ICE with associated existential type

[ambiso]

Hi,

I'm guessing this error is known, but it appears to fail in a different way compared to #54899.

Code:

#![feature(existential_type)]


trait IterBits {
    type BitsIter: Iterator<Item = u8>;
    fn iter_bits(self, n: u8) -> Self::BitsIter;
}

existential type IterBitsIter<T, E, I>: std::iter::Iterator<Item = I>;

impl<T, E> IterBits for T where T: std::ops::Shr<Output = T> + std::ops::BitAnd<T, Output = T> + std::convert::From<u8> + std::convert::TryInto<u8, Error = E>,
                                E: std::fmt::Debug
{
    type BitsIter = IterBitsIter<T, E, u8>;
    fn iter_bits(self, n: u8) -> Self::BitsIter {
        (0u8..n)
            .rev()
            .map(move |shift| ((self >> T::from(shift)) & T::from(1)).try_into().unwrap())
    }
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error: type parameter `E` is part of concrete type but not used in parameter list for existential type
  --> src/lib.rs:14:49
   |
14 |       fn iter_bits(self, n: u8) -> Self::BitsIter {
   |  _________________________________________________^
15 | |         (0u8..n)
16 | |             .rev()
17 | |             .map(move |shift| ((self >> T::from(shift)) & T::from(1)).try_into().unwrap())
18 | |     }
   | |_____^

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', src/libcore/option.rs:347:21
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:59
             at src/libstd/panicking.rs:197
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:211
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:478
   6: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:381
   7: rust_begin_unwind
             at src/libstd/panicking.rs:308
   8: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
   9: core::panicking::panic
             at src/libcore/panicking.rs:49
  10: rustc_typeck::check::writeback::WritebackCx::visit_opaque_types
  11: rustc_typeck::check::writeback::<impl rustc_typeck::check::FnCtxt>::resolve_type_vars_in_body
  12: rustc::ty::context::GlobalCtxt::enter_local
  13: rustc_typeck::check::typeck_tables_of
  14: rustc::ty::query::__query_compute::typeck_tables_of
  15: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::typeck_tables_of>::compute
  16: rustc::dep_graph::graph::DepGraph::with_task_impl
  17: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  18: rustc_typeck::collect::find_existential_constraints::ConstraintLocator::check
  19: rustc::hir::intravisit::Visitor::visit_nested_impl_item
  20: rustc::hir::intravisit::walk_item
  21: rustc_typeck::collect::find_existential_constraints
  22: rustc_typeck::collect::checked_type_of
  23: rustc_typeck::collect::type_of
  24: rustc::ty::query::__query_compute::type_of
  25: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::type_of>::compute
  26: rustc::dep_graph::graph::DepGraph::with_task_impl
  27: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  28: <rustc_typeck::collect::CollectItemTypesVisitor as rustc::hir::intravisit::Visitor>::visit_item
  29: rustc::hir::map::Map::visit_item_likes_in_module
  30: rustc_typeck::collect::collect_mod_item_types
  31: rustc::ty::query::__query_compute::collect_mod_item_types
  32: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::collect_mod_item_types>::compute
  33: rustc::dep_graph::graph::DepGraph::with_task_impl
  34: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  35: rustc_typeck::check_crate::{{closure}}::{{closure}}
  36: rustc::util::common::time
  37: rustc_typeck::check_crate
  38: rustc_interface::passes::analysis
  39: rustc::ty::query::__query_compute::analysis
  40: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::analysis>::compute
  41: rustc::dep_graph::graph::DepGraph::with_task_impl
  42: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  43: rustc::ty::context::tls::enter_global
  44: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  45: rustc_interface::passes::create_global_ctxt::{{closure}}
  46: rustc_interface::passes::BoxedGlobalCtxt::enter
  47: rustc_interface::interface::run_compiler_in_existing_thread_pool
  48: std::thread::local::LocalKey<T>::with
  49: scoped_tls::ScopedKey<T>::set
  50: syntax::with_globals
query stack during panic:
#0 [typeck_tables_of] processing `<T as IterBits>::iter_bits`
#1 [type_of] processing `IterBitsIter`
#2 [collect_mod_item_types] collecting item types in top-level module
#3 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error


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: rustc 1.36.0-nightly (8dd4aae9a 2019-05-04) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type lib

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.

Warnings

No main function was detected, so your code was compiled
but not run. If you'd like to execute your code, please
add a main function.

[Centril]

Suggested P-Medium due to unstable buggy feature.

[cramertj]

cc @oli-obk

[nikomatsakis]

triage: P-medium

Marking as P-medium. I wanted to put this on a tracking issue as a blocker to investigate but it's not that we have a suitable one.

[Aaron1011]

This no longer causes an ICE on the latest nightly.

[samuelpilz]

It does not fail indeed. However, I have no idea what the error message is trying to tell me. Can you verify that this is indeed the intended error message?