Programming Tutorials

  You have just finished refactoring your direct struct calls into a clean, abstract interface. You define an   async fn   inside your trait, update your dependency injection logic, and compile. Then, the Rust compiler halts with the error that keeps backend engineers awake at night: error: future cannot be sent between threads safely Specifically, you see that the trait bound  impl Future: Send  is not satisfied. When working with Tokio, this error stops you cold. It usually happens because you are trying to execute a trait method inside a  tokio::spawn  block, and the compiler cannot guarantee that the future generated by your trait implementation is thread-safe. This guide covers the root cause of this thread-safety violation in async traits and provides the modern, idiomatic fix for Rust 2024/2026 editions. The Root Cause: Opaque State Machines To fix the error, you must understand what an  async fn  actually is. When you compile an async...