Type checking for this incorrect impl trait panics with incremental compilation

[BattyBoopers]

When trying to compile this incorrect code using cargo, nightly rustc panics.
When compiling using rustc directly without further arguments (or using the same command cargo calls with -C incremental=... removed), it outputs an error message instead.

Code

trait XTrait<A> {}

struct X<T: XTrait<A>, A> (T, A);

trait Y<'t> {
    type M;
    type N: 't;
}

impl<'t, T: XTrait<A>, A> Y<'t> for X<T, A> {
    type M = X<T, Self::N>;
    type N = &'t ();
}

fn main() {
    println!("Hello, world!");
}

I also noticed that changing type N = &'t (); to type N = (); will also result in an error message instead of a panic.

Meta

rustc --version --verbose:

rustc 1.56.0-nightly (2faabf579 2021-07-27)
binary: rustc
commit-hash: 2faabf579323f5252329264cc53ba9ff803429a3
commit-date: 2021-07-27
host: x86_64-unknown-linux-gnu
release: 1.56.0-nightly
LLVM version: 12.0.1

Error output

cargo +nightly --verbose run

   Compiling crash v0.1.0 (/home/dinky/code/rust/tmp/crash)
     Running `rustc --crate-name crash --edition=2018 src/main.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type bin --emit=dep-info,link -C embed-bitcode=no -C debuginfo=2 -C metadata=d474572ba93d44de -C extra-filename=-d474572ba93d44de --out-dir /home/dinky/code/rust/tmp/crash/target/debug/deps -C incremental=/home/dinky/code/rust/tmp/crash/target/debug/incremental -L dependency=/home/dinky/code/rust/tmp/crash/target/debug/deps`
error: internal compiler error: compiler/rustc_middle/src/ich/impls_ty.rs:94:17: StableHasher: unexpected region '_#0r

thread 'rustc' panicked at 'Box<dyn Any>', compiler/rustc_errors/src/lib.rs:1034:9
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 1.56.0-nightly (2faabf579 2021-07-27) running on x86_64-unknown-linux-gnu

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

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

query stack during panic:
#0 [check_impl_item_well_formed] checking that `<impl at src/main.rs:10:1: 13:2>::M` is well-formed
#1 [analysis] running analysis passes on this crate
end of query stack
error: could not compile `crash`

Caused by:
  process didn't exit successfully: `rustc --crate-name crash --edition=2018 src/main.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type bin --emit=dep-info,link -C embed-bitcode=no -C debuginfo=2 -C metadata=d474572ba93d44de -C extra-filename=-d474572ba93d44de --out-dir /home/dinky/code/rust/tmp/crash/target/debug/deps -C incremental=/home/dinky/code/rust/tmp/crash/target/debug/incremental -L dependency=/home/dinky/code/rust/tmp/crash/target/debug/deps` (exit status: 101)

Backtrace

RUST_BACKTRACE=1 cargo +nightly --verbose run

   Compiling crash v0.1.0 (/home/dinky/code/rust/tmp/crash)
     Running `rustc --crate-name crash --edition=2018 src/main.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type bin --emit=dep-info,link -C embed-bitcode=no -C debuginfo=2 -C metadata=d474572ba93d44de -C extra-filename=-d474572ba93d44de --out-dir /home/dinky/code/rust/tmp/crash/target/debug/deps -C incremental=/home/dinky/code/rust/tmp/crash/target/debug/incremental -L dependency=/home/dinky/code/rust/tmp/crash/target/debug/deps`
error: internal compiler error: compiler/rustc_middle/src/ich/impls_ty.rs:94:17: StableHasher: unexpected region '_#0r

thread 'rustc' panicked at 'Box<dyn Any>', compiler/rustc_errors/src/lib.rs:1034:9
stack backtrace:
   0: std::panicking::begin_panic
   1: std::panic::panic_any
   2: rustc_errors::HandlerInner::bug
   3: rustc_errors::Handler::bug
   4: rustc_middle::ty::context::tls::with_opt
   5: rustc_middle::util::bug::opt_span_bug_fmt
   6: rustc_middle::util::bug::bug_fmt
   7: 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
   8: <rustc_middle::ty::TyS as rustc_data_structures::stable_hasher::HashStable<rustc_middle::ich::hcx::StableHashingContext>>::hash_stable
   9: std::thread::local::LocalKey<T>::with
  10: rustc_middle::ich::impls_ty::<impl rustc_data_structures::stable_hasher::HashStable<rustc_middle::ich::hcx::StableHashingContext> for rustc_middle::ty::sty::Binder<T>>::hash_stable
  11: rustc_query_system::dep_graph::dep_node::DepNode<K>::construct
  12: rustc_query_system::query::plumbing::get_query_impl
  13: rustc_query_system::query::plumbing::get_query
  14: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::error_reporting::InferCtxtExt>::report_selection_error
  15: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::error_reporting::InferCtxtPrivExt>::report_fulfillment_error
  16: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::error_reporting::InferCtxtExt>::report_fulfillment_errors
  17: rustc_typeck::check::fn_ctxt::_impl::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::select_all_obligations_or_error
  18: rustc_infer::infer::InferCtxtBuilder::enter
  19: rustc_typeck::check::wfcheck::check_associated_item
  20: rustc_typeck::check::check::check_impl_item_well_formed
  21: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  22: rustc_data_structures::stack::ensure_sufficient_stack
  23: rustc_query_system::query::plumbing::force_query_with_job
  24: rustc_query_system::query::plumbing::get_query_impl
  25: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::check_impl_item_well_formed
  26: <rustc_typeck::check::wfcheck::CheckTypeWellFormedVisitor as rustc_hir::intravisit::Visitor>::visit_impl_item
  27: std::panicking::try
  28: rustc_data_structures::sync::par_for_each_in
  29: rustc_session::session::Session::track_errors
  30: rustc_typeck::check_crate
  31: rustc_interface::passes::analysis
  32: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  33: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  34: rustc_query_system::dep_graph::graph::DepGraph<K>::with_eval_always_task
  35: rustc_data_structures::stack::ensure_sufficient_stack
  36: rustc_query_system::query::plumbing::force_query_with_job
  37: rustc_query_system::query::plumbing::get_query_impl
  38: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::analysis
  39: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  40: rustc_span::with_source_map
  41: rustc_interface::interface::create_compiler_and_run
  42: scoped_tls::ScopedKey<T>::set
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 1.56.0-nightly (2faabf579 2021-07-27) running on x86_64-unknown-linux-gnu

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

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

query stack during panic:
#0 [check_impl_item_well_formed] checking that `<impl at src/main.rs:10:1: 13:2>::M` is well-formed
#1 [analysis] running analysis passes on this crate
end of query stack
error: could not compile `crash`

Caused by:
  process didn't exit successfully: `rustc --crate-name crash --edition=2018 src/main.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type bin --emit=dep-info,link -C embed-bitcode=no -C debuginfo=2 -C metadata=d474572ba93d44de -C extra-filename=-d474572ba93d44de --out-dir /home/dinky/code/rust/tmp/crash/target/debug/deps -C incremental=/home/dinky/code/rust/tmp/crash/target/debug/incremental -L dependency=/home/dinky/code/rust/tmp/crash/target/debug/deps` (exit status: 101)

[Luro02]

I seem to have run into the same issue with this code:

pub struct Table<T, const N: usize>([Option<T>; N]);

impl<'a, T, const N: usize> IntoIterator for &'a Table<T, N> {
    type IntoIter = ::core::iter::Flatten<::core::slice::Iter<'a, T>>;
    type Item = &'a T;

    fn into_iter(self) -> Self::IntoIter {
        unimplemented!()
    }
}

cargo +nightly --verbose check

     Running `rustc --crate-name ice --edition=2018 src\lib.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type lib --emit=dep-info,metadata -C embed-bitcode=no -C debuginfo=2 -C metadata=1c3553409c4d9998 -C extra-filename=-1c3553409c4d9998 --out-dir "target\debug\deps" -C "incremental=target\debug\incremental" -L "dependency=target\debug\deps"`
error: internal compiler error: compiler\rustc_middle\src\ich\impls_ty.rs:94:17: StableHasher: unexpected region '_#0r

thread 'rustc' panicked at 'Box<dyn Any>', compiler\rustc_errors\src\lib.rs:1034:9
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 1.56.0-nightly (08095fc1f 2021-07-26) 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

query stack during panic:
#0 [check_impl_item_well_formed] checking that `<impl at src\lib.rs:3:1: 10:2>::IntoIter` is well-formed
#1 [analysis] running analysis passes on this crate
end of query stack
error[E0277]: `&T` is not an iterator
   --> src\lib.rs:7:27
    |
7   |     fn into_iter(self) -> Self::IntoIter {
    |                           ^^^^^^^^^^^^^^ `&T` is not an iterator
    |
   ::: .rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib/rustlib/src/rust\library\core\src\iter\adapters\flatten.rs:150:38
    |
150 | pub struct Flatten<I: Iterator<Item: IntoIterator>> {
    |                                      ------------ required by this bound in `Flatten`
    |
    = help: the trait `Iterator` is not implemented for `&T`
    = note: required because of the requirements on the impl of `IntoIterator` for `&T`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `ice` due to previous error

Caused by:
  process didn't exit successfully: `rustc --crate-name ice --edition=2018 src\lib.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type lib --emit=dep-info,metadata -C embed-bitcode=no -C debuginfo=2 -C metadata=1c3553409c4d9998 -C extra-filename=-1c3553409c4d9998 --out-dir "target\debug\deps" -C "incremental=target\debug\incremental" -L "dependency=target\debug\deps"` (exit code: 101)

rustc --version --verbose

rustc 1.56.0-nightly (08095fc1f 2021-07-26)
binary: rustc
commit-hash: 08095fc1f875c89e507f17cf6c6a780c8ffa4c01
commit-date: 2021-07-26
host: x86_64-pc-windows-msvc
release: 1.56.0-nightly
LLVM version: 12.0.1

Backtrace

thread 'rustc' panicked at 'Box<dyn Any>', compiler\rustc_errors\src\lib.rs:1034:9
stack backtrace:
   0:     0x7fffef6f72af - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hf8f9ccaaf737268c
   1:     0x7fffef7202ea - core::fmt::write::h7e004fee938dc3bd
   2:     0x7fffef6eaa48 - <std::io::IoSlice as core::fmt::Debug>::fmt::h5e8f50613d775ee8
   3:     0x7fffef6faed6 - std::panicking::take_hook::hb09930640d1f28d1
   4:     0x7fffef6fa9b9 - std::panicking::take_hook::hb09930640d1f28d1
   5:     0x7fffe65b210e - <sha2::sha512::Sha512 as std::io::Write>::flush::h720d3aa9a35641a4
   6:     0x7fffef6fb7d0 - std::panicking::rust_panic_with_hook::ha77067ca1c3aa04d
   7:     0x7fffea84dff0 - rustc_errors::diagnostic_builder::DiagnosticBuilder::code::h17316800dfe2f643
   8:     0x7fffea84dfc9 - rustc_errors::diagnostic_builder::DiagnosticBuilder::code::h17316800dfe2f643
   9:     0x7fffeab0c6a1 - rustc_query_system::query::job::report_cycle::h37e4bf0df2043575
  10:     0x7fffea880290 - <rustc_errors::json::Diagnostic::from_errors_diagnostic::BufWriter as std::io::Write>::flush::h40a088e8c152c98f
  11:     0x7fffea886b15 - rustc_errors::HandlerInner::emit_diagnostic::h605f2fc202d4eea0
  12:     0x7fffea884622 - rustc_errors::Handler::bug::ha197644133cfa24e
  13:     0x7fffea7426d7 - rustc_middle::ty::walk::<impl rustc_middle::ty::subst::GenericArg>::walk_shallow::h2e0646470201bd9e
  14:     0x7fffea741fa0 - rustc_middle::ty::walk::<impl rustc_middle::ty::subst::GenericArg>::walk_shallow::h2e0646470201bd9e
  15:     0x7fffea741f48 - rustc_middle::ty::walk::<impl rustc_middle::ty::subst::GenericArg>::walk_shallow::h2e0646470201bd9e
  16:     0x7fffea742609 - rustc_middle::ty::walk::<impl rustc_middle::ty::subst::GenericArg>::walk_shallow::h2e0646470201bd9e
  17:     0x7fffeab08be7 - rustc_middle::util::bug::bug_fmt::h66e69068a677c29e
  18:     0x7fffea724198 - 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::h06caf495d24d485b
  19:     0x7fffea6e26e4 - <rustc_middle::ty::TyS as rustc_data_structures::stable_hasher::HashStable<rustc_middle::ich::hcx::StableHashingContext>>::hash_stable::h8bcc894d8e8fafcd
  20:     0x7fffea57b5df - <rustc_infer::traits::project::ProjectionCacheEntry as core::fmt::Debug>::fmt::h0f137517fe2c6105
  21:     0x7fffea72466e - 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::h06caf495d24d485b
  22:     0x7fffe99a2922 - <aho_corasick::packed::rabinkarp::RabinKarp as core::fmt::Debug>::fmt::hde8c8300ff16d93a
  23:     0x7fffe97ef07c - <aho_corasick::packed::rabinkarp::RabinKarp as core::fmt::Debug>::fmt::hde8c8300ff16d93a
  24:     0x7fffe9a7603b - <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::try_mark_green::h00bfb97540d859ae
  25:     0x7fffea347d84 - <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::error_reporting::InferCtxtExt>::report_selection_error::h7556b0cf2e4f5cc4
  26:     0x7fffea3517fd - <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::error_reporting::InferCtxtPrivExt>::report_fulfillment_error::h763b7d46de7cfc90
  27:     0x7fffea346071 - <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::error_reporting::InferCtxtExt>::report_fulfillment_errors::ha2f2c168eee417d3
  28:     0x7fffe8c5da5d - rustc_typeck::check::fn_ctxt::_impl::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::field_ty::h3cbfaf9ed5d45b96
  29:     0x7fffe8d57501 - <rustc_typeck::check::regionck::RegionCtxt as rustc_hir::intravisit::Visitor>::visit_expr::h02adde056c81ec89
  30:     0x7fffe8cf7c7f - <rustc_typeck::check::method::probe::ProbeScope as core::fmt::Debug>::fmt::hb5b3726d1521d739
  31:     0x7fffe8e3ca40 - rustc_typeck::check::check::check_item_type::ha94a19386e8638d1
  32:     0x7fffe9aee0a6 - <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::try_mark_green::h00bfb97540d859ae
  33:     0x7fffe9ac3059 - <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::try_mark_green::h00bfb97540d859ae
  34:     0x7fffe9bccdcf - rustc_query_impl::on_disk_cache::__ty_decoder_impl::<impl rustc_serialize::serialize::Decoder for rustc_query_impl::on_disk_cache::CacheDecoder>::error::h1345e6f615df1d2d
  35:     0x7fffe98d5971 - <aho_corasick::packed::rabinkarp::RabinKarp as core::fmt::Debug>::fmt::hde8c8300ff16d93a
  36:     0x7fffe985580d - <aho_corasick::packed::rabinkarp::RabinKarp as core::fmt::Debug>::fmt::hde8c8300ff16d93a
  37:     0x7fffe9a71391 - <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::try_mark_green::h00bfb97540d859ae
  38:     0x7fffe8ce337c - <rustc_typeck::check::wfcheck::CheckTypeWellFormedVisitor as rustc_hir::intravisit::Visitor>::visit_impl_item::h9e73050a69cf68be
  39:     0x7fffe8cc5dd3 - rustc_typeck::check::method::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::resolve_fully_qualified_call::h0e4aa83e76db2063
  40:     0x7fffe8dd715a - <rustc_typeck::constrained_generic_params::Parameter as core::fmt::Debug>::fmt::h3ea920eb9c7a4ad7
  41:     0x7fffe8d24175 - <rustc_typeck::outlives::explicit::ExplicitPredicatesMap as core::fmt::Debug>::fmt::he1d4e3e051288eac
  42:     0x7fffe8cf4fb1 - rustc_typeck::check_crate::h6bf0390329652898
  43:     0x7fffe66facd2 - rustc_interface::passes::analysis::h3a1cc36134a75432
  44:     0x7fffe9aef70b - <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::try_mark_green::h00bfb97540d859ae
  45:     0x7fffe9aae79d - <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::try_mark_green::h00bfb97540d859ae
  46:     0x7fffe9bcf745 - rustc_query_impl::on_disk_cache::__ty_decoder_impl::<impl rustc_serialize::serialize::Decoder for rustc_query_impl::on_disk_cache::CacheDecoder>::error::h1345e6f615df1d2d
  47:     0x7fffe98bdf63 - <aho_corasick::packed::rabinkarp::RabinKarp as core::fmt::Debug>::fmt::hde8c8300ff16d93a
  48:     0x7fffe97aa9dc - <aho_corasick::packed::rabinkarp::RabinKarp as core::fmt::Debug>::fmt::hde8c8300ff16d93a
  49:     0x7fffe9a6852c - <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::try_mark_green::h00bfb97540d859ae
  50:     0x7fffe6606f01 - <rustc_driver::args::Error as core::fmt::Debug>::fmt::h5df0c4d8fb3d3c5e
  51:     0x7fffe65cbb35 - chalk_engine::TimeStamp::increment::h8dcb95b70021502b
  52:     0x7fffe6608759 - <rustc_driver::args::Error as core::fmt::Debug>::fmt::h5df0c4d8fb3d3c5e
  53:     0x7fffe65d7b20 - rustc_driver::pretty::print_after_hir_lowering::h5170af9d47465e28
  54:     0x7fffe65c6d7d - <rustc_driver::Compilation as core::fmt::Debug>::fmt::h64cba24964385564
  55:     0x7fffef70931c - std::sys::windows::thread::Thread::new::h0a6e657a03ae2a48
  56:     0x7ff873fc7034 - BaseThreadInitThunk
  57:     0x7ff875f22651 - RtlUserThreadStart

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

[lqd]

I believe these ICEs trigger because of / are made visible by #83898 cc @Aaron1011 @estebank.

[estebank]

I'm not able to repro this with rustc 1.56.0-nightly (e3b1c12 2021-08-02):

% rustc +nightly f33.rs --edition=2018
error[E0277]: the trait bound `T: XTrait<&()>` is not satisfied
  --> f33.rs:11:14
   |
3  | struct X<T: XTrait<A>, A> (T, A);
   |             --------- required by this bound in `X`
...
11 |     type M = X<T, Self::N>;
   |              ^^^^^^^^^^^^^ the trait `XTrait<&()>` is not implemented for `T`
   |
help: consider further restricting this bound
   |
10 | impl<'t, T: XTrait<A> + XTrait<&()>, A> Y<'t> for X<T, A> {
   |                       ^^^^^^^^^^^^^

Could you verify that you see it with a recent nightly? There have been significant changes in this area recently.

---

Edit: ah! with incremental enabled I see it:

% rustc +nightly f33.rs -C incremental=true
error: internal compiler error: compiler/rustc_middle/src/ich/impls_ty.rs:94:17: StableHasher: unexpected region '_#0r

thread 'rustc' panicked at 'Box<dyn Any>', compiler/rustc_errors/src/lib.rs:1034:9
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 1.56.0-nightly (e3b1c12be 2021-08-02) running on x86_64-apple-darwin

note: compiler flags: -C incremental

query stack during panic:
#0 [check_impl_item_well_formed] checking that `<impl at f33.rs:10:1: 13:2>::M` is well-formed
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

[Luro02]

I am able to reproduce my issue and the other one on rustc 1.56.0-nightly (e3b1c12be 2021-08-02) by simply running cargo +nightly check:
reproduce.zip