make `fn() -> _ { .. }` suggestion MachineApplicable
This might not be valid, but it would be nice to promote this to `MachineApplicable` so people can use rustfix here.
Also de65fcf009d07019689cfad7f327667e390a325d is to [restore the suggestion for `issue-77179.rs`](de65fcf009 (diff-12e43fb5d6d12ec7cb5c6b48204a18d113cf5de0e12eb71a358b639bd9aadaf0R8)). (though in this case, the code in that issue still doesn't compile, so it's not marked with rustfix).
4a7483c905