Programming Tutorials

  If you are writing async Rust using Tokio, you have likely encountered this compiler error chain: error: future cannot be sent between threads safely --> src/main.rs:15:5 | 15 | tokio::spawn(async move { | ^^^^^^^^^^^^ future created by async block is not `Send` | = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `std::sync::MutexGuard<'_, Data>` = note: required because it's used within this `async` block This error usually occurs when you hold a standard library  std::sync::MutexGuard  across an  .await  point. The Root Cause: Async State Machines and  Send To understand why this breaks, you must understand how Rust compiles  async  blocks and how the Tokio runtime schedules tasks. The State Machine:  When you write an  async  block, the compiler transforms it into a state machine generated as an anonymous  struct . Any variable that must exist...