ICE with misused conservative impl trait

[leonardo-m]

This is wrong code:

#![feature(conservative_impl_trait)]
fn foo(filename: &str) -> impl Iterator<Item=&str> {
    use std::fs::File;
    use std::io::{BufReader, BufRead};
    BufReader::new(File::open(filename).unwrap()).lines()
}
fn main() {}

The compiler gives:

error: internal compiler error: src\librustc_typeck\check\mod.rs:626: escaping regions in predicate Obligation(predicate=Binder(ProjectionPredicate(ProjectionTy { substs: Slice([_]), item_def_id: DefId { krate: CrateNum(2), node: DefIndex(1588) => core/571a388::iter[0]::iterator[0]::Iterator[0]::Item[0] } }, &str)),depth=0)
 --> ...\test.rs:2:27
  |
2 | fn foo(filename: &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 1.21.0-nightly (469a6f9bd 2017-08-22) running on x86_64-pc-windows-gnu

note: run with `RUST_BACKTRACE=1` for a backtrace

thread 'rustc' panicked at 'Box<Any>', src\librustc_errors\lib.rs:436:8
stack backtrace:
   0: mingw_set_invalid_parameter_handler
...

[kennytm]

Duplicate of #42479...