Respect stacked borrows when polling

[eholk]

Polling the wrapped future previously had several reborrow operations, such as in the use of map_unchecked_mut and in the implementation of as_mut_ptr. This caused issues when running under miri in the stress_drop_sender test.

This change re-implements these problematic cases to avoid reborrows, either through using Pin::get_unchecked_mut or transmuting pointers rather than borrowing and casting.

Fixes #9

[daprilik]

As much as I'd love to rubber-stamp this PR, I'm not all too familiar with stacked borrow rules myself, so i'll leave it to Wesley to sign off here.

[yoshuawuyts]

I see Miri is enabled on CI, but the Miri docs mention stacked borrows is still experimental — does Miri in CI here validate against the stacked borrows rules, or does that need to be enabled somehow?

[eholk]

I've just been running Miri in the default settings and was able to repro the bug locally. We're using the defaults in CI too, so we should be good there too.

[yoshuawuyts]

Gotcha! Yay, great!

[wesleywiser]

Looks good to me!

[wesleywiser]

I see Miri is enabled on CI, but the Miri docs mention stacked borrows is still experimental — does Miri in CI here validate against the stacked borrows rules, or does that need to be enabled somehow?

Stacked borrows is the model used to detect aliasing violations so it's enabled by default. There's actually an option to disable it -Zmiri-disable-stacked-borrows.