Directly using constant from default specialisation of type causes "erroneous constant used" error

[LAK132]

This works:

#![allow(incomplete_features)]
#![feature(specialization)]
trait ThingImpl {
    const VALUE: i32;
}

struct ThingDefault {
}

impl ThingImpl for ThingDefault {
    const VALUE: i32 = -100;
}

trait ThingTrait {
    type Type: ThingImpl;
}

struct Thing<const T: u32>;

impl<const T: u32> ThingTrait for Thing<T> {
    default type Type = ThingDefault;
}

//

fn main() {
    println!("{}", value::<Thing<0>>());
    println!("{}", value::<Thing<1>>());
    println!("{}", value::<Thing<2>>());
    println!("{}", value::<Thing<3>>());
    println!("{}", value2::<<Thing<3> as ThingTrait>::Type>());
}

fn value<T: ThingTrait>() -> i32 {
    T::Type::VALUE
}

fn value2<T: ThingImpl>() -> i32 {
    T::VALUE
}

// Thing<0>

struct Thing0 {
}

impl ThingImpl for Thing0 {
    const VALUE: i32 = 0;
}

impl ThingTrait for Thing<0> {
    type Type = Thing0;
}

This fails:

#![allow(incomplete_features)]
#![feature(specialization)]

//

trait ThingImpl {
    const VALUE: i32;
}

struct ThingDefault {
}

impl ThingImpl for ThingDefault {
    const VALUE: i32 = -100;
}

trait ThingTrait {
    type Type: ThingImpl;
}

struct Thing<const T: u32>;

impl<const T: u32> ThingTrait for Thing<T> {
    default type Type = ThingDefault;
}

//

fn main() {
    println!("{}", <Thing<0> as ThingTrait>::Type::VALUE);
    println!("{}", <Thing<1> as ThingTrait>::Type::VALUE);
}

// Thing<0>

struct Thing0 {
}

impl ThingImpl for Thing0 {
    const VALUE: i32 = 0;
}

impl ThingTrait for Thing<0> {
    type Type = Thing0;
}

with this error

error: erroneous constant used
  --> src/main.rs:31:20
   |
31 |     println!("{}", <Thing<1> as ThingTrait>::Type::VALUE);
   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ referenced constant has errors
   |
   = note: `#[deny(const_err)]` on by default
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>

Meta

Build using the Nightly version: 1.57.0-nightly (2021-09-28 8f8092c) (rust playground)

[LAK132]

more details now that I have a local copy of the compiler:

error: internal compiler error: Encountered error `Unimplemented` selecting `Binder(<<Thing<1_u32> as ThingTrait>::Type as ThingImpl>, [])` during codegen

thread 'rustc' panicked at 'aborting due to `-Z treat-err-as-bug=1`', compiler/rustc_errors/src/lib.rs:1181:27
stack backtrace:
   0: rust_begin_unwind
   1: core::panicking::panic_fmt
   2: rustc_errors::HandlerInner::emit_diagnostic
   3: rustc_errors::HandlerInner::emit_diag_at_span
   4: rustc_errors::HandlerInner::span_bug
   5: rustc_session::session::Session::delay_span_bug
   6: rustc_infer::infer::InferCtxtBuilder::enter
   7: rustc_trait_selection::traits::codegen::codegen_fulfill_obligation
   8: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
   9: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  10: rustc_data_structures::stack::ensure_sufficient_stack
  11: rustc_query_system::query::plumbing::get_query
  12: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::codegen_fulfill_obligation
  13: rustc_ty_utils::instance::inner_resolve_instance
  14: rustc_ty_utils::instance::resolve_instance
  15: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  16: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  17: rustc_data_structures::stack::ensure_sufficient_stack
  18: rustc_query_system::query::plumbing::get_query
  19: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::resolve_instance
  20: rustc_middle::ty::instance::Instance::resolve_opt_const_arg
  21: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_resolve
  22: <rustc_trait_selection::traits::query::normalize::QueryNormalizer as rustc_middle::ty::fold::TypeFolder>::fold_const
  23: <rustc_trait_selection::traits::query::normalize::QueryNormalizer as rustc_middle::ty::fold::TypeFolder>::fold_mir_const
  24: <rustc_infer::infer::at::At as rustc_trait_selection::traits::query::normalize::AtExt>::normalize
  25: rustc_infer::infer::InferCtxtBuilder::enter
  26: core::ops::function::FnOnce::call_once
  27: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  28: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  29: rustc_data_structures::stack::ensure_sufficient_stack
  30: rustc_query_system::query::plumbing::get_query
  31: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::normalize_mir_const_after_erasing_regions
  32: rustc_middle::ty::normalize_erasing_regions::<impl rustc_middle::ty::context::TyCtxt>::normalize_erasing_regions
  33: rustc_const_eval::interpret::operand::<impl rustc_const_eval::interpret::eval_context::InterpCx<M>>::eval_operand
  34: rustc_const_eval::interpret::step::<impl rustc_const_eval::interpret::eval_context::InterpCx<M>>::run
  35: rustc_const_eval::const_eval::eval_queries::eval_to_allocation_raw_provider
  36: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  37: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  38: rustc_data_structures::stack::ensure_sufficient_stack
  39: rustc_query_system::query::plumbing::get_query
  40: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::eval_to_allocation_raw
  41: rustc_const_eval::const_eval::eval_queries::eval_to_const_value_raw_provider
  42: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  43: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  44: rustc_data_structures::stack::ensure_sufficient_stack
  45: rustc_query_system::query::plumbing::get_query
  46: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::eval_to_const_value_raw
  47: rustc_const_eval::const_eval::eval_queries::eval_to_const_value_raw_provider
  48: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  49: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  50: rustc_data_structures::stack::ensure_sufficient_stack
  51: rustc_query_system::query::plumbing::get_query
  52: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::eval_to_const_value_raw
  53: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_global_id
  54: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_resolve
  55: <rustc_trait_selection::traits::query::normalize::QueryNormalizer as rustc_middle::ty::fold::TypeFolder>::fold_const
  56: <rustc_trait_selection::traits::query::normalize::QueryNormalizer as rustc_middle::ty::fold::TypeFolder>::fold_mir_const
  57: <rustc_infer::infer::at::At as rustc_trait_selection::traits::query::normalize::AtExt>::normalize
  58: rustc_infer::infer::InferCtxtBuilder::enter
  59: core::ops::function::FnOnce::call_once
  60: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  61: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  62: rustc_data_structures::stack::ensure_sufficient_stack
  63: rustc_query_system::query::plumbing::get_query
  64: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::normalize_mir_const_after_erasing_regions
  65: rustc_middle::ty::normalize_erasing_regions::<impl rustc_middle::ty::context::TyCtxt>::normalize_erasing_regions
  66: rustc_const_eval::interpret::operand::<impl rustc_const_eval::interpret::eval_context::InterpCx<M>>::eval_operand
  67: rustc_const_eval::interpret::step::<impl rustc_const_eval::interpret::eval_context::InterpCx<M>>::eval_rvalue_into_place
  68: <rustc_mir_transform::const_prop::ConstPropagator as rustc_middle::mir::visit::MutVisitor>::visit_statement
  69: <rustc_mir_transform::const_prop::ConstPropagator as rustc_middle::mir::visit::MutVisitor>::visit_body
  70: <rustc_mir_transform::const_prop::ConstProp as rustc_middle::mir::MirPass>::run_pass
  71: rustc_mir_transform::run_passes
  72: rustc_mir_transform::optimized_mir
  73: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  74: rustc_data_structures::stack::ensure_sufficient_stack
  75: rustc_query_system::query::plumbing::try_execute_query
  76: rustc_query_system::query::plumbing::get_query
  77: rustc_middle::ty::<impl rustc_middle::ty::context::TyCtxt>::instance_mir
  78: rustc_monomorphize::collector::collect_neighbours
  79: rustc_monomorphize::collector::collect_items_rec
  80: rustc_session::utils::<impl rustc_session::session::Session>::time
  81: rustc_monomorphize::collector::collect_crate_mono_items
  82: rustc_monomorphize::partitioning::collect_and_partition_mono_items
  83: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  84: rustc_data_structures::stack::ensure_sufficient_stack
  85: rustc_query_system::query::plumbing::try_execute_query
  86: rustc_query_system::query::plumbing::get_query
  87: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::collect_and_partition_mono_items
  88: rustc_codegen_ssa::base::codegen_crate
  89: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  90: rustc_session::utils::<impl rustc_session::session::Session>::time
  91: rustc_interface::queries::Queries::ongoing_codegen

query stack during panic:
#0 [codegen_fulfill_obligation] checking if `ThingImpl` fulfills its obligations
#1 [resolve_instance] resolving instance `<<Thing<1_u32> as ThingTrait>::Type as ThingImpl>::VALUE`
#2 [normalize_mir_const_after_erasing_regions] normalizing `<<Thing<1_u32> as ThingTrait>::Type as ThingImpl>::VALUE`
#3 [eval_to_allocation_raw] const-evaluating + checking `main::promoted[0]`
#4 [eval_to_const_value_raw] simplifying constant for the type system `main::promoted[0]`
#5 [eval_to_const_value_raw] simplifying constant for the type system `main::promoted[0]`
#6 [normalize_mir_const_after_erasing_regions] normalizing `main::promoted[0]`
#7 [optimized_mir] optimizing MIR for `main`
#8 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack

[LAK132]

Debug build of rustc gets me

thread 'rustc' panicked at 'assertion failed: `(left == right)`
  left: `Binder(<<Thing<1_u32> as ThingTrait>::Type as ThingImpl>, [])`,
 right: `Binder(<ThingDefault as ThingImpl>, [])`', compiler/rustc_trait_selection/src/traits/codegen.rs:30:5

Replacing

rust/compiler/rustc_trait_selection/src/traits/codegen.rs

Line 30 in d3e6770

debug_assert_eq!(trait_ref, tcx.normalize_erasing_regions(param_env, trait_ref));

with let trait_ref = tcx.normalize_erasing_regions(param_env, trait_ref); seems to stop it from hitting

rust/compiler/rustc_trait_selection/src/traits/codegen.rs

Lines 65 to 76 in d3e6770

	Err(Unimplemented) => {
	// This can trigger when we probe for the source of a `'static` lifetime requirement
	// on a trait object: `impl Foo for dyn Trait {}` has an implicit `'static` bound.
	infcx.tcx.sess.delay_span_bug(
	rustc_span::DUMMY_SP,
	&format!(
	"Encountered error `Unimplemented` selecting `{:?}` during codegen",
	trait_ref
	),
	);
	return Err(ErrorReported);
	}

which is what was causing the ICE in the release compiler.

[LAK132]

Would this change be accepted as a PR? It's unclear to me if this assertion was testing a precondition for the argument passed to the function or just a postcondition to protect the code that caused the ICE in release.

[estebank]

I believe that an earlier pass of normalization should have made the assert successful, so the change as is is likely not going to be accepted, but we clearly have a bug that needs fixing (glances at the open issue count... understatement of the century).

CC @rust-lang/compiler

Edit: I think that the problem is in the following:

rust/compiler/rustc_ty_utils/src/instance.rs

Lines 221 to 227 in ce1143e

	let trait_ref = ty::TraitRef::from_method(tcx, trait_id, rcvr_substs);
	// See FIXME on `BoundVarsCollector`.
	let mut bound_vars_collector = BoundVarsCollector::new();
	trait_ref.visit_with(&mut bound_vars_collector);
	let trait_binder = ty::Binder::bind_with_vars(trait_ref, bound_vars_collector.into_vars(tcx));
	let vtbl = tcx.codegen_fulfill_obligation((param_env, trait_binder))?;

I wonder if the following has anything to do with this (but don't know if at all):

rust/compiler/rustc_ty_utils/src/instance.rs

Lines 18 to 24 in ce1143e

	// FIXME(#86795): `BoundVarsCollector` here should **NOT** be used
	// outside of `resolve_associated_item`. It's just to address #64494,
	// #83765, and #85848 which are creating bound types/regions that lose
	// their `Binder` *unintentionally*.
	// It's ideal to remove `BoundVarsCollector` and just use
	// `ty::Binder::*` methods but we use this stopgap until we figure out
	// the "real" fix.

Edit 2: as expected (almost tautologically), adding let trait_binder = tcx.normalize_erasing_regions(param_env, trait_binder); before the call to codegen_fulfill_obligation highlighted above makes the repro case compile. From a quick glance it looks like it might be a reasonable patch, but someone with better understanding of why that assert was there in the first place (like @lcnr), will be a better judge of it.

[LAK132]

This appears to be fixed :D