ICE on stable: could not fully normalize `<&Struct as Trait>::Type` as enum variant

[mamins1376]

The error message strongly suggests that this is coming from the same origin as #91231, but the reproduction is different.

In this one, due to lack of reliance on #![feature(extern_types)] it is possible to trigger it on stable channel as well as nightly.

I made a repo to have both codes at one place.

Code

struct Struct;

trait Trait {
    type Type;
}

enum Enum<'a> where &'a Struct: Trait {
    Variant(<&'a Struct as Trait>::Type)
}

Meta

Stable:

rustc --version --verbose:

rustc 1.56.1 (59eed8a2a 2021-11-01)
binary: rustc
commit-hash: 59eed8a2aac0230a8b53e89d4e99d55912ba6b35
commit-date: 2021-11-01
host: x86_64-unknown-linux-gnu
release: 1.56.1
LLVM version: 13.0.0

Nightly:

rustc --version --verbose
rustc 1.58.0-nightly (b426445c6 2021-11-24)
binary: rustc
commit-hash: b426445c60b4faab6e96d2b866164d478680abf6
commit-date: 2021-11-24
host: x86_64-unknown-linux-gnu
release: 1.58.0-nightly
LLVM version: 13.0.0

Error output

For both nightly and stable it is the same. To keep it short, I removed dead_code warnings.

error: internal compiler error: compiler/rustc_traits/src/normalize_erasing_regions.rs:54:32: could not fully normalize `<&Struct as Trait>::Type`

thread 'rustc' panicked at 'Box<dyn Any>', compiler/rustc_errors/src/lib.rs:1169:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.58.0-nightly (b426445c6 2021-11-24) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib

query stack during panic:
#0 [normalize_generic_arg_after_erasing_regions] normalizing `<&Struct as Trait>::Type`
#1 [layout_of] computing layout of `<&Struct as Trait>::Type`
end of query stack
error: aborting due to previous error; 2 warnings emitted

Backtrace

error: internal compiler error: compiler/rustc_traits/src/normalize_erasing_regions.rs:54:32: could not fully normalize `<&Struct as Trait>::Type`

thread 'rustc' panicked at 'Box<dyn Any>', compiler/rustc_errors/src/lib.rs:1169:9
stack backtrace:
   0: std::panicking::begin_panic::<rustc_errors::ExplicitBug>
   1: std::panic::panic_any::<rustc_errors::ExplicitBug>
   2: <rustc_errors::HandlerInner>::bug
   3: <rustc_errors::Handler>::bug
   4: rustc_middle::ty::context::tls::with_opt::<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, ()>
   5: rustc_middle::util::bug::opt_span_bug_fmt::<rustc_span::span_encoding::Span>
   6: rustc_middle::util::bug::bug_fmt
   7: <rustc_infer::infer::InferCtxtBuilder>::enter::<rustc_middle::ty::subst::GenericArg, rustc_traits::normalize_erasing_regions::normalize_after_erasing_regions<rustc_middle::ty::subst::GenericArg>::{closure#0}>
   8: <rustc_traits::normalize_erasing_regions::provide::{closure#0} as core::ops::function::FnOnce<(rustc_middle::ty::context::TyCtxt, rustc_middle::ty::ParamEnvAnd<rustc_middle::ty::subst::GenericArg>)>>::call_once
   9: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<rustc_middle::ty::ParamEnvAnd<rustc_middle::ty::subst::GenericArg>, rustc_middle::ty::subst::GenericArg>>
  10: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::normalize_generic_arg_after_erasing_regions
  11: <&rustc_middle::ty::TyS as rustc_middle::ty::fold::TypeFoldable>::fold_with::<rustc_middle::ty::normalize_erasing_regions::NormalizeAfterErasingRegionsFolder>
  12: rustc_middle::ty::layout::layout_of
  13: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::layout_of
  14: <alloc::vec::Vec<rustc_target::abi::TyAndLayout<&rustc_middle::ty::TyS>> as alloc::vec::spec_from_iter::SpecFromIter<rustc_target::abi::TyAndLayout<&rustc_middle::ty::TyS>, core::iter::adapters::ResultShunt<core::iter::adapters::map::Map<core::slice::iter::Iter<rustc_middle::ty::FieldDef>, <rustc_middle::ty::layout::LayoutCx<rustc_middle::ty::context::TyCtxt>>::layout_of_uncached::{closure#5}::{closure#0}>, rustc_middle::ty::layout::LayoutError>>>::from_iter
  15: <alloc::vec::Vec<alloc::vec::Vec<rustc_target::abi::TyAndLayout<&rustc_middle::ty::TyS>>> as alloc::vec::spec_from_iter::SpecFromIter<alloc::vec::Vec<rustc_target::abi::TyAndLayout<&rustc_middle::ty::TyS>>, core::iter::adapters::ResultShunt<core::iter::adapters::map::Map<core::slice::iter::Iter<rustc_middle::ty::VariantDef>, <rustc_middle::ty::layout::LayoutCx<rustc_middle::ty::context::TyCtxt>>::layout_of_uncached::{closure#5}>, rustc_middle::ty::layout::LayoutError>>>::from_iter
  16: rustc_middle::ty::layout::layout_of
  17: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::layout_of
  18: <rustc_lint::context::LateContext as rustc_middle::ty::layout::LayoutOf>::layout_of
  19: <rustc_lint::BuiltinCombinedModuleLateLintPass as rustc_lint::passes::LateLintPass>::check_item
  20: <rustc_lint::late::LateContextAndPass<rustc_lint::BuiltinCombinedModuleLateLintPass> as rustc_hir::intravisit::Visitor>::visit_nested_item
  21: rustc_lint::late::late_lint_mod::<rustc_lint::BuiltinCombinedModuleLateLintPass>
  22: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<rustc_span::def_id::LocalDefId, ()>>
  23: rustc_query_system::query::plumbing::get_query::<rustc_query_impl::queries::lint_mod, rustc_query_impl::plumbing::QueryCtxt>
  24: <rustc_session::session::Session>::time::<(), rustc_lint::late::check_crate<rustc_lint::BuiltinCombinedLateLintPass, rustc_interface::passes::analysis::{closure#5}::{closure#0}::{closure#3}::{closure#0}::{closure#0}>::{closure#1}::{closure#0}>
  25: <rustc_session::session::Session>::time::<(), rustc_interface::passes::analysis::{closure#5}::{closure#0}::{closure#3}::{closure#0}>
  26: <core::panic::unwind_safe::AssertUnwindSafe<rustc_interface::passes::analysis::{closure#5}::{closure#0}> as core::ops::function::FnOnce<()>>::call_once
  27: <rustc_session::session::Session>::time::<(), rustc_interface::passes::analysis::{closure#5}>
  28: rustc_interface::passes::analysis
  29: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<(), core::result::Result<(), rustc_errors::ErrorReported>>>
  30: rustc_query_system::query::plumbing::get_query::<rustc_query_impl::queries::analysis, rustc_query_impl::plumbing::QueryCtxt>
  31: <rustc_interface::passes::QueryContext>::enter::<rustc_driver::run_compiler::{closure#1}::{closure#2}::{closure#3}, core::result::Result<(), rustc_errors::ErrorReported>>
  32: <rustc_interface::interface::Compiler>::enter::<rustc_driver::run_compiler::{closure#1}::{closure#2}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_errors::ErrorReported>>
  33: rustc_span::with_source_map::<core::result::Result<(), rustc_errors::ErrorReported>, rustc_interface::interface::create_compiler_and_run<core::result::Result<(), rustc_errors::ErrorReported>, rustc_driver::run_compiler::{closure#1}>::{closure#1}>
  34: rustc_interface::interface::create_compiler_and_run::<core::result::Result<(), rustc_errors::ErrorReported>, rustc_driver::run_compiler::{closure#1}>
  35: <scoped_tls::ScopedKey<rustc_span::SessionGlobals>>::set::<rustc_interface::util::setup_callbacks_and_run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_errors::ErrorReported>, rustc_driver::run_compiler::{closure#1}>::{closure#0}, core::result::Result<(), rustc_errors::ErrorReported>>::{closure#0}::{closure#0}, core::result::Result<(), rustc_errors::ErrorReported>>
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.58.0-nightly (b426445c6 2021-11-24) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib

query stack during panic:
#0 [normalize_generic_arg_after_erasing_regions] normalizing `<&Struct as Trait>::Type`
#1 [layout_of] computing layout of `<&Struct as Trait>::Type`
#2 [layout_of] computing layout of `Enum`
#3 [lint_mod] linting top-level module
#4 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error; 2 warnings emitted

[b-naber]

Seems as if layout_of shouldn't expect the normalization in normalize_erasing_regions to succeed, since normalize_after_erasing_regions, which is internally called by normalize_erasing_regions calls bug if the normalization fails.

This actually results in ICEs whenever layout_of is called on an associated type that isn't provable, e.g. this also ICEs:

#![feature(repr_transparent)]

struct Struct;

trait Trait {
    type Type;
}

#[repr(transparent)]
struct NumberWithUnit<'a>(<&'a Struct as Trait>::Type) where &'a Struct: Trait;

fn main() {}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=5c81ddf7f591aeef94a97bb7d5957b36

I can try to fix this issue.

[b-naber]

@rustbot claim

[mamins1376]

@b-naber There are quite a few of seemingly related issues, so fixing one might eradicate many.

[camelid]

It looks like adding the impl used in the bound makes compilation succeed:

impl Trait for &Struct {
    type Type = i32;
}

So am I correct that this is an ICE that occurs when an error should've been emitted?

[b-naber]

@camelid That's what I also thought initially, but it seems we never reject these during typecheck, e.g. this also compiles:

struct Struct;

trait Trait {
    type Type;
}

fn foo<'a>(_arg: <&'a Struct as Trait>::Type) where &'a Struct: Trait {}

Using this function or the enum above would yield an error then.

[camelid]

Ah, that's a good point! The where bound is impossible to fulfill, but the definition is still valid.