ManuallyDrop causes pinning to be unsound?

[asajeffrey]

ManuallyDrop<Pin<T>> causes Pin<T> not to call its destructor, oh dear. This is different from mem::forget(T) because that has to be called after the pinning (so the borrowchecker will complain about lifetimes) whereas ManuallyDrop::new is called before. Sigh.

[asajeffrey]

IRC chat: https://mozilla.logbot.info/servo/20180130#c14216445 and https://botbot.me/mozilla/rust-lang/2018-01-30/?msg=96321031&page=2

RFC issue: rust-lang/rfcs#1860

cc @nox

[asajeffrey]

Some more IRC chat: https://botbot.me/mozilla/rust-lang/2018-02-01/?msg=96382526&page=2

[asajeffrey]

And yet more, this time with a possible solution! https://mozilla.logbot.info/servo/20180201#c14230291

[asajeffrey]

A very rough draft of what a fixed API would look like: https://play.rust-lang.org/?gist=6791dbfd630b20c8cabda5511e43e089&version=stable

[asajeffrey]

Posted a "lessons learned" message to the unsafe code guidelines discussion forum: https://internals.rust-lang.org/t/rust-1-20-caused-pinning-to-become-incorrect/6695