eliding lifetimes in impl trait leads to a compiler panic

[0x7CFE]

My code contained the following method:

pub fn iter<'b>(&'b self) -> impl Iterator<Item=(&'b FragmentKey, &'b Box<Fragment>)> {
    self.map.iter()
}

Clippy suggested me that lifetime specifiers are redundant here and they may be elided completely. So I simplified the code like this:

pub fn iter(&self) -> impl Iterator<Item=(&FragmentKey, &Box<Fragment>)> {
    self.map.iter()
}

But when it tried to rebuild the project, compiler crashed:

$ RUST_BACKTRACE=1  cargo build                   
   Compiling cortex v0.1.0 (file:///home/korvin/work/research/redozubov/cortex)
error: internal compiler error: unexpected panic

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: run with `RUST_BACKTRACE=1` for a backtrace

thread 'rustc' panicked at 'assertion failed: match *region { ty::ReLateBound(..) => false, _ => true, }', /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/librustc/infer/higher_ranked/mod.rs:492
stack backtrace:
   1:     0x7f0cee64145c - std::sys::imp::backtrace::tracing::imp::write::hf7294f5e24536b4a
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:42
   2:     0x7f0cee64f90e - std::panicking::default_hook::{{closure}}::h9a07d0b00c43fbee
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/panicking.rs:351
   3:     0x7f0cee64f4b3 - std::panicking::default_hook::hf25feff2d08bf39b
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/panicking.rs:361
   4:     0x7f0cee64fdab - std::panicking::rust_panic_with_hook::h4cb8c6fbb8386ccf
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/panicking.rs:555
   5:     0x7f0ceb5be72f - std::panicking::begin_panic::h9e2767cec6fae47f
   6:     0x7f0ceb6bcae7 - rustc::infer::higher_ranked::fold_regions_in::{{closure}}::hfdfe0b774688fbf9
   7:     0x7f0ceb7b3ebe - rustc::ty::fold::TypeFolder::fold_ty::h9973aa2c13a5eff1
   8:     0x7f0ceb5b1fa4 - <rustc_data_structures::accumulate_vec::AccumulateVec<A> as core::iter::traits::FromIterator<<A as rustc_data_structures::array_vec::Array>::Element>>::from_iter::h3c0bd810cda7086d
   9:     0x7f0ceb7f1499 - rustc::ty::structural_impls::<impl rustc::ty::fold::TypeFoldable<'tcx> for &'tcx rustc::ty::Slice<&'tcx rustc::ty::TyS<'tcx>>>::super_fold_with::h1b503259b5d3b322
  10:     0x7f0ceb7b3dc4 - rustc::ty::fold::TypeFolder::fold_ty::h9973aa2c13a5eff1
  11:     0x7f0ceb6e5bf4 - rustc::infer::InferCtxt::match_poly_projection_predicate::h9b3155fdc343189c
  12:     0x7f0ceb78e6fd - rustc::traits::project::confirm_param_env_candidate::h824a2cb6e91e204f
  13:     0x7f0ceb78b03f - rustc::traits::project::opt_normalize_projection_type::h9f3de06b07283d8b
  14:     0x7f0ceb7886ef - rustc::traits::project::normalize_projection_type::h79a91c393ed89b1d
  15:     0x7f0ceb78836c - <rustc::traits::project::AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty::hdd75b6b51662ff62
  16:     0x7f0ceb789c94 - rustc::traits::project::opt_normalize_projection_type::h9f3de06b07283d8b
  17:     0x7f0ceb78604c - rustc::traits::project::project_and_unify_type::hac56b6032c41964c
  18:     0x7f0ceb6dd9e2 - rustc::infer::InferCtxt::commit_if_ok::hd2912d20d4e08c55
  19:     0x7f0ceb783d13 - <rustc::traits::fulfill::FulfillProcessor<'a, 'b, 'gcx, 'tcx> as rustc_data_structures::obligation_forest::ObligationProcessor>::process_obligation::hcaed46acd96ba11a
  20:     0x7f0ceb6210d7 - <rustc_data_structures::obligation_forest::ObligationForest<O>>::process_obligations::h0a39d2a7dea0d69b
  21:     0x7f0ceb7825cf - rustc::traits::fulfill::FulfillmentContext::select_where_possible::h1de446b26cc56aea
  22:     0x7f0cebe28439 - rustc_typeck::check::FnCtxt::select_obligations_where_possible::h3d9dc81d4ec899ba
  23:     0x7f0cebe29700 - rustc_typeck::check::FnCtxt::check_argument_types::h1645086727026c10
  24:     0x7f0cebe02be6 - rustc_typeck::check::callee::<impl rustc_typeck::check::FnCtxt<'a, 'gcx, 'tcx>>::confirm_builtin_call::h4317ad5b85dc5c65
  25:     0x7f0cebe01ac5 - rustc_typeck::check::callee::<impl rustc_typeck::check::FnCtxt<'a, 'gcx, 'tcx>>::check_call::h1be9195a210d695b
  26:     0x7f0cebe3599f - rustc_typeck::check::FnCtxt::check_expr_kind::h2c29b2633d90fef5
  27:     0x7f0cebe3488d - rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_lvalue_pref::hc0ab5cf329ed3bb9
  28:     0x7f0cebdb85b4 - rustc_typeck::check::_match::<impl rustc_typeck::check::FnCtxt<'a, 'gcx, 'tcx>>::check_match::h5d3c9633e773daac
  29:     0x7f0cebe34fd1 - rustc_typeck::check::FnCtxt::check_expr_kind::h2c29b2633d90fef5
  30:     0x7f0cebe3488d - rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_lvalue_pref::hc0ab5cf329ed3bb9
  31:     0x7f0cebe4618d - rustc_typeck::check::FnCtxt::check_decl_initializer::h5e55f2dd6081fe5f
  32:     0x7f0cebe46294 - rustc_typeck::check::FnCtxt::check_decl_local::h1a92ec3283fc5732
  33:     0x7f0cebe465c6 - rustc_typeck::check::FnCtxt::check_stmt::hd5c04fea4080a98c
  34:     0x7f0cebe4691e - rustc_typeck::check::FnCtxt::check_block_with_expected::h03584e43fc04a13e
  35:     0x7f0cebe34cc6 - rustc_typeck::check::FnCtxt::check_expr_kind::h2c29b2633d90fef5
  36:     0x7f0cebe3488d - rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_lvalue_pref::hc0ab5cf329ed3bb9
  37:     0x7f0cebe4655c - rustc_typeck::check::FnCtxt::check_stmt::hd5c04fea4080a98c
  38:     0x7f0cebe4691e - rustc_typeck::check::FnCtxt::check_block_with_expected::h03584e43fc04a13e
  39:     0x7f0cebe34cc6 - rustc_typeck::check::FnCtxt::check_expr_kind::h2c29b2633d90fef5
  40:     0x7f0cebe3488d - rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_lvalue_pref::hc0ab5cf329ed3bb9
  41:     0x7f0cebe1aef1 - rustc_typeck::check::check_fn::hdcc283c26b22bc9e
  42:     0x7f0cebe19f04 - rustc_typeck::check::check_bare_fn::hf15cf554a01d6e48
  43:     0x7f0cebe170f5 - rustc_typeck::check::check_item_bodies::ha330650a58a36448
  44:     0x7f0cebe84a47 - rustc_typeck::check_crate::h06f4ceb61c7f6528
  45:     0x7f0cee9ede57 - rustc_driver::driver::phase_3_run_analysis_passes::{{closure}}::h074ad5edbf4a7db7
  46:     0x7f0cee9e0286 - rustc_driver::driver::phase_3_run_analysis_passes::h380fa931cb354859
  47:     0x7f0cee9ce090 - rustc_driver::driver::compile_input::hab977ae496b3a6f1
  48:     0x7f0ceea189c4 - rustc_driver::run_compiler::h81290683db66a63c
  49:     0x7f0cee924deb - std::panicking::try::do_call::h00942d7a5d04424f
  50:     0x7f0cee658bea - __rust_maybe_catch_panic
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libpanic_unwind/lib.rs:98
  51:     0x7f0cee94d052 - <F as alloc::boxed::FnBox<A>>::call_box::hd87f7ab2fccbd670
  52:     0x7f0cee64e764 - std::sys::imp::thread::Thread::new::thread_start::hc16926852e47c008
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/liballoc/boxed.rs:623
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/sys_common/thread.rs:21
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/sys/unix/thread.rs:84
  53:     0x7f0ce640a6b9 - start_thread
  54:     0x7f0cee30482c - clone
  55:                0x0 - <unknown>

error: Could not compile `cortex`.

$ rustc --version --verbose
rustc 1.17.0-nightly (0648517fa 2017-02-03)
binary: rustc
commit-hash: 0648517faf1e2cf37c8b6770cbd0180a816ed9a0
commit-date: 2017-02-03
host: x86_64-unknown-linux-gnu
release: 1.17.0-nightly
LLVM version: 3.9

[Mark-Simulacrum]

No longer ICEs today. E-needstest.

[venkatagiri]

@Mark-Simulacrum, the example attached in duplicate issue #39929 is stilling ICEing. https://is.gd/yWRQML

[Mark-Simulacrum]

I'll reopen that one then since it apparently was not a duplicate.

[venkatagiri]

It still ICEs on nightly.

$ git diff
diff --git a/src/memory.rs b/src/memory.rs
index 09529ab..0eb0aa2 100644
--- a/src/memory.rs
+++ b/src/memory.rs
@@ -558,7 +558,7 @@ impl Dictionary {
         prototype.get_key(freq_range, detectors)
     }

-    pub fn iter<'b>(&'b self) -> impl Iterator<Item=(&'b FragmentKey, &'b Box<Fragment>)> {
+    pub fn iter(&self) -> impl Iterator<Item=(&FragmentKey, &Box<Fragment>)> {
         self.map.iter()
     }

$ cargo +nightly build
   Compiling cortex v0.1.0 (file:///tmp/cortex)
error: internal compiler error: unexpected panic

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

thread 'rustc' panicked at 'assertion failed: match *region { ty::ReLateBound(..) => false, _ => true, }', /checkout/src/librustc/infer/higher_ranked/mod.rs:493
note: Run with `RUST_BACKTRACE=1` for a backtrace.

error: Could not compile `cortex`.

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

$ rustup run nightly rustc --version --verbose
rustc 1.19.0-nightly (5b13bff52 2017-05-23)
binary: rustc
commit-hash: 5b13bff5203c1bdc6ac6dc87f69b5359a9503078
commit-date: 2017-05-23
host: x86_64-unknown-linux-gnu
release: 1.19.0-nightly
LLVM version: 4.0

[Mark-Simulacrum]

Minimal test case (possibly also above, but I found it hard to find last time I triaged..)

#![feature(conservative_impl_trait)]

pub fn iter(x: &usize) -> impl Iterator<Item=(&i32, &Box<i32>)> {
    unimplemented!()
}

fn main() {}