Programming Tutorials

  One of the most notoriously difficult hurdles in async Rust—specifically when implementing manual   Future   types or middleware—is the compilation error:   cannot move out of ... which is behind a shared reference   or   cannot borrow data in a '&' reference as mutable . When you encounter this inside a  Future::poll  implementation, it is usually a symptom of a misunderstood memory model regarding  Pin . You are attempting to access a field of a struct that is structurally pinned, but you are not projecting that pin correctly to the field. This post dissects why naive field access fails in async contexts and demonstrates the correct implementation using structural pinning. The Root Cause: Pinning and Memory Stability To understand the error, we must look at the signature of the  Future  trait: pub trait Future { type Output; fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::O...