ICE combining universal plus existential impl Trait

[nikomatsakis]

The following program will ICE:

#![feature(conservative_impl_trait)]
#![feature(universal_impl_trait)]

use std::fmt::Debug;

fn foo<'a>(x: &'a impl Debug) -> impl Into<&'a impl Debug> {
    x
}

fn main() { }

The problem derives from the fact that universal impl traits, during HIR lowering, do not generate entries in the hir::Generics structure -- they are synthesized after the fact, in the ty::Generics. This interferes with the resolve_lifetime code, which expects to use the HIR generics to determine the indices of things.

I'm not sure the best fix here. The easiest fix is probably to count the number of universal impl traits and take them into account in resolve_lifetime. The best fix feels like it would be to actually insert the type parameters for universal impl trait instances into hir::Generics.

cc @cramertj @chrisvittal

[cramertj]

The best fix feels like it would be to actually insert the type parameters for universal impl trait instances into hir::Generics

This actually might not be that hard-- you can probably re-use a lot of the code I set up for lowering in-band lifetime bindings. You'd have to synthesize names for them, though, and make sure there aren't any conflicting names, plus the names can't escape into error messages.