Invalid usage of `&mut self` in `std::sync::mpsc`

[alexcrichton]

Almost all of the internal code in the mpsc module makes use of &mut self, which isn't valid because it's all explicitly designed to work across multiple threads, meaning the guarantees of &mut self don't hold. This entire implementation dates back to Fall 2013, which is long before the multithreading of &self vs &mut self was even decided!

We should update these implementation details to basically never use &mut self but instead use UnsafeCell where appropriate.

[jnicholls]

Could this be a root cause for why this unreachable is being hit, due to a spurious wake up? https://github.com/rust-lang/rust/blob/1.12.0/src/libstd/sync/mpsc/mod.rs#L880 Seeing Empty condition in 1.12.0.

[alexcrichton]

@jnicholls perhaps yeah, but AFAIK #38421 didn't have any substantive change beyond a standard refactor, so my guess is that it'd be unlikely