Add get_pin_ref and get_pin_mut methods to slice

[taiki-e]

This adds methods to return a pinned reference to an element or subslice from a pinned slice.

Motivation: https://users.rust-lang.org/t/working-with-pinned-slices-are-there-any-structurally-pinning-vec-like-collection-types/50634

Suffix (*_pin_{ref, mut}) is based on Option's as_pin_{ref, mut} methods.

[rust-highfive]

r? @joshtriplett

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[Aaron1011]

The existence of this method means that unsafe code needs to be careful when projecting from a Pin<&mut [T]> to a &mut T (a mutable reference to one of the slice elements). It can still be done soundly, but the &mut T cannot be moved out of if the Pin<&mut [T]> is ever exposed to safe code (which could call get_pin_mut).

I think it would be good to mention this in the docs.

[taiki-e]

That is correct. Option::as_pin_mut also has the same issue. We need to add docs to both.

[JohnCSimon]

taiki-e on Nov 21, 2020
That is correct. Option::as_pin_mut also has the same issue. We need to add docs to both.

assigning back to author - not sure if this is done

[crlf0710]

@taiki-e Ping from triage. It seems the next step for this PR would be update the docs and switching this PR to S-waiting-on-review?

[Dylan-DPC-zz]

Closing this due to inactivity.