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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

<details>

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

</details>