RS.CLIPPY.FUTURE_NOT_SEND

Public Futures must be Send

This checker is a Clippy lint created by The Rust Project Contributors. The documentation shown here is a copy of the original documentation for: future_not_send. Copyright ©2025 The Rust Team. All rights reserved.

What it does

This lint requires Future implementations returned from functions and methods to implement the Send marker trait, ignoring type parameters.

If a function is generic and its Future conditionally implements Send based on a generic parameter then it is considered Send and no warning is emitted.

This can be used by library authors (public and internal) to ensure their functions are compatible with both multi-threaded runtimes that require Send futures, as well as single-threaded runtimes where callers may choose !Send types for generic parameters.

Why is this bad?

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.

Example

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

Use instead:

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