Tracking issue for `lifetime_underscore` compatibility lint

[petrochenkov]

What is this lint about

Lifetime name '_ is reserved for possible future use as a lifetime placeholder, similarly to how _ is used as a placeholder in types or patterns (see, for example, rust-lang/rfcs#1177).
Earlier versions of the compiler accepted '_ as a normal lifetime or label name, #33794 fixed this oversight.

How to fix this warning/error

Rename the lifetime or label named '_ into something else, like 'a or 'z.

Current status

• Add AST validation pass and move some checks to it #33794 introduces the lifetime_underscore lint as warn-by-default
• Make sufficiently old or low-impact compatibility lints deny-by-default #36894 makes the lifetime_underscore lint deny-by-default
• Turn sufficiently old compatibility lints into hard errors #42136 makes the lifetime_underscore lint a hard error