If you are working with Tokio and async Rust, you have likely encountered this compiler error. It usually looks like a wall of text ending with note: required by 'tokio::spawn', but the core message is specific:
error[E0277]: `Rc<...>` cannot be sent between threads safely
// OR
error[E0277]: `std::sync::MutexGuard<'_, ...>` cannot be sent between threads safely
This error prevents you from spawning tasks onto the Tokio runtime. It is not a syntax error; it is a fundamental architectural constraint of work-stealing runtimes enforced by Rust's type system.
The Root Cause: Async State Machines and the Send Trait
To understand the fix, you must understand what the compiler does with an async block.
When you write an async block, the Rust compiler transforms your code into a State Machine. This state machine is implemented as an enum where every .await point represents a variant transition.
Crucially, any variable that lives across an .await point must be stored inside this generated enum/struct.
Tokio's tokio::spawn function requires that the Future you pass it implements the Send trait. This is because Tokio uses a multi-threaded, work-stealing scheduler. A task might start executing on Thread A, hit an .await, yield execution, and later be woken up and resumed on Thread B.
Therefore, the entire generated State Machine (the Future) must be Send. By transitivity, every single field stored within that State Machine must be Send.
If you hold a non-Send type (like Rc or std::sync::MutexGuard) across an .await point, the compiler includes it in the state machine, rendering the entire Future !Send.
Scenario 1: The Rc Mistake
std::rc::Rc is a Reference Counted pointer designed for single-threaded scenarios. It uses non-atomic arithmetic for the reference counter. If moved between threads, a race condition could corrupt the count, leading to memory leaks or double-frees.
The Fix: Switch to Arc
If you need shared ownership in an async environment, you must use std::sync::Arc (Atomic Reference Counted).
Bad Code (Compilation Fail):
use std::rc::Rc;
use tokio::task;
#[tokio::main]
async fn main() {
let data = Rc::new(5);
// Error: `Rc` cannot be sent between threads safely
task::spawn(async move {
// The Rc is moved into the async block
println!("Data: {}", data);
}).await.unwrap();
}
The Solution: Simply replace Rc with Arc. Arc uses atomic CPU instructions to update the count, making it safe to share across threads.
use std::sync::Arc;
use tokio::task;
#[tokio::main]
async fn main() {
let data = Arc::new(5);
task::spawn(async move {
println!("Data: {}", data);
}).await.unwrap();
}
Scenario 2: The MutexGuard Trap
This is the more subtle and common issue for systems engineers. std::sync::Mutex itself is Send (assuming the data inside is Send). However, std::sync::MutexGuard (the object returned by .lock().unwrap()) is NOT Send.
This is not just a Rust quirk; it is a POSIX requirement. In many operating systems, the thread that acquires a mutex must be the one to release it. If you hold a MutexGuard across an .await, the task might resume on a different thread, attempting to unlock a mutex owned by the previous thread.
The Broken Pattern
use std::sync::{Arc, Mutex};
struct State {
count: u64,
}
#[tokio::main]
async fn main() {
let state = Arc::new(Mutex::new(State { count: 0 }));
let state_clone = state.clone();
tokio::spawn(async move {
// 1. We acquire the lock here.
// `guard` is type std::sync::MutexGuard
let mut guard = state_clone.lock().unwrap();
guard.count += 1;
// 2. We await here.
// CRITICAL: `guard` is still in scope (alive).
// The compiler MUST save `guard` into the Future's state machine.
// Since MutexGuard is !Send, the Future becomes !Send.
some_io_operation().await;
// 3. We use guard again (or it simply drops here).
println!("Count: {}", guard.count);
});
}
async fn some_io_operation() {
tokio::time::sleep(tokio::time::Duration::from_millis(10)).await;
}
Solution A: Restrict the Scope (Recommended)
The most performant fix is to ensure the MutexGuard is dropped before the .await call. You can force this using a purely lexical block. This ensures the lock is never held while the task is yielded, preventing deadlocks and satisfying the Send requirement.
use std::sync::{Arc, Mutex};
struct State {
count: u64,
}
#[tokio::main]
async fn main() {
let state = Arc::new(Mutex::new(State { count: 0 }));
let state_clone = state.clone();
tokio::spawn(async move {
{
// Scope created specifically for the lock
let mut guard = state_clone.lock().unwrap();
guard.count += 1;
// 'guard' is dropped here, unlocking the mutex
}
// The state machine no longer holds the MutexGuard
some_io_operation().await;
// If we need to read it again, we lock again
let count = state_clone.lock().unwrap().count;
println!("Count: {}", count);
});
}
async fn some_io_operation() {
tokio::time::sleep(tokio::time::Duration::from_millis(10)).await;
}
Solution B: Use tokio::sync::Mutex
If you absolutely must hold a lock across an .await point (e.g., you need to ensure exclusive access to a resource while performing I/O), you cannot use the standard library Mutex. You must use Tokio's asynchronous Mutex.
Tokio's Mutex is not based on OS primitives in the same way; it is an asynchronous semaphore. Its MutexGuard is Send.
Warning: This comes with a performance cost. It is slower than std::sync::Mutex. Only use this if you strictly need to hold the lock while awaiting.
use std::sync::Arc;
use tokio::sync::Mutex; // Note the import change
struct State {
count: u64,
}
#[tokio::main]
async fn main() {
// Wrap in Tokio's Mutex
let state = Arc::new(Mutex::new(State { count: 0 }));
let state_clone = state.clone();
tokio::spawn(async move {
// .lock() is now an async method!
let mut guard = state_clone.lock().await;
guard.count += 1;
// This is now SAFE.
// tokio::sync::MutexGuard IS Send.
some_io_operation().await;
println!("Count: {}", guard.count);
});
}
async fn some_io_operation() {
tokio::time::sleep(tokio::time::Duration::from_millis(10)).await;
}
Summary
When you see E0277 ... cannot be sent between threads safely involving a Future in Tokio:
- Look for non-
Sendtypes stored in variables within theasyncblock. - If it is an
Rc, replace it withArc. - If it is a
std::sync::MutexGuard:- Ideally: Scope the lock so it drops before any
.await. - Alternatively: Switch to
tokio::sync::Mutexif the lock must be held during the await.
- Ideally: Scope the lock so it drops before any