They will ICE during typechecking if used, because they depend on trait reform. This is part of unboxed closures.
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]
used to indicate when anonymous regions (i.e., &T) are permitted