rust-lang / rust-lang/rust

ICE :coherence: impl was matchable against Binder but now it is not, otherwise hang

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C-bug F-specialization fixed-by-next-solver I-ICE S-bug-has-test T-compiler
Dominant language
Rust
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Description

This one is a bit special because without coherence, it looks like rustc just hangs and with coherence, we ICE.
Not sure which is better though 😆

Code
#![feature(specialization)]

trait Foo {}

impl Foo for <u16 as Assoc>::Output {}

impl Foo for u32 {}

trait Assoc {
    type Output;
}
impl Output for u32 {}
impl Assoc for <u16 as Assoc>::Output {
    default type Output = bool;
}
Meta

rustc --version --verbose:

rustc 1.80.0-nightly (6e1d94708 2024-05-10)
binary: rustc
commit-hash: 6e1d94708a0a4a35ca7e46c6cac98adf62fe800e
commit-date: 2024-05-10
host: x86_64-unknown-linux-gnu
release: 1.80.0-nightly
LLVM version: 18.1.4

rustc 2.rs -Znext-solver=coherence

Error output
error[E0405]: cannot find trait `Output` in this scope
  --> 2.rs:12:6
   |
12 | impl Output for u32 {}
   |      ^^^^^^ not found in this scope

warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
 --> 2.rs:1:12
  |
1 | #![feature(specialization)]
  |            ^^^^^^^^^^^^^^
  |
  = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
  = help: consider using `min_specialization` instead, which is more stable and complete
  = note: `#[warn(incomplete_features)]` on by default

error[E0601]: `main` function not found in crate `2`
  --> 2.rs:15:2
   |
15 | }
   |  ^ consider adding a `main` function to `2.rs`

error[E0119]: conflicting implementations of trait `Foo` for type `<u16 as Assoc>::Output`
 --> 2.rs:7:1
  |
5 | impl Foo for <u16 as Assoc>::Output {}
  | ----------------------------------- first implementation here
6 |
7 | impl Foo for u32 {}
  | ^^^^^^^^^^^^^^^^ conflicting implementation for `<u16 as Assoc>::Output`
Backtrace 


error: internal compiler error: compiler/rustc_trait_selection/src/traits/select/mod.rs:2484:17: impl DefId(0:9 ~ 2[eac6]::{impl#3}) was matchable against Binder { value: TraitPredicate(<u16 as Assoc>, polarity:Positive), bound_vars: [] } but now is not

thread 'rustc' panicked at compiler/rustc_trait_selection/src/traits/select/mod.rs:2484:17:
Box<dyn Any>
stack backtrace:
   0:     0x70d6652fec45 - std::backtrace_rs::backtrace::libunwind::trace::h076b7d335d3f5d8e
                               at /rustc/6e1d94708a0a4a35ca7e46c6cac98adf62fe800e/library/std/src/../../backtrace/src/backtrace/libunwind.rs:105:5
   1:     0x70d6652fec45 - std::backtrace_rs::backtrace::trace_unsynchronized::h690cc670e4b53e26
                               at /rustc/6e1d94708a0a4a35ca7e46c6cac98adf62fe800e/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x70d6652fec45 - std::sys_common::backtrace::_print_fmt::h1c4819ca31a9a7e4
                               at /rustc/6e1d94708a0a4a35ca7e46c6cac98adf62fe800e/library/std/src/sys_common/backtrace.rs:68:5
   3:     0x70d6652fec45 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h80477bfb9d3c4c0e
                               at /rustc/6e1d94708a0a4a35ca7e46c6cac98adf62fe800e/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x70d66534de0b - core::fmt::rt::Argument::fmt::h15c65d56ade59385
                               at /rustc/6e1d94708a0a4a35ca7e46c6cac98adf62fe800e/library/core/src/fmt/rt.rs:165:63
   5:     0x70d66534de0b - core::fmt::write::haea2d0f40af65206
                               at /rustc/6e1d94708a0a4a35ca7e46c6cac98adf62fe800e/library/core/src/fmt/mod.rs:1157:21
   6:     0x70d6652f398f - std::io::Write::write_fmt::h3669847656ea0615
                               at /rustc/6e1d94708a0a4a35ca7e46c6cac98adf62fe800e/library/std/src/io/mod.rs:1835:15
   7:     0x70d6652fea1e - std::sys_common::backtrace::_print::ha73eb023eb111e4d
                               at /rustc/6e1d94708a0a4a35ca7e46c6cac98adf62fe800e/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x70d6652fea1e - std::sys_common::backtrace::print::hbff52728084c2b90
                               at /rustc/6e1d94708a0a4a35ca7e46c6cac98adf62fe800e/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x70d665301389 - std::panicking::default_hook::{{closure}}::h0d17646ea03cf057
  10:     0x70d6653010cd - std::panicking::default_hook::h40601e5e85aa667a
                               at /rustc/6e1d94708a0a4a35ca7e46c6cac98adf62fe800e/library/std/src/panicking.rs:298:9
  11:     0x70d661cceab0 - std[420967f48346b39c]::panicking::update_hook::<alloc[478b9a7ca3809208]::boxed::Box<rustc_driver_impl[9e6d636adc327e66]::install_ice_hook::{closure#0}>>::{closure#0}
  12:     0x70d665301a86 - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::hd12a80ecb6b6cfe8
                               at /rustc/6e1d94708a0a4a35ca7e46c6cac98adf62fe800e/library/alloc/src/boxed.rs:2036:9
  13:     0x70d665301a86 - std::panicking::rust_panic_with_hook::h33d302c7fc861cff
                               at /rustc/6e1d94708a0a4a35ca7e46c6cac98adf62fe800e/library/std/src/panicking.rs:799:13
  14:     0x70d661cfead4 - std[420967f48346b39c]::panicking::begin_panic::<rustc_errors[45d68b0688aaeb54]::ExplicitBug>::{closure#0}
  15:     0x70d661cfb476 - std[420967f48346b39c]::sys_common::backtrace::__rust_end_short_backtrace::<std[420967f48346b39c]::panicking::begin_panic<rustc_errors[45d68b0688aaeb54]::ExplicitBug>::{closure#0}, !>
  16:     0x70d661cf6a26 - std[420967f48346b39c]::panicking::begin_panic::<rustc_errors[45d68b0688aaeb54]::ExplicitBug>
  17:     0x70d661d07c61 - <rustc_errors[45d68b0688aaeb54]::diagnostic::BugAbort as rustc_errors[45d68b0688aaeb54]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  18:     0x70d6621bee9c - rustc_middle[b29a689460622aa4]::util::bug::opt_span_bug_fmt::<rustc_span[7f36b8f4ab5c669f]::span_encoding::Span>::{closure#0}
  19:     0x70d6621a387a - rustc_middle[b29a689460622aa4]::ty::context::tls::with_opt::<rustc_middle[b29a689460622aa4]::util::bug::opt_span_bug_fmt<rustc_span[7f36b8f4ab5c669f]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  20:     0x70d6621a371b - rustc_middle[b29a689460622aa4]::ty::context::tls::with_context_opt::<rustc_middle[b29a689460622aa4]::ty::context::tls::with_opt<rustc_middle[b29a689460622aa4]::util::bug::opt_span_bug_fmt<rustc_span[7f36b8f4ab5c669f]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  21:     0x70d660498d20 - rustc_middle[b29a689460622aa4]::util::bug::bug_fmt
  22:     0x70d6637ab30e - <rustc_trait_selection[6d54a2dd8c1020d6]::traits::select::SelectionContext>::confirm_candidate
  23:     0x70d66356f516 - <rustc_trait_selection[6d54a2dd8c1020d6]::traits::select::SelectionContext>::evaluate_trait_predicate_recursively
  24:     0x70d663566dc0 - <rustc_trait_selection[6d54a2dd8c1020d6]::traits::select::SelectionContext>::evaluate_root_obligation
  25:     0x70d66356538b - rustc_traits[8c7b2acb6abdc47]::evaluate_obligation::evaluate_obligation
  26:     0x70d663564d29 - rustc_query_impl[afd3850f0842b024]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[afd3850f0842b024]::query_impl::evaluate_obligation::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b29a689460622aa4]::query::erase::Erased<[u8; 2usize]>>
  27:     0x70d663564292 - rustc_query_system[6b56706b6a133fc0]::query::plumbing::try_execute_query::<rustc_query_impl[afd3850f0842b024]::DynamicConfig<rustc_query_system[6b56706b6a133fc0]::query::caches::DefaultCache<rustc_type_ir[f4c1b7e92dc1a1]::canonical::Canonical<rustc_middle[b29a689460622aa4]::ty::context::TyCtxt, rustc_middle[b29a689460622aa4]::ty::ParamEnvAnd<rustc_middle[b29a689460622aa4]::ty::predicate::Predicate>>, rustc_middle[b29a689460622aa4]::query::erase::Erased<[u8; 2usize]>>, false, false, false>, rustc_query_impl[afd3850f0842b024]::plumbing::QueryCtxt, false>
  28:     0x70d663563eda - rustc_query_impl[afd3850f0842b024]::query_impl::evaluate_obligation::get_query_non_incr::__rust_end_short_backtrace
  29:     0x70d65fc962fb - <rustc_trait_selection[6d54a2dd8c1020d6]::traits::fulfill::FulfillProcessor as rustc_data_structures[fdcc01f8f4e370e]::obligation_forest::ObligationProcessor>::process_obligation
  30:     0x70d66300d6e3 - <rustc_data_structures[fdcc01f8f4e370e]::obligation_forest::ObligationForest<rustc_trait_selection[6d54a2dd8c1020d6]::traits::fulfill::PendingPredicateObligation>>::process_obligations::<rustc_trait_selection[6d54a2dd8c1020d6]::traits::fulfill::FulfillProcessor>
  31:     0x70d6633f7e7a - <rustc_trait_selection[6d54a2dd8c1020d6]::traits::engine::ObligationCtxt>::assumed_wf_types
  32:     0x70d663654ea7 - rustc_hir_analysis[4c558df9982554fc]::check::wfcheck::check_well_formed
  33:     0x70d663653b9d - rustc_query_impl[afd3850f0842b024]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[afd3850f0842b024]::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b29a689460622aa4]::query::erase::Erased<[u8; 1usize]>>
  34:     0x70d663653340 - rustc_query_system[6b56706b6a133fc0]::query::plumbing::try_execute_query::<rustc_query_impl[afd3850f0842b024]::DynamicConfig<rustc_query_system[6b56706b6a133fc0]::query::caches::VecCache<rustc_hir[e33a67695c771d92]::hir_id::OwnerId, rustc_middle[b29a689460622aa4]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[afd3850f0842b024]::plumbing::QueryCtxt, false>
  35:     0x70d6636530bf - rustc_query_impl[afd3850f0842b024]::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
  36:     0x70d663650ff7 - rustc_hir_analysis[4c558df9982554fc]::check::wfcheck::check_mod_type_wf
  37:     0x70d663650e1f - rustc_query_impl[afd3850f0842b024]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[afd3850f0842b024]::query_impl::check_mod_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b29a689460622aa4]::query::erase::Erased<[u8; 1usize]>>
  38:     0x70d663aeda70 - rustc_query_system[6b56706b6a133fc0]::query::plumbing::try_execute_query::<rustc_query_impl[afd3850f0842b024]::DynamicConfig<rustc_query_system[6b56706b6a133fc0]::query::caches::DefaultCache<rustc_span[7f36b8f4ab5c669f]::def_id::LocalModDefId, rustc_middle[b29a689460622aa4]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[afd3850f0842b024]::plumbing::QueryCtxt, false>
  39:     0x70d663aed81b - rustc_query_impl[afd3850f0842b024]::query_impl::check_mod_type_wf::get_query_non_incr::__rust_end_short_backtrace
  40:     0x70d66338fc46 - rustc_hir_analysis[4c558df9982554fc]::check_crate
  41:     0x70d663385f07 - rustc_interface[d156b119c0ee28cb]::passes::analysis
  42:     0x70d663385a55 - rustc_query_impl[afd3850f0842b024]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[afd3850f0842b024]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b29a689460622aa4]::query::erase::Erased<[u8; 1usize]>>
  43:     0x70d663dcb565 - rustc_query_system[6b56706b6a133fc0]::query::plumbing::try_execute_query::<rustc_query_impl[afd3850f0842b024]::DynamicConfig<rustc_query_system[6b56706b6a133fc0]::query::caches::SingleCache<rustc_middle[b29a689460622aa4]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[afd3850f0842b024]::plumbing::QueryCtxt, false>
  44:     0x70d663dcb2c9 - rustc_query_impl[afd3850f0842b024]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  45:     0x70d663c0324e - rustc_interface[d156b119c0ee28cb]::interface::run_compiler::<core[554a336f897370d5]::result::Result<(), rustc_span[7f36b8f4ab5c669f]::ErrorGuaranteed>, rustc_driver_impl[9e6d636adc327e66]::run_compiler::{closure#0}>::{closure#1}
  46:     0x70d663bdfe09 - std[420967f48346b39c]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[d156b119c0ee28cb]::util::run_in_thread_with_globals<rustc_interface[d156b119c0ee28cb]::util::run_in_thread_pool_with_globals<rustc_interface[d156b119c0ee28cb]::interface::run_compiler<core[554a336f897370d5]::result::Result<(), rustc_span[7f36b8f4ab5c669f]::ErrorGuaranteed>, rustc_driver_impl[9e6d636adc327e66]::run_compiler::{closure#0}>::{closure#1}, core[554a336f897370d5]::result::Result<(), rustc_span[7f36b8f4ab5c669f]::ErrorGuaranteed>>::{closure#0}, core[554a336f897370d5]::result::Result<(), rustc_span[7f36b8f4ab5c669f]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[554a336f897370d5]::result::Result<(), rustc_span[7f36b8f4ab5c669f]::ErrorGuaranteed>>
  47:     0x70d663bdfbb6 - <<std[420967f48346b39c]::thread::Builder>::spawn_unchecked_<rustc_interface[d156b119c0ee28cb]::util::run_in_thread_with_globals<rustc_interface[d156b119c0ee28cb]::util::run_in_thread_pool_with_globals<rustc_interface[d156b119c0ee28cb]::interface::run_compiler<core[554a336f897370d5]::result::Result<(), rustc_span[7f36b8f4ab5c669f]::ErrorGuaranteed>, rustc_driver_impl[9e6d636adc327e66]::run_compiler::{closure#0}>::{closure#1}, core[554a336f897370d5]::result::Result<(), rustc_span[7f36b8f4ab5c669f]::ErrorGuaranteed>>::{closure#0}, core[554a336f897370d5]::result::Result<(), rustc_span[7f36b8f4ab5c669f]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[554a336f897370d5]::result::Result<(), rustc_span[7f36b8f4ab5c669f]::ErrorGuaranteed>>::{closure#2} as core[554a336f897370d5]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  48:     0x70d66530b8cb - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h56757e55bbcfa856
                               at /rustc/6e1d94708a0a4a35ca7e46c6cac98adf62fe800e/library/alloc/src/boxed.rs:2022:9
  49:     0x70d66530b8cb - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hf230e264012c630a
                               at /rustc/6e1d94708a0a4a35ca7e46c6cac98adf62fe800e/library/alloc/src/boxed.rs:2022:9
  50:     0x70d66530b8cb - std::sys::pal::unix::thread::Thread::new::thread_start::hfcc46c6a7fe0b042
                               at /rustc/6e1d94708a0a4a35ca7e46c6cac98adf62fe800e/library/std/src/sys/pal/unix/thread.rs:108:17
  51:     0x70d65eaa955a - <unknown>
  52:     0x70d65eb26a3c - <unknown>
  53:                0x0 - <unknown>

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: please make sure that you have updated to the latest nightly

note: please attach the file at `/tmp/im/rustc-ice-2024-05-11T13_50_03-227107.txt` to your bug report

note: compiler flags: -Z next-solver=coherence

query stack during panic:
#0 [evaluate_obligation] evaluating trait selection obligation `u16: Assoc`
#1 [check_well_formed] checking that `<impl at 2.rs:13:1: 13:38>` is well-formed
#2 [check_mod_type_wf] checking that types are well-formed in top-level module
#3 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 4 previous errors; 1 warning emitted

Some errors have detailed explanations: E0119, E0405, E0601.
For more information about an error, try `rustc --explain E0119`.

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Reproduce the minimal example with rustc 1.80.0-nightly and -Znext-solver=coherence, then compare the ICE and hang behavior without coherence. Start at compiler/rustc_trait_selection/src/traits/select/mod.rs:2484 and trace the reported candidate-matching path; done means the example no longer causes an internal compiler error or hangs.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Needs clarification
Newbie friendliness
25/100

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