ICE when trying to reference an associated trait constant

[jyn514]

trait ToBytes {
    const SIZE: usize;

    fn a(self) -> [u8; ToBytes::SIZE];
    // error happens because a previous type error happened I think
    fn b(self) -> [u8; ToBytes::SIZE];
}

Error message

error: internal compiler error: src/librustc/traits/codegen/mod.rs:51: Encountered ambiguity selecting Binder(<[type error] as ToBytes>) during codegen, presuming due to overflow

This only happens if there are two erroneous functions, if I take out b it works fine.

It seems like I was trying to do this wrong in the first place since the first function gives an error error[E0283]: type annotations required: cannot resolve '_: ToBytes'. Not sure what the right way to do this is.

Traceback

error: internal compiler error: src/librustc/traits/codegen/mod.rs:51: Encountered ambiguity selecting `Binder(<[type error] as ToBytes>)` during codegen, presuming due to overflow

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:637:9
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::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::bug_fmt
  14: rustc::ty::context::GlobalCtxt::enter_local
  15: rustc::traits::codegen::codegen_fulfill_obligation
  16: rustc::ty::query::__query_compute::codegen_fulfill_obligation
  17: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::codegen_fulfill_obligation>::compute
  18: rustc::dep_graph::graph::DepGraph::with_task_impl
  19: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  20: rustc::ty::instance::Instance::resolve
  21: rustc_mir::interpret::eval_context::InterpretCx<M>::resolve
  22: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpretCx<M>>::eval_const_to_op
  23: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpretCx<M>>::run
  24: rustc_mir::const_eval::eval_body_using_ecx
  25: rustc_mir::const_eval::const_eval_raw_provider
  26: rustc::ty::query::__query_compute::const_eval_raw
  27: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::const_eval_raw>::compute
  28: rustc::dep_graph::graph::DepGraph::with_task_impl
  29: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  30: rustc_mir::const_eval::const_eval_provider
  31: rustc::ty::query::__query_compute::const_eval
  32: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::const_eval>::compute
  33: rustc::dep_graph::graph::DepGraph::with_task_impl
  34: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  35: <rustc::traits::project::AssociatedTypeNormalizer as rustc::ty::fold::TypeFolder>::fold_const
  36: rustc::ty::structural_impls::<impl rustc::ty::fold::TypeFoldable for &rustc::ty::TyS>::super_fold_with
  37: <rustc::traits::project::AssociatedTypeNormalizer as rustc::ty::fold::TypeFolder>::fold_ty
  38: <smallvec::SmallVec<A> as core::iter::traits::collect::FromIterator<<A as smallvec::Array>::Item>>::from_iter
  39: rustc::ty::fold::TypeFoldable::fold_with
  40: rustc::traits::project::normalize
  41: rustc_typeck::check::FnCtxt::normalize_associated_types_in
  42: rustc::ty::context::GlobalCtxt::enter_local
  43: rustc_typeck::check::wfcheck::check_associated_item
  44: rustc::ty::query::__query_compute::check_trait_item_well_formed
  45: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::check_trait_item_well_formed>::compute
  46: rustc::dep_graph::graph::DepGraph::with_task_impl
  47: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  48: <rustc_typeck::check::wfcheck::CheckTypeWellFormedVisitor as rustc::hir::itemlikevisit::ParItemLikeVisitor>::visit_trait_item
  49: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:85
  50: rustc_data_structures::sync::par_for_each_in
  51: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:85
  52: rustc::hir::Crate::par_visit_all_item_likes
  53: rustc::util::common::time
  54: rustc_typeck::check_crate
  55: rustc_interface::passes::analysis
  56: rustc::ty::query::__query_compute::analysis
  57: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::analysis>::compute
  58: rustc::dep_graph::graph::DepGraph::with_task_impl
  59: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  60: rustc::ty::context::tls::enter_global
  61: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  62: rustc_interface::passes::create_global_ctxt::{{closure}}
  63: rustc_interface::interface::run_compiler_in_existing_thread_pool
  64: std::thread::local::LocalKey<T>::with
  65: scoped_tls::ScopedKey<T>::set
  66: syntax::with_globals
query stack during panic:
#0 [codegen_fulfill_obligation] checking if `ToBytes` fulfills its obligations
#1 [const_eval_raw] const-evaluating `ToBytes::b::{{constant}}#0`
#2 [const_eval] const-evaluating + checking `ToBytes::b::{{constant}}#0`
#3 [check_trait_item_well_formed] processing `ToBytes::b`
#4 [analysis] running analysis passes on this crate
end of query stack

Version

$ rustc --version -v
rustc 1.36.0 (a53f9df32 2019-07-03)
binary: rustc
commit-hash: a53f9df32fbb0b5f4382caaad8f1a46f36ea887c
commit-date: 2019-07-03
host: x86_64-unknown-linux-gnu
release: 1.36.0
LLVM version: 8.0

[ackxolotl]

Tried to reduce it and found a similar case with the same ICE:

trait A {
    const C: usize;
    
    fn f() -> ([u8; A::C], [u8; A::C]);
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=233300ab42b3986b556a0d38e4d398a5

[nikomatsakis]

Compiler triage: Seems likely to be caused by the need for lazy norm. Marking as p-medium for now.

[JohnTitor]

The ICE doesn't appear on the latest nightly, marked as E-needstest