Compiler panic when using associated constants for array length

[nikolads]

The following code causes a compiler panic:

struct Foo<'a>([&'a (); 2]);

impl<'a> Foo<'a> {
    const LEN: usize = 2;

    fn new() -> Self {
        Foo({
            let arr: [_; Self::LEN] = [&(), &()];
            arr
        })
    }
}

Error:

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `Some(NodeId(15))`,
 right: `None`: free_scope: DefId(0/0:4 ~ rustc_ice[3d2b]::{{impl}}[0]) not recognized by the region scope tree for None / Some(DefId(0/0:6 ~ rustc_ice[3d2b]::{{impl}}[0]::new[0]))', librustc/middle/region.rs:734:13

Using the constant on the right hand side as in let arr = [&(); Self::LEN]; causes the same error.

The lifetimes are required. If I replace the struct with struct Foo([&'static (); 2]) the code compiles fine.

Rustc version: rustc 1.29.0-nightly (6a1c0637c 2018-07-23).
Also panics on 1.27.2-stable.

Backtrace:

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
             at libstd/sys_common/backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             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:479
   6: std::panicking::continue_panic_fmt
             at libstd/panicking.rs:390
   7: std::panicking::begin_panic_fmt
             at libstd/panicking.rs:345
   8: rustc::middle::region::ScopeTree::early_free_scope
   9: rustc::middle::free_region::RegionRelations::is_subregion_of
  10: rustc::infer::lexical_region_resolve::resolve
  11: rustc::infer::InferCtxt::resolve_regions_and_report_errors_inner
  12: rustc_typeck::check::regionck::<impl rustc_typeck::check::FnCtxt<'a, 'gcx, 'tcx>>::regionck_expr
  13: rustc::ty::context::tls::with_related_context
  14: rustc::infer::InferCtxtBuilder::enter
  15: rustc_typeck::check::typeck_tables_of
  16: rustc::ty::query::__query_compute::typeck_tables_of
  17: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::typeck_tables_of<'tcx>>::compute
  18: rustc::ty::context::tls::with_context
  19: rustc::dep_graph::graph::DepGraph::with_task_impl
  20: rustc::ty::context::tls::with_related_context
  21: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  22: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  23: rustc::ty::query::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::typeck_tables_of
  24: rustc_mir::interpret::const_eval::const_eval_provider
  25: rustc::ty::query::__query_compute::const_eval
  26: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::const_eval<'tcx>>::compute
  27: rustc::ty::context::tls::with_context
  28: rustc::dep_graph::graph::DepGraph::with_task_impl
  29: rustc::ty::context::tls::with_related_context
  30: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  31: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  32: <rustc::traits::project::AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_const
  33: rustc::ty::structural_impls::<impl rustc::ty::fold::TypeFoldable<'tcx> for &'tcx rustc::ty::TyS<'tcx>>::super_fold_with
  34: <rustc::traits::project::AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
  35: rustc::infer::InferCtxt::partially_normalize_associated_types_in
  36: <rustc_typeck::check::FnCtxt<'a, 'gcx, 'tcx> as rustc_typeck::astconv::AstConv<'gcx, 'tcx>>::normalize_ty
  37: <(dyn rustc_typeck::astconv::AstConv<'gcx, 'tcx> + 'o)>::ast_ty_to_ty
  38: <rustc_typeck::check::GatherLocalsVisitor<'a, 'gcx, 'tcx> as rustc::hir::intravisit::Visitor<'gcx>>::visit_local
  39: rustc::hir::intravisit::walk_expr
  40: rustc::hir::intravisit::walk_expr
  41: rustc_typeck::check::check_fn
  42: rustc::ty::context::tls::with_related_context
  43: rustc::infer::InferCtxtBuilder::enter
  44: rustc_typeck::check::typeck_tables_of
  45: rustc::ty::query::__query_compute::typeck_tables_of
  46: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::typeck_tables_of<'tcx>>::compute
  47: rustc::ty::context::tls::with_context
  48: rustc::dep_graph::graph::DepGraph::with_task_impl
  49: rustc::ty::context::tls::with_related_context
  50: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  51: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  52: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::ensure_query
  53: rustc_typeck::check::typeck_item_bodies
  54: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::typeck_item_bodies<'tcx>>::compute
  55: rustc::ty::context::tls::with_context
  56: rustc::dep_graph::graph::DepGraph::with_task_impl
  57: rustc::ty::context::tls::with_related_context
  58: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  59: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  60: rustc::util::common::time
  61: rustc_typeck::check_crate
  62: rustc::ty::context::tls::enter_context
  63: <std::thread::local::LocalKey<T>>::with
  64: rustc::ty::context::TyCtxt::create_and_enter
  65: rustc_driver::driver::compile_input
  66: rustc_driver::run_compiler_with_pool
  67: <scoped_tls::ScopedKey<T>>::set
  68: <scoped_tls::ScopedKey<T>>::set
  69: syntax::with_globals
  70: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
  71: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:106
  72: rustc_driver::run
  73: rustc_driver::main
  74: std::rt::lang_start::{{closure}}
  75: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  76: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:106
  77: std::rt::lang_start_internal
             at libstd/panicking.rs:289
             at libstd/panic.rs:392
             at libstd/rt.rs:58
  78: main
  79: __libc_start_main
  80: <unknown>
query stack during panic:
#0 [typeck_tables_of] processing `Foo::new::{{constant}}`
#1 [const_eval] const-evaluating `Foo::new::{{constant}}`
#2 [typeck_tables_of] processing `Foo::new`
#3 [typeck_item_bodies] type-checking all item bodies
end of query stack

[huonw]

Reduced slightly:

struct Foo<'a> { x: &'a () }

impl<'a> Foo<'a> {
    const X: usize = 0;

    fn foo(_: [u8; Self::X]) {}
}


fn main() {}

[pnkfelix]

closing as duplicate of #56445 (since I think the latter has more useful info in its comment thread.)