How do I fix "future cannot be sent between threads safely" in Rust?

tokio::spawn needs a future that can move between threads (Send). Something not thread-safe, often a std MutexGuard or an Rc, is held across an .await. Drop the guard before awaiting. Use tokio's Mutex when you must hold it across await. Use Arc instead of Rc.

Rust errors / Build and Cargo

Rust build error · checked with Rust 1.99.0

error: future cannot be sent between threads safely

error: future cannot be sent between threads safely

What it meanstokio::spawn needs a future that can move between threads (Send). Something not thread-safe, often a std MutexGuard or an Rc, is held across an .await.

Why it happens

Holding a std::sync::MutexGuard across .await.
Keeping an Rc, RefCell borrow or raw pointer alive across .await.

How to fix it

1. Drop the guard before awaiting

Scope it so it ends before the await.

let value = {
    let g = state.lock().unwrap();
    g.clone()
}; // guard dropped here
send(value).await;

2. Use tokio's Mutex when you must hold it across await

let g = state.lock().await;  // tokio::sync::Mutex

3. Use Arc instead of Rc

let shared = std::sync::Arc::new(data);

Other wordings of this error

future cannot be sent between threads safely, future is not Send, has type which is not Send, MutexGuard is not Send tokio spawn

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