Existential type ICE without feature specified

[DutchGhost]

Another ICE that uses a nightly only feature, but still crashes on stable, beta and nightly without the #![feature(existential_type)] flag.

trait Bug {
    type Item: Bug;
    
    const FUN: fn() -> Self::Item;
}

impl Bug for &() {
    existential type Item: Bug;
    
    const FUN: fn() -> Self::Item = || ();
}

According to godbolt, this sneaked in with 1.34. 1.33 and lower are not affected.

Backtrace:

 Compiling playground v0.0.1 (/playground)
error[E0658]: existential types are unstable (see issue #34511)
  --> src/lib.rs:10:5
   |
10 |     existential type Item: Bug;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0277]: the trait bound `(): Bug` is not satisfied
  --> src/lib.rs:10:5
   |
10 |     existential type Item: Bug;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Bug` is not implemented for `()`
   |
   = help: the following implementations were found:
             <&() as Bug>
   = note: the return type of a function must have a statically known size

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', src/libcore/option.rs:345:21
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:70
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:58
             at src/libstd/panicking.rs:200
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:215
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:482
   6: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:385
   7: rust_begin_unwind
             at src/libstd/panicking.rs:312
   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<'a, 'gcx, 'tcx>>::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<'tcx> for rustc::ty::query::queries::typeck_tables_of<'tcx>>::compute
  16: rustc::dep_graph::graph::DepGraph::with_task_impl
  17: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  18: rustc_typeck::collect::find_existential_constraints::ConstraintLocator::check
  19: <rustc_typeck::collect::find_existential_constraints::ConstraintLocator<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_impl_item
  20: rustc::hir::intravisit::walk_item
  21: rustc_typeck::collect::find_existential_constraints
  22: rustc_typeck::collect::type_of
  23: rustc::ty::query::__query_compute::type_of
  24: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::type_of<'tcx>>::compute
  25: rustc::dep_graph::graph::DepGraph::with_task_impl
  26: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  27: <rustc_typeck::collect::CollectItemTypesVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_impl_item
  28: rustc::hir::map::Map::visit_item_likes_in_module
  29: rustc_typeck::collect::collect_mod_item_types
  30: rustc::ty::query::__query_compute::collect_mod_item_types
  31: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::collect_mod_item_types<'tcx>>::compute
  32: rustc::dep_graph::graph::DepGraph::with_task_impl
  33: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  34: rustc_typeck::collect::collect_item_types
  35: rustc::util::common::time
  36: rustc_typeck::check_crate
  37: <std::thread::local::LocalKey<T>>::with
  38: rustc::ty::context::TyCtxt::create_and_enter
  39: rustc_driver::driver::compile_input
  40: rustc_driver::run_compiler_with_pool
  41: <scoped_tls::ScopedKey<T>>::set
  42: rustc_driver::run_compiler
  43: syntax::with_globals
  44: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:87
  45: <F as alloc::boxed::FnBox<A>>::call_box
  46: std::sys::unix::thread::Thread::new::thread_start
             at /rustc/fc50f328b0353b285421b8ff5d4100966387a997/src/liballoc/boxed.rs:759
             at src/libstd/sys_common/thread.rs:14
             at src/libstd/sys/unix/thread.rs:81
  47: start_thread
  48: __clone
query stack during panic:
#0 [typeck_tables_of] processing `<&() as Bug>::FUN`
#1 [type_of] processing `<&() as Bug>::Item`
#2 [collect_mod_item_types] collecting item types in top-level module
end of query stack
error: aborting due to 2 previous errors

Some errors occurred: E0277, E0658.
For more information about an error, try `rustc --explain E0277`.

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 1.34.1 (fc50f328b 2019-04-24) 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.

[jonas-schievink]

Pretty sure this is a duplicate, but I can't find the original right now nevermind I was thinking of #60263, which you also filed

[DutchGhost]

Perhaps you mean #57272 ? Being able to ICE a feature, without the feature gate turned on?

I found this verry related:
#58887

[DutchGhost]

Again this is kinda 2 issue's in 1, so I dont know if this should have it's own issue...

After some more trying, I found this also panics, but only on nightly:

#![feature(existential_type)]

trait Bug {
    type Item: Bug;
    
    const FUN: fn() -> Self::Item;
}

impl Bug for () {
    existential type Item: Bug;
    
    const FUN: fn() -> Self::Item = || {};
}

Notice I removed the & on the impl Bug for () line

Backtrace:

error: internal compiler error: broken MIR in DefId(0/0:8 ~ playground[2d7c]::{{impl}}[0]::FUN[0]) (bb0[0]): equate_inputs_and_outputs: `fn() -> Item==fn() -> Item` failed with `NoSolution`
  --> src/lib.rs:12:5
   |
12 |     const FUN: fn() -> Self::Item = || {};
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:355:17
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::begin_panic
   7: <rustc_errors::Handler as core::ops::drop::Drop>::drop
   8: core::ptr::real_drop_in_place
   9: <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop
  10: core::ptr::real_drop_in_place
  11: rustc_interface::interface::run_compiler_in_existing_thread_pool
  12: std::thread::local::LocalKey<T>::with
  13: scoped_tls::ScopedKey<T>::set
  14: syntax::with_globals
query stack during panic:
end of query stack

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 (938d4ffe1 2019-04-27) 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.

[DutchGhost]

Another one, stable, beta & nightly:

trait Bug {
    type Item;
    
    const FUN: Self::Item;
}

impl Bug for () {
    existential type Item: Fn() -> Self::Item;
    
    const FUN: fn() -> Self::Item = || { || {} }; // <---- double closure!
}

Backtrace:

  Compiling playground v0.0.1 (/playground)
error[E0658]: existential types are unstable
 --> src/lib.rs:8:5
  |
8 |     existential type Item: Fn() -> Self::Item;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: for more information, see https://github.com/rust-lang/rust/issues/34511
  = help: add #![feature(existential_type)] to the crate attributes to enable

thread 'rustc' panicked at 'instantiated twice: DefId(0/0:7 ~ playground[2d7c]::{{impl}}[0]::Item[0])/OpaqueTypeDecl { substs: [], concrete_ty: _, has_required_region_bounds: false, origin: ExistentialType }', src/librustc_typeck/check/mod.rs:2354:13
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: std::panicking::begin_panic_fmt
             at src/libstd/panicking.rs:336
   8: rustc_typeck::check::FnCtxt::instantiate_opaque_types_from_value
   9: rustc_typeck::check::check_fn
  10: rustc_typeck::check::closure::<impl rustc_typeck::check::FnCtxt>::check_expr_closure
  11: rustc_typeck::check::FnCtxt::check_expr_kind
  12: rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_needs
  13: rustc_typeck::check::FnCtxt::check_block_with_expected
  14: rustc_typeck::check::FnCtxt::check_expr_kind
  15: rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_needs
  16: rustc_typeck::check::FnCtxt::check_return_expr
  17: rustc_typeck::check::check_fn
  18: rustc_typeck::check::closure::<impl rustc_typeck::check::FnCtxt>::check_expr_closure
  19: rustc_typeck::check::FnCtxt::check_expr_kind
  20: rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_needs
  21: rustc::ty::context::GlobalCtxt::enter_local
  22: rustc_typeck::check::typeck_tables_of
  23: rustc::ty::query::__query_compute::typeck_tables_of
  24: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::typeck_tables_of>::compute
  25: rustc::dep_graph::graph::DepGraph::with_task_impl
  26: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  27: rustc_typeck::collect::find_existential_constraints::ConstraintLocator::check
  28: rustc::hir::intravisit::Visitor::visit_nested_impl_item
  29: rustc::hir::intravisit::walk_item
  30: rustc_typeck::collect::find_existential_constraints
  31: rustc_typeck::collect::checked_type_of
  32: rustc_typeck::collect::type_of
  33: rustc::ty::query::__query_compute::type_of
  34: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::type_of>::compute
  35: rustc::dep_graph::graph::DepGraph::with_task_impl
  36: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  37: <rustc_typeck::collect::CollectItemTypesVisitor as rustc::hir::intravisit::Visitor>::visit_impl_item
  38: rustc::hir::map::Map::visit_item_likes_in_module
  39: rustc_typeck::collect::collect_mod_item_types
  40: rustc::ty::query::__query_compute::collect_mod_item_types
  41: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::collect_mod_item_types>::compute
  42: rustc::dep_graph::graph::DepGraph::with_task_impl
  43: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  44: rustc_typeck::check_crate::{{closure}}::{{closure}}
  45: rustc::util::common::time
  46: rustc_typeck::check_crate
  47: rustc_interface::passes::analysis
  48: rustc::ty::query::__query_compute::analysis
  49: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::analysis>::compute
  50: rustc::dep_graph::graph::DepGraph::with_task_impl
  51: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  52: rustc::ty::context::tls::enter_global
  53: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  54: rustc_interface::passes::create_global_ctxt::{{closure}}
  55: rustc_interface::interface::run_compiler_in_existing_thread_pool
  56: std::thread::local::LocalKey<T>::with
  57: scoped_tls::ScopedKey<T>::set
  58: syntax::with_globals
query stack during panic:
#0 [typeck_tables_of] processing `<() as Bug>::FUN`
#1 [type_of] processing `<() as Bug>::Item`
#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

For more information about this error, try `rustc --explain E0658`.

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 (938d4ffe1 2019-04-27) 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.

[pnkfelix]

triage: P-high due to ICE; removing nomination.

[nikomatsakis]

Likely duplicate from #60473

#![feature(impl_trait_in_bindings)]

struct A<'a>(&'a ());

trait Trait<T> {
}

impl<T> Trait<T> for () {
}

fn main() {
    let x: impl Trait<A> = ();
}