StableHasher: unexpected region with impl trait for type with complex generic constant

[zeyonaut]

Code

Assuming the following header:

#![feature(const_generics, const_evaluatable_checked)]
#![allow(incomplete_features)]
use core::{convert::TryFrom, num::NonZeroUsize};

struct A<const N: NonZeroUsize>([u8; N.get()]) where [u8; N.get()]: Sized;

The following example in trait impl causes an ICE:

impl<'a, const N: NonZeroUsize> TryFrom<&'a [u8]> for A<N> where [u8; N.get()]: Sized {
	type Error = ();
	fn try_from(slice: &'a [u8]) -> Result<A<N>, ()> {
		let _x = <&[u8; N.get()] as TryFrom<&[u8]>>::try_from(slice);
		unimplemented!();
	}
}

The following without a trait implementation does not:

fn try_from<'a, const N: NonZeroUsize>(slice: &'a [u8]) -> Result<A<N>, ()> where [u8; N.get()]: Sized {
	let x = <&[u8; N.get()] as TryFrom<&[u8]>>::try_from(slice);
	unimplemented!();
}

Neither does the equivalent implementation without const_evaluatable_checked:

struct B<const N: usize>([u8; N]) where [u8; N]: Sized;

impl<'a, const N: usize> TryFrom<&'a [u8]> for B<N> where [u8; N]: Sized {
	type Error = ();
	fn try_from(slice: &'a [u8]) -> Result<B<N>, ()> {
		let _x = <&[u8; N] as TryFrom<&[u8]>>::try_from(slice);
		unimplemented!();
	}
}

Meta

rustc --version --verbose:

rustc 1.49.0-nightly (91a79fb29 2020-10-07)
binary: rustc
commit-hash: 91a79fb29ac78d057d04dbe86be13d5dcc64309a
commit-date: 2020-10-07
host: x86_64-pc-windows-msvc
release: 1.49.0-nightly
LLVM version: 11.0

Error output

cargo check

error: internal compiler error: compiler\rustc_middle\src\ich\impls_ty.rs:94:17: StableHasher: unexpected region '_#2r

thread 'rustc' panicked at 'Box<Any>', compiler\rustc_errors\src\lib.rs:945:9
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 1.49.0-nightly (91a79fb29 2020-10-07) running on x86_64-pc-windows-msvc

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type lib

note: some of the compiler flags provided by cargo are hidden

error: aborting due to previous error; 2 warnings emitted

Backtrace

stack backtrace:
   0:     0x7ff947a09989 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hc20a26e0f7659afd
   1:     0x7ff947a370bb - core::fmt::write::h435be716304d26a1
   2:     0x7ff9479fb1d8 - <std::io::IoSlice as core::fmt::Debug>::fmt::h997d94c78f1ba97b
   3:     0x7ff947a0ed74 - std::panicking::take_hook::hbf7e33d26fc3ca30
   4:     0x7ff947a0e958 - std::panicking::take_hook::hbf7e33d26fc3ca30
   5:     0x7ff8fbdcf5a7 - rustc_driver::report_ice::h901ed469a4afd632
   6:     0x7ff947a0f7d0 - std::panicking::rust_panic_with_hook::h19068ccbe4b337a1
   7:     0x7ff900537870 - <rustc_errors::snippet::Style as core::fmt::Debug>::fmt::h154d3f8b7cc27034
   8:     0x7ff900537799 - <rustc_errors::snippet::Style as core::fmt::Debug>::fmt::h154d3f8b7cc27034
   9:     0x7ff9005377e1 - <rustc_errors::snippet::Style as core::fmt::Debug>::fmt::h154d3f8b7cc27034
  10:     0x7ff90056a80f - rustc_errors::HandlerInner::err_count::hc7ff30e58befa0c0
  11:     0x7ff900568ec2 - rustc_errors::Handler::bug::h9f679e586b05b709
  12:     0x7ff900043d4f - rustc_middle::util::bug::bug_fmt::h333e4a32fa9b0768
  13:     0x7ff90003fe70 - <rustc_middle::ty::consts::int::ConstInt as core::fmt::Debug>::fmt::he16df1c1cd9a0739
  14:     0x7ff90003fe13 - <rustc_middle::ty::consts::int::ConstInt as core::fmt::Debug>::fmt::he16df1c1cd9a0739
  15:     0x7ff900043c7c - rustc_middle::util::bug::bug_fmt::h333e4a32fa9b0768
  16:     0x7ff900043be0 - rustc_middle::util::bug::bug_fmt::h333e4a32fa9b0768
  17:     0x7ff90003bd54 - rustc_middle::ich::impls_ty::<impl rustc_data_structures::stable_hasher::HashStable<rustc_middle::ich::hcx::StableHashingContext> for rustc_middle::ty::sty::RegionKind>::hash_stable::h1b2fcdca26fe65c7
  18:     0x7ff8fe089e72 - rustc_ty::provide::h0935123a64901122
  19:     0x7ff8ffccce57 - <rustc_trait_selection::traits::fulfill::PendingPredicateObligation as core::fmt::Debug>::fmt::h193f29610c75bb2a
  20:     0x7ff8ffb69daf - <rustc_trait_selection::traits::TraitQueryMode as core::fmt::Debug>::fmt::h933d8945b4748b17
  21:     0x7ff8ffc24e3d - <rustc_trait_selection::traits::specialize::OverlapError as core::fmt::Debug>::fmt::hb9cecb2f776e6243
  22:     0x7ff8ffcca571 - rustc_trait_selection::traits::fulfill::FulfillProcessor::progress_changed_obligations::h6e0b9a7a32cf419d
  23:     0x7ff8ffcf7944 - <rustc_trait_selection::traits::fulfill::PendingPredicateObligation as core::fmt::Debug>::fmt::h193f29610c75bb2a
  24:     0x7ff8ffcc9b3f - <rustc_trait_selection::traits::fulfill::FulfillmentContext as rustc_infer::traits::engine::TraitEngine>::select_where_possible::h1148d87c05c2aec0
  25:     0x7ff8fe12fb9f - <rustc_middle::ty::sty::TraitRef as rustc_traits::chalk::lowering::LowerInto<chalk_solve::rust_ir::TraitBound<rustc_middle::traits::chalk::RustInterner>>>::lower_into::h8ff0d70e3d0188ac
  26:     0x7ff8fe1234e1 - <rustc_middle::ty::sty::TraitRef as rustc_traits::chalk::lowering::LowerInto<chalk_solve::rust_ir::TraitBound<rustc_middle::traits::chalk::RustInterner>>>::lower_into::h8ff0d70e3d0188ac
  27:     0x7ff8fe1f56da - <rustc_traits::chalk::db::RustIrDatabase as chalk_solve::RustIrDatabase<rustc_middle::traits::chalk::RustInterner>>::generator_witness_datum::h56e9b357d981663c
  28:     0x7ff8ffca08b2 - <rustc_trait_selection::traits::specialize::OverlapError as core::fmt::Debug>::fmt::hb9cecb2f776e6243
  29:     0x7ff8ffca476b - <rustc_trait_selection::traits::specialize::OverlapError as core::fmt::Debug>::fmt::hb9cecb2f776e6243
  30:     0x7ff8ffb34698 - <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::query::outlives_bounds::InferCtxtExt>::implied_outlives_bounds::h491f1d8abd137254
  31:     0x7ff8ffca8e4b - <rustc_trait_selection::traits::specialize::OverlapError as core::fmt::Debug>::fmt::hb9cecb2f776e6243
  32:     0x7ff8ffc536c3 - <rustc_trait_selection::traits::specialize::OverlapError as core::fmt::Debug>::fmt::hb9cecb2f776e6243
  33:     0x7ff8ffccc165 - rustc_trait_selection::traits::query::type_op::ascribe_user_type::<impl rustc_trait_selection::traits::query::type_op::QueryTypeOp for rustc_middle::traits::query::type_op::AscribeUserType>::perform_query::h79d7e0f340ea47eb
  34:     0x7ff8ff026445 - <rustc_mir::transform::dest_prop::CandidateAssignment as core::fmt::Debug>::fmt::h52c7195478494fc1
  35:     0x7ff8fec586c0 - <rustc_middle::mir::traversal::ReversePostorder as core::iter::traits::iterator::Iterator>::size_hint::h344f471fd1cfc36d
  36:     0x7ff8fef20f53 - <rustc_middle::ty::sty::RegionVid as rustc_mir::borrow_check::facts::FactRow>::write::h2cc1e203cd8b8d95
  37:     0x7ff8ff0c5d07 - rustc_mir::borrow_check::provide::hca974026c7954712
  38:     0x7ff8febc3d7b - <rustc_mir_build::thir::pattern::_match::split_grouped_constructors::Border as core::fmt::Debug>::fmt::h2468913b704d8b89
  39:     0x7ff8ff0c2e00 - rustc_mir::borrow_check::provide::hca974026c7954712
  40:     0x7ff8ff092e9f - <rustc_mir::util::spanview::SpanViewable as core::fmt::Debug>::fmt::h8fc74e5ad64e2a6b
  41:     0x7ff8fbfc9fae - rustc_interface::interface::parse_cfgspecs::hf1434c0a659b9ddd
  42:     0x7ff8fbfb7326 - <rls_span::serde_expanded::_IMPL_DESERIALIZE_FOR_OneIndexed::<impl serde::de::Deserialize for rls_span::OneIndexed>::deserialize::__Visitor as serde::de::Visitor>::expecting::h9a9b74719e191e7e
  43:     0x7ff8fbf7c877 - <rls_span::serde_expanded::_IMPL_DESERIALIZE_FOR_OneIndexed::<impl serde::de::Deserialize for rls_span::OneIndexed>::deserialize::__Visitor as serde::de::Visitor>::expecting::h9a9b74719e191e7e
  44:     0x7ff8fbfcc3dd - rustc_interface::interface::parse_cfgspecs::hf1434c0a659b9ddd
  45:     0x7ff8fbf69f5e - <rls_span::serde_expanded::_IMPL_DESERIALIZE_FOR_OneIndexed::<impl serde::de::Deserialize for rls_span::OneIndexed>::deserialize::__Visitor as serde::de::Visitor>::expecting::h9a9b74719e191e7e
  46:     0x7ff8fbf7566f - <rls_span::serde_expanded::_IMPL_DESERIALIZE_FOR_OneIndexed::<impl serde::de::Deserialize for rls_span::OneIndexed>::deserialize::__Visitor as serde::de::Visitor>::expecting::h9a9b74719e191e7e
  47:     0x7ff8fbfbc034 - <rls_span::serde_expanded::_IMPL_DESERIALIZE_FOR_OneIndexed::<impl serde::de::Deserialize for rls_span::OneIndexed>::deserialize::__Visitor as serde::de::Visitor>::expecting::h9a9b74719e191e7e
  48:     0x7ff8fbff5d3d - rustc_interface::passes::QueryContext::print_stats::hd29879a8b3af7832
  49:     0x7ff8fbe1957b - tracing_core::event::Event::metadata::h86572d73d587b6bd
  50:     0x7ff8fbe1ddb6 - tracing_core::event::Event::metadata::h86572d73d587b6bd
  51:     0x7ff8fbdddf02 - <rustc_driver::Compilation as core::fmt::Debug>::fmt::h234a3cece69c9eed
  52:     0x7ff8fbdf8ba6 - <rustc_driver::Compilation as core::fmt::Debug>::fmt::h234a3cece69c9eed
  53:     0x7ff8fbd81b26 - rustc_errors::snippet::MultilineAnnotation::increase_depth::h93baa802966d31ca
  54:     0x7ff8fbe1e53b - tracing_core::event::Event::metadata::h86572d73d587b6bd
  55:     0x7ff8fbdf4a34 - <rustc_driver::Compilation as core::fmt::Debug>::fmt::h234a3cece69c9eed
  56:     0x7ff8fbd9074c - <rustc_driver::args::Error as core::fmt::Debug>::fmt::h500583d00d12c2d9
  57:     0x7ff8fbdf6b28 - <rustc_driver::Compilation as core::fmt::Debug>::fmt::h234a3cece69c9eed
  58:     0x7ff8fbdd3a30 - <rustc_driver::Compilation as core::fmt::Debug>::fmt::h234a3cece69c9eed
  59:     0x7ff8fbdfa123 - <rustc_driver::Compilation as core::fmt::Debug>::fmt::h234a3cece69c9eed
  60:     0x7ff8fbd77523 - rustc_errors::snippet::MultilineAnnotation::increase_depth::h93baa802966d31ca
  61:     0x7ff947a1f6a7 - std::sys::windows::thread::Thread::new::h04914a10b014a9d7
  62:     0x7ff99b6c7bd4 - BaseThreadInitThunk
  63:     0x7ff99d2ace51 - RtlUserThreadStart

Query Stack

query stack during panic:
#0 [type_op_ascribe_user_type] evaluating `type_op_ascribe_user_type` `Canonical { max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }, CanonicalVarInfo { kind: Region(U0) }, CanonicalVarInfo { kind: Region(U0) }, CanonicalVarInfo { kind: Region(U0) }], value: ParamEnvAnd { param_env: ParamEnv { caller_bounds: [ConstEvaluatable(WithOptConstParam { did: DefId(0:15 ~ bugtest[35b2]::{impl#0}::{constant#0}), const_param_did: None }, [ReLateBound(DebruijnIndex(0), BrAnon(0)), Const { ty: std::num::NonZeroUsize, val: Param(N/#1) }]), TraitPredicate(<[u8; _] as std::marker::Sized>)], reveal: UserFacing }, value: AscribeUserType { mir_ty: fn(&[u8]) -> std::result::Result<&[u8; _], <&[u8; _] as std::convert::TryFrom<&[u8]>>::Error> {<&[u8; _] as std::convert::TryFrom<&[u8]>>::try_from}, def_id: DefId(2:1966 ~ core[5e46]::convert::TryFrom::try_from), user_substs: UserSubsts { substs: [&[u8; _], &[u8]], user_self_ty: None } } } }`
#1 [mir_borrowck] borrow-checking `<A<N> as std::convert::TryFrom<&'a [u8]>>::try_from`
#2 [analysis] running analysis passes on this crate
end of query stack

Possibly related to #77650

[fanninpm]

I'm trying to add this ICE to the glacier. Can you confirm if this still causes an ICE with the latest nightly?

[varkor]

It appears to be working now.

[dmit]

Just like in #72442, this bug is only present when building in debug mode and with incremental compilation enabled.

If you copy the first example from this ticket to an empty project, then:

• ❌ cargo build fails.
• ✅ cargo build --release works.
• ✅ CARGO_INCREMENTAL=0 cargo build works.

Tested just now with rustc 1.50.0-nightly (f745834 2020-12-18).

The playground sets incremental = false, which explains why it compiles there.

[moelife-coder]

It's still present in the latest build:

rustc 1.53.0-nightly (9d9c2c92b 2021-04-19)
binary: rustc
commit-hash: 9d9c2c92b834c430f102ea96f65119e37320776e
commit-date: 2021-04-19
host: x86_64-unknown-linux-gnu
release: 1.53.0-nightly
LLVM version: 12.0.0

Related issue: #84355

[hellow554]

this is my current MCVE that also panics on stable: rustc +stable -C incremental=1 const_incr.rs
It seems to be important that T is a reference to something. Can't trigger the error when T = Z

#![feature(const_generics)]
#![feature(const_evaluatable_checked)]

struct Z;
const fn one() -> usize { 1 }

fn from_a_to_b<T>(source: [u8; one()]) -> T {
    todo!()
}

fn main() {
    let _: &Z = from_a_to_b([0; 1]);
}

Regression in nightly-2020-09-19 f3c923a...bbc6774

@rustbot modify labels: A-incr-comp regression-from-stable-to-stable

[hellow554]

I have another example while I was reducing #85848

#![feature(const_generics, const_evaluatable_checked)]

trait Delegates<T> {}

struct FileCap<const Op: bool> {}

fn writes_to_path<C>(cap: &C)
where
    C: Delegates<FileCap<{ false }>>,
{
    writes_to_specific_path(&cap);
}

fn writes_to_specific_path<C>(cap: &C)
where
    C: Delegates<FileCap<{ false }>>,
{
}

fn main() {}

Funny enough, the braces around false are mandatory for the ICE, although rust suggests removing them: unnecessary braces around const expression. If I remove them, the ICE goes away.