Fix `thread` `park`/`unpark` synchronization#54174

Merged

bors merged 3 commits intorust-lang:masterfrom

Sep 19, 2018

Commits on Sep 12, 2018

Fix `thread` `park`/`unpark` synchronization
James Duley
committed

Commits on Sep 14, 2018

Add comments and assertion to `park`/`unpark`
James Duley
committed

Commits on Sep 18, 2018

Expand synchronization comments in `park`/`unpark`
James Duley
committed