This gives at least some explanation for why a borrow is expected to
last for a certain free region. Also:
* Reports E0373: "closure may outlive the current function" with NLL.
* Special cases the case of returning a reference to (or value
referencing) a local variable or temporary (E0515).
* Special case assigning a reference to a local variable in a closure
to a captured variable.
This commit updates the test output for the updated NLL compare mode
that uses `-Z borrowck=migrate` rather than `-Z borrowck=mir`. The
previous commit changes `compiletest` and this commit only updates
`.nll.stderr` files.
rustc_typeck: turn `where Type:,` into a WF(Type) predicate, instead of ignoring it.
Fixes#53696, assuming crater doesn't catch anyone using this syntax already.
Allowing an empty list of bounds in the grammar was done for the benefit of macro authors, most of which would *probably* be using it for bounds on type parameters, which are always WF.
r? @nikomatsakis cc @petrochenkov
