How to project a pin through a field to an inner field?

[quinedot]

@philomathic_life

This should apply to any type though; thus if Bar contained Result<T, E> instead of Foo<T>; it would not be allowed to project to T since Result doesn't offer a projection method, correct?

Not if you made the Result structually pinned (as you have in the OP), is how I understand it.

@Yandros

So, the question is:

Can we go from Pin<&[mut] Wrapper<Field>> to Pin<&[mut] Field>?

The point that may still be unclear in the documentation is the following: it is up to the creator of Wrapper to decide!

Let's say Bar contained a Result<T, E>. You've provided a safe method to get from Pin<&mut Bar> to Pin<&mut Result<T, E>>. It's up to Result<T, E> to decide if there's a safe way to get from there to Pin<&mut T>... or to &mut T. If they decide on the latter, and you also provided the safe projection from Pin<&mut Bar> to Pin<&mut T>, one could (without unsafe) create the Pin<&mut T>, then the Pin<&mut Result<T, E>>, then the &mut T, and then swap out the T without calling drop.

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What if you hadn't provided a way to get a Pin<&mut Result<T, E>>? Then, as I understand it, you could provide the Pin<&mut T> projection (provided you meet all the other requirements).