippy::arc-with-non-send-sync

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.

Suspiciousgroup

warndefault level

nofixed by cargo clippy --fix

1.72.0added in Rust

Clippy's own documentation

What it does.

This lint warns when you use Arc with a type that does not implement Send or Sync.

Why is this bad?

Arc<T> is a thread-safe Rc<T> and guarantees that updates to the reference counter use atomic operations. To send an Arc<T> across thread boundaries and share ownership between multiple threads, T must be both Send and Sync, so either T should be made Send + Sync or an Rc should be used instead of an Arc.

Example


fn main() {
    // This is fine, as `i32` implements `Send` and `Sync`.
    let a = Arc::new(42);

    // `RefCell` is `!Sync`, so either the `Arc` should be replaced with an `Rc`
    // or the `RefCell` replaced with something like a `RwLock`
    let b = Arc::new(RefCell::new(42));
}

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

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