ICE with generators and NLL

[DutchGhost]

The following ICE's on nightly 2018:

#![feature(generators, generator_trait)]

use std::ops::Generator;
use std::ops::GeneratorState::Yielded;

pub struct GenIter<G>(G);

impl <G> Iterator for GenIter<G>
where
    G: Generator,
{
    type Item = G::Yield;
    
    fn next(&mut self) -> Option<Self::Item> {
        unsafe {
            match self.0.resume() {
                Yielded(y) => Some(y),
                _ => None
            }
        }
    }
}

fn bug<'a>() -> impl Iterator<Item = &'a str> {
    GenIter(move || {
        let mut s = String::new();
        yield &s[..]
    })
}

fn main() {
    bug();
}

This is an NLL issue, on 2015 nightly without NLL turned on, the compiler complains that s does not live long enough. with NLL turned on in 2015, I get the same ICE.

Backtrace:

error: internal compiler error: librustc_mir/borrow_check/nll/region_infer/error_reporting/region_name.rs:181: can't make a name for free region '_#2r
  --> src/main.rs:25:13
   |
25 |       GenIter(move || {
   |  _____________^
26 | |         let mut s = String::new();
27 | |         yield &s[..]
28 | |     })
   | |_____^

thread 'main' panicked at 'Box<Any>', librustc_errors/lib.rs:538:9
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at libstd/sys_common/backtrace.rs:59
             at libstd/panicking.rs:211
   3: std::panicking::default_hook
             at libstd/panicking.rs:227
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:480
   6: std::panicking::begin_panic
   7: rustc_errors::Handler::span_bug
   8: rustc::util::bug::opt_span_bug_fmt::{{closure}}
   9: rustc::ty::context::tls::with_opt::{{closure}}
  10: rustc::ty::context::tls::with_context_opt
  11: rustc::ty::context::tls::with_opt
  12: rustc::util::bug::opt_span_bug_fmt
  13: rustc::util::bug::span_bug_fmt
  14: rustc_mir::borrow_check::nll::region_infer::error_reporting::region_name::<impl rustc_mir::borrow_check::nll::region_infer::RegionInferenceContext<'tcx>>::give_region_a_name
  15: rustc_mir::borrow_check::nll::region_infer::error_reporting::<impl rustc_mir::borrow_check::nll::region_infer::RegionInferenceContext<'tcx>>::free_region_constraint_info
  16: rustc_mir::borrow_check::nll::explain_borrow::<impl rustc_mir::borrow_check::MirBorrowckCtxt<'cx, 'gcx, 'tcx>>::explain_why_borrow_contains_point
  17: rustc_mir::borrow_check::error_reporting::<impl rustc_mir::borrow_check::MirBorrowckCtxt<'cx, 'gcx, 'tcx>>::report_borrowed_value_does_not_live_long_enough
  18: rustc_mir::borrow_check::path_utils::each_borrow_involving_path
  19: rustc_mir::borrow_check::MirBorrowckCtxt::access_place
  20: <rustc_mir::borrow_check::MirBorrowckCtxt<'cx, 'gcx, 'tcx> as rustc_mir::dataflow::DataflowResultsConsumer<'cx, 'tcx>>::visit_terminator_entry
  21: rustc_mir::borrow_check::do_mir_borrowck
  22: rustc::ty::context::tls::with_related_context
  23: rustc::infer::InferCtxtBuilder::enter
  24: rustc_mir::borrow_check::mir_borrowck
  25: rustc::ty::query::__query_compute::mir_borrowck
  26: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::mir_borrowck<'tcx>>::compute
  27: rustc::dep_graph::graph::DepGraph::with_task_impl
  28: rustc::ty::context::tls::with_related_context
  29: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  30: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  31: rustc::ty::query::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::mir_borrowck
  32: rustc_mir::borrow_check::nll::type_check::TypeChecker::check_stmt
  33: rustc_mir::borrow_check::nll::type_check::TypeChecker::typeck_mir
  34: rustc_mir::borrow_check::nll::type_check::type_check
  35: rustc_mir::borrow_check::do_mir_borrowck
  36: rustc::ty::context::tls::with_related_context
  37: rustc::infer::InferCtxtBuilder::enter
  38: rustc_mir::borrow_check::mir_borrowck
  39: rustc::ty::query::__query_compute::mir_borrowck
  40: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::mir_borrowck<'tcx>>::compute
  41: rustc::dep_graph::graph::DepGraph::with_task_impl
  42: rustc::ty::context::tls::with_related_context
  43: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  44: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  45: rustc::ty::query::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::mir_borrowck
  46: rustc::ty::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::par_body_owners
  47: rustc::util::common::time
  48: rustc::ty::context::tls::enter_context
  49: <std::thread::local::LocalKey<T>>::with
  50: rustc::ty::context::TyCtxt::create_and_enter
  51: rustc_driver::driver::compile_input
  52: rustc_driver::run_compiler_with_pool
  53: <scoped_tls::ScopedKey<T>>::set
  54: rustc_driver::run_compiler
  55: syntax::with_globals
  56: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:102
  57: rustc_driver::run
  58: rustc_driver::main
  59: std::rt::lang_start::{{closure}}
  60: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  61: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:102
  62: std::rt::lang_start_internal
             at libstd/panicking.rs:289
             at libstd/panic.rs:398
             at libstd/rt.rs:58
  63: main
  64: __libc_start_main
  65: <unknown>
query stack during panic:
#0 [mir_borrowck] processing `bug::{{closure}}`
#1 [mir_borrowck] processing `bug`
end of query stack
error: aborting due to previous error


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 1.32.0-nightly (36a50c29f 2018-11-09) running on x86_64-unknown-linux-gnu

note: compiler flags: -C opt-level=3 -C codegen-units=1 --crate-type bin

note: some of the compiler flags provided by cargo are hidden

error: Could not compile `playground`.

To learn more, run the command again with --verbose.