support code such as `fn(x: !) -> ! { x }` with #![feature(never_type)]

[durka]

This kind of code worked with the original implementation of #![feature(never_type)], but that's because the check for diverging functions that converge was removed from liveness. With that restored, it incorrectly fires here.

@eddyb believes the fix involves:

<eddyb> and if you want to fix it *separately*, basically have a check like that that
        changes succ to be self.s.exit_ln if there's a NeverToAny adjustment for that
        expression, here https://github.com/rust-lang/rust/blob/master/src/librustc/middle/liveness.rs#L930
<eddyb> durka42: oh and if you do that separate PR, you can remove the ExprCall and
        ExprMethod call is_never checks

cc @canndrew. Discovered during #35883.

[arielb1]

You can also skip the check in liveness if the return expression has type TyNever - that's how MIR typeck does it.

[durka]

@arielb1 isn't that the check that we put back in at #35883? If you don't check you end up with fn foo() -> ! { 42 } being accepted.

[Mark-Simulacrum]

The below is now accepted. src/test/run-fail/call-fn-never-arg.rs tests for this.

#![feature(never_type)]

fn x(y: !) -> ! { y }