The Hook You have written an asynchronous Rust service. You are trying to store a collection of Futures in a Vec to run them concurrently, or perhaps you are attempting to race two async tasks using tokio::select! within a loop. Suddenly, the compiler halts with a distinct error: the trait bound 'impl Future: Unpin' is not satisfied or the trait 'Unpin' is not implemented for 'dyn Future<Output = ...>' This error is a rite of passage for Rust systems engineers. It usually leads to a frantic application of Box::pin until the red squiggles disappear, often without a clear understanding of why it was necessary or what performance cost was just incurred. The Why: Self-Referential Structs To understand Unpin , you must understand how Rust models async/await under the hood. When you write an async fn , the compiler transforms that function into a generated state machine (an enum ). If your async function has local ...
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