sync: Once is no longer unsafe, update docs
Show impl for Ref & RefMut
auto merge of #14811 : forticulous/rust/refcell-show, r=alexcrichton

Show impl for RefCell and friends
auto merge of #14813 : cmr/rust/once-docs-unsafe, r=alexcrichton
librustc: Fix the issue with labels shadowing variable names by making
the leading quote part of the identifier for the purposes of hygiene.

This adopts @jbclements' solution to #14539.

I'm not sure if this is a breaking change or not.

Closes #12512.

[breaking-change]