delegation: fix zero-args nested delegation ICE

[aerooneqq]

This PR fixes an ICE when during lowering of nested delegation we need to access information about its parent, who is also inside body of another delegation. As a fix we lower delegation body even if there are no arguments in signature function, in this case we will see an error this function takes 0 arguments but 1 argument was supplied. Fixes #154332. Part of #118212.

r? @petrochenkov

[petrochenkov]

The current behavior exists to support glob reuse.

#![feature(fn_delegation)]
#![allow(incomplete_features)]

trait Trait {
    fn method(&self) {}
    fn static_method() {}
}

impl Trait for u8 {}

struct Wrapper { inner: u8 }

reuse impl Trait for Wrapper { self.inner } // should probably be ok

When Trait methods have self, then self.inner is passed instead of self, when other methods are static, then self.inner is not passed at all.

The current desugaring doesn't support it though, and produces error[E0283]: type annotations needed, so this fix is ok in the meantime.

[petrochenkov]

@bors r+

[rust-bors]

📌 Commit 11a338d has been approved by petrochenkov

It is now in the queue for this repository.