ICE with impl_trait

[leonardo-m]

This little program:

#![feature(conservative_impl_trait)]

fn suffixes(txt: &str) -> impl Iterator<Item=&str> {
    txt.char_indices().skip(1).map(|&(j, _)| &txt[j ..])
}
fn main() {}

Generates:

error: internal compiler error: src\librustc_typeck\check\mod.rs:617: escaping regions in predicate Obligation(predicate=Binder(ProjectionPredicate(ProjectionTy { trait_ref: <_ as std::iter::Iterator>, item_def_id: DefId { krate: CrateNum(2), node: DefIndex(1577) => core/a635d0d::iter[0]::iterator[0]::Iterator[0]::Item[0] } }, &str)),depth=0)
 --> test.rs:3:27
  |
3 | fn suffixes(txt: &str) -> impl Iterator<Item=&str> {
  |                           ^^^^^^^^^^^^^^^^^^^^^^^^

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.20.0-nightly (2fbba5bdb 2017-07-04) running on x86_64-pc-windows-gnu

[kennytm]

Duplicate of #43021 or #42479.

[arielb1]

Closing as duplicate.