Unpin trait

  • Unpin type allows types to move freely, even when they’re wrapped by a Pin
  • Most types implement Unpin, because it is an “auto trait”
  • auto trait behavior can be changed:
    • !Unpin types must never move
    • Types containing a PhantomPinned field do not implement Unpin by default

Explain that when a trait implements Unpin, the pinning behavior of Pin<Ptr> does not get invoked. The value is free to move.

Explain that almost all types implement Unpin; automatically implemented by the compiler.

Types implementing Unpin are saying: ‘I promise I have no self-references, so moving me is always safe.’

Ask: What types might be !Unpin?

  • Compiler-generated futures
  • Types containing a PhantomPinned field
  • Some types wrapping C++ objects

!Unpin types cannot be moved once pinned