Programming Tutorials

  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. Crucial...

  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...