Fix ICE in msg_span_from_free_region on ReEmpty

[pnkfelix]

On an example like this:

#![feature(conservative_impl_trait)]

fn server() -> impl FilterBase2 {
    segment2(|| { loop { } }).map2(|| "")
}

trait FilterBase2 {
    fn map2<F>(self, _fn: F) -> Map2<F> where Self: Sized { loop { } }
}

struct Map2<F> { _func: F }

impl<F> FilterBase2 for Map2<F> { }

fn segment2<F>(_fn: F) -> Map2<F> where F: Fn() -> Result<(), ()> {
    loop { }
}

we now, instead of ICE'ing, get a diagnostic like:

error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
 --> issue-55608.rs:3:16
  |
3 | fn server() -> impl FilterBase2 {
  |                ^^^^^^^^^^^^^^^^
  |
  = note: hidden type `Map2<[closure@issue-55608.rs:4:36: 4:41]>` captures an empty lifetime

Fix #55608

[rust-highfive]

r? @estebank

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[pnkfelix]

looking at the regression test and the resulting error diagnostic, you might well ask: "Where is the lifetime coming from that is being captured? Should we be accepting this code rather than erroring?"

And these are good questions.

[estebank]

@bors r+ rollup

[bors]

📌 Commit cc33aec has been approved by estebank