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Remove P: Unpin bound on impl Future for Pin
The `Unpin` bound was originally added in #56939 following the recommendation of @withoutboats in #55766 (comment) That comment does not give explicit justification for why the bound should be added. The relevant context was: > [ ] Remove `impl<P> Unpin for Pin<P>` > > This impl is not justified by our standard justification for unpin > impls: there is no pointer direction between `Pin<P>` and `P`. Its > usefulness is covered by the impls for pointers themselves. > > This futures impl (link to the impl changed in this PR) will need to > change to add a `P: Unpin` bound. The decision to remove the unconditional impl of `Unpin for Pin` is sound (these days there is just an auto-impl for when `P: Unpin`). But, I think the decision to also add the `Unpin` bound for `impl Future` may have been unnecessary. Or if that's not the case, I'd be very interested to have the argument for why written down somewhere. The bound _appears_ to not be needed, since the presence of a `Pin<P>` should indicate that it's safe to project to `Pin<&mut P::Target>` just like for `Pin::as_mut`.
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