clippy::future_not_send: future cannot be sent between threads safely

What you see

warning: future cannot be sent between threads safely
 --> main.rs:3:44
  |
3 | async fn not_send(bytes: std::rc::Rc<[u8]>) {}
  |                                            ^ future returned by `not_send` is not `Send`
  |
note: captured value is not `Send`
 --> main.rs:3:19
  |
3 | async fn not_send(bytes: std::rc::Rc<[u8]>) {}
  |                   ^^^^^ has type `std::rc::Rc<[u8]>` which is not `Send`
  = note: `std::rc::Rc<[u8]>` doesn't implement `std::marker::Send`
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#future_not_send
  = note: `-W clippy::future-not-send` implied by `-W clippy::nursery`

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?

A Future implementation captures some state that it needs to eventually produce its final value. When targeting a multithreaded executor (which is the norm on non-embedded devices) this means that this state may need to be transported to other threads, in other words the whole Future needs to implement the Send marker trait. If it does not, then the resulting Future cannot be submitted to a thread pool in the end user’s code. Especially for generic functions it can be confusing to leave the discovery of this problem to the end user: the reported error location will be far from its cause and can in many cases not even be fixed without modifying the library where the offending Future implementation is produced.

New lints still being finished. Off by default and may have false positives.

How to fix it

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

Flagged

async fn not_send(bytes: std::rc::Rc<[u8]>) {}

Write this instead

async fn is_send(bytes: std::sync::Arc<[u8]>) {}

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

[lints.clippy]
future_not_send = "warn"

#![warn(clippy::future_not_send)]

cargo clippy -- -W clippy::future-not-send

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.