Lifetimes on nested `impl Trait` causes ICE

[Osspial]

Compiling this code:

fn foo<'a>() -> impl Iterator<Item = impl 'a + Iterator<Item=()>> {
    // Body is irrelevant but needed to create a properly-typed return value
    Some(None.into_iter()).into_iter()
}

Crashes the compiler, with the given error message:

error: internal compiler error: librustc\ty\subst.rs:424: Region parameter out of range when substituting in region 'a (root type=None) (index=1)

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 1.25.0-nightly (4e3901d35 2018-01-23) running on x86_64-pc-windows-msvc

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

thread 'rustc' panicked at 'Box<Any>', librustc_errors\lib.rs:456:9
stack backtrace:
   0: <std::sync::condvar::Condvar as core::fmt::Debug>::fmt
   1: std::panicking::Location::column
   2: std::panicking::Location::column
   3: std::panicking::rust_panic_with_hook
   4: rustc::ty::context::TyCtxt::_intern_substs
   5: rustc::util::ppaux::<impl core::fmt::Debug for rustc::ty::Predicate<'tcx>>::fmt
   6: rustc::util::ppaux::<impl core::fmt::Debug for rustc::ty::Predicate<'tcx>>::fmt
   7: rustc::ty::context::tls::span_debug
   8: rustc::session::bug_fmt
   9: rustc::session::bug_fmt
  10: <rustc::ty::subst::SubstFolder<'a, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_region
  11: rustc::ty::error::<impl rustc::ty::TyS<'tcx>>::sort_string
  12: rustc::middle::lang_items::LanguageItems::items
  13: rustc::ty::GenericPredicates::instantiate_own
  14: rustc::infer::anon_types::Instantiator::fold_anon_ty
  15: rustc::ty::error::<impl rustc::ty::TyS<'tcx>>::sort_string
  16: rustc::ty::error::<impl rustc::ty::TyS<'tcx>>::sort_string
  17: rustc::ty::error::<impl rustc::ty::TyS<'tcx>>::sort_string
  18: rustc::ty::error::<impl rustc::ty::TyS<'tcx>>::sort_string
  19: rustc::infer::anon_types::Instantiator::fold_anon_ty
  20: <rustc_typeck::namespace::Namespace as core::fmt::Debug>::fmt
  21: <rustc_typeck::check::GatherLocalsVisitor<'a, 'gcx, 'tcx> as rustc::hir::intravisit::Visitor<'gcx>>::visit_pat
  22: <rustc::hir::Arm as rustc_typeck::check::coercion::AsCoercionSite>::as_coercion_site
  23: <unknown>
  24: <rustc_typeck::check::CheckItemTypesVisitor<'a, 'tcx> as rustc::hir::itemlikevisit::ItemLikeVisitor<'tcx>>::visit_item
  25: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_tables_of<'tcx>>::ensure
  26: rustc::dep_graph::graph::DepGraph::assert_ignored
  27: rustc::util::ppaux::<impl core::fmt::Debug for rustc::ty::Predicate<'tcx>>::fmt
  28: <rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx> as rustc::ty::layout::HasTyCtxt<'gcx>>::tcx
  29: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_tables_of<'tcx>>::ensure
  30: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_tables_of<'tcx>>::try_get
  31: rustc::ty::maps::TyCtxtAt::typeck_tables_of
  32: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_tables_of<'tcx>>::ensure
  33: <rustc_typeck::check::CheckItemTypesVisitor<'a, 'tcx> as rustc::hir::itemlikevisit::ItemLikeVisitor<'tcx>>::visit_item
  34: rustc::dep_graph::graph::DepGraph::assert_ignored
  35: rustc::util::ppaux::<impl core::fmt::Debug for rustc::ty::Predicate<'tcx>>::fmt
  36: <rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx> as rustc::ty::layout::HasTyCtxt<'gcx>>::tcx
  37: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_item_bodies<'tcx>>::ensure
  38: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_item_bodies<'tcx>>::try_get
  39: rustc::ty::maps::TyCtxtAt::typeck_item_bodies
  40: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::typeck_item_bodies
  41: rustc_typeck::check_crate
  42: <env_logger::filter::Filter as core::fmt::Display>::fmt
  43: <env_logger::filter::Filter as core::fmt::Display>::fmt
  44: rustc_driver::driver::compile_input
  45: rustc_driver::run_compiler
  46: <unknown>
  47: _rust_maybe_catch_panic
  48: <unknown>
  49: <std::sync::condvar::Condvar as core::fmt::Debug>::fmt
  50: std::sys::windows::thread::Thread::new
  51: BaseThreadInitThunk

Removing the lifetime on the inner impl Trait, like so, lets it compile without error:

fn foo<'a>() -> impl Iterator<Item = impl Iterator<Item=()>> {
    Some(None.into_iter()).into_iter()
}

Meta

> rustc --version --verbose
rustc 1.25.0-nightly (4e3901d35 2018-01-23)
binary: rustc
commit-hash: 4e3901d35f6a8652f67111e7272263c9e62ab3e1
commit-date: 2018-01-23
host: x86_64-pc-windows-msvc
release: 1.25.0-nightly
LLVM version: 4.0

Related to #34511 .

[cramertj]

Dup of #46464