ICE: `delayed bugs were created` - `failed region resolution while normalizing ParamEnv`

[steffahn]

Code

#![allow(unused)]

trait Supertrait<T> {}

trait Other {
    fn method(&self) {}
}

impl WithAssoc for &'static () {
    type As = ();
}

trait WithAssoc {
    type As;
}

trait Trait<P: WithAssoc>: Supertrait<P::As> {
    fn method(&self) {}
}

fn hrtb<T: for<'a> Trait<&'a ()>>() {}

(playground)

Meta

tested stable 1.84.1 and nightly a9730c3b5 2025-02-05¹

Error output

   Compiling playground v0.0.1 (/playground)
note: no errors encountered even though delayed bugs were created

note: those delayed bugs will now be shown as internal compiler errors

error: internal compiler error: failed region resolution while normalizing ParamEnv { caller_bounds: [Binder { value: TraitPredicate(<T as Trait<&'a ()>>, polarity:Positive), bound_vars: [Region(BrNamed(DefId(0:16 ~ playground[53aa]::hrtb::'a), 'a))] }, Binder { value: TraitPredicate(<T as Supertrait<<&'a () as WithAssoc>::As>>, polarity:Positive), bound_vars: [Region(BrNamed(DefId(0:16 ~ playground[53aa]::hrtb::'a), 'a))] }, Binder { value: TraitPredicate(<T as std::marker::Sized>, polarity:Positive), bound_vars: [] }] }: [ConcreteFailure(Subtype(TypeTrace { cause: ObligationCause { span: src/lib.rs:21:1: 21:36 (#0), body_id: DefId(0:14 ~ playground[53aa]::hrtb), code: MatchImpl(ObligationCause { span: src/lib.rs:21:1: 21:36 (#0), body_id: DefId(0:14 ~ playground[53aa]::hrtb), code: Misc }, DefId(0:7 ~ playground[53aa]::{impl#0})) }, values: TraitRefs(ExpectedFound { expected: <&'a () as WithAssoc>, found: <&'static () as WithAssoc> }) }), 'static, !1_0.Named(DefId(0:16 ~ playground[53aa]::hrtb::'a), "'a"))]
  --> src/lib.rs:21:1
   |
21 | fn hrtb<T: for<'a> Trait<&'a ()>>() {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: delayed at compiler/rustc_trait_selection/src/traits/mod.rs:293:19 - disabled backtrace
  --> src/lib.rs:21:1
   |
21 | fn hrtb<T: for<'a> Trait<&'a ()>>() {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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 `/playground/rustc-ice-2025-02-06T21_30_27-21.txt` to your bug report

note: compiler flags: --crate-type lib -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2

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

query stack during panic:
end of query stack
error: could not compile `playground` (lib)

Backtrace

note: delayed at compiler/rustc_trait_selection/src/traits/mod.rs:293:19
         0: <rustc_errors::DiagCtxtInner>::emit_diagnostic
         1: <rustc_errors::DiagCtxtHandle>::emit_diagnostic
         2: <rustc_span::ErrorGuaranteed as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
         3: <rustc_errors::DiagCtxtHandle>::span_delayed_bug::<rustc_span::span_encoding::Span, alloc::string::String>
         4: rustc_trait_selection::traits::do_normalize_predicates
         5: rustc_trait_selection::traits::normalize_param_env_or_error
         6: rustc_ty_utils::ty::param_env
         7: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::param_env::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
         8: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::DefIdCache<rustc_middle::query::erase::Erased<[u8; 8]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, true>
         9: rustc_query_impl::query_impl::param_env::get_query_incr::__rust_end_short_backtrace
        10: rustc_middle::query::plumbing::query_get_at::<rustc_query_system::query::caches::DefIdCache<rustc_middle::query::erase::Erased<[u8; 8]>>>
        11: rustc_hir_analysis::check::wfcheck::check_item_fn
        12: rustc_hir_analysis::check::wfcheck::check_well_formed
        13: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 1]>>
        14: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::Erased<[u8; 1]>, rustc_query_system::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, true>
        15: rustc_query_impl::query_impl::check_well_formed::get_query_incr::__rust_end_short_backtrace
        16: rustc_hir_analysis::check::wfcheck::check_mod_type_wf
        17: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::check_mod_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 1]>>
        18: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::DefaultCache<rustc_span::def_id::LocalModDefId, rustc_middle::query::erase::Erased<[u8; 1]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, true>
        19: rustc_query_impl::query_impl::check_mod_type_wf::get_query_incr::__rust_end_short_backtrace
        20: rustc_hir_analysis::check_crate
        21: rustc_interface::passes::run_required_analyses
        22: rustc_interface::passes::analysis
        23: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 0]>>
        24: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::SingleCache<rustc_middle::query::erase::Erased<[u8; 0]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, true>
        25: rustc_query_impl::query_impl::analysis::get_query_incr::__rust_end_short_backtrace
        26: rustc_interface::passes::create_and_enter_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
        27: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
        28: std::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
        29: <<std::thread::Builder>::spawn_unchecked_<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
        30: std::sys::pal::unix::thread::Thread::new::thread_start
        31: start_thread
                   at ./nptl/pthread_create.c:447:8
        32: clone3
                   at ./misc/../sysdeps/unix/sysv/linux/x86_64/clone3.S:78:0
  --> src/main.rs:21:1
   |
21 | fn hrtb<T: for<'a> Trait<&'a ()>>() {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Footnotes

1. and also cf8986d which hasn't quite reached nightly yet ↩

[compiler-errors]

This has to do with the fact that we don't check the well-formedness of the trait goal T: for<'a> Trait<&'a ()> since it has higher-ranked regions. We currently opt out of checking a lot of things with higher-ranked regions due to a lack of higher-ranked implied bounds.

To be specific:

fn hrtb<T: for<'a> Trait<&'a ()>>() {}

... should additionally require that for<'a> &'a (): WithAssoc holds. That would be an error, since we know that the goal only holds when 'a = 'static. But we don't check it.

I don't expect this to get solved any time soon. The new trait solver side-steps this ICE, but it doesn't "fix" the issue necessarily as fn hrtb ideally should error.

[matthiaskrgr]

this seems to crash since nightly-2022-07-16 , before:

error: implementation of `WithAssoc` is not general enough
  --> /tmp/crash.rs:21:1
   |
21 | fn hrtb<T: for<'a> Trait<&'a ()>>() {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `WithAssoc` is not general enough
   |
   = note: `&'0 ()` must implement `WithAssoc`, for any lifetime `'0`...
   = note: ...but `WithAssoc` is actually implemented for the type `&'static ()`

error: aborting due to previous error

commit[0] 2022-07-14: Auto merge of #99252 - lqd:win-dwarf5, r=eddyb
commit[1] 2022-07-14: Auto merge of #96544 - m-ysk:feature/issue-96358, r=cjgillot
commit[2] 2022-07-15: Auto merge of #98203 - kckeiks:gather-body-owners-in-hir-item-queries, r=cjgillot
commit[3] 2022-07-15: Auto merge of #97406 - aliemjay:verify-bounds-fix-master, r=oli-obk
commit[4] 2022-07-15: Auto merge of #99267 - Dylan-DPC:rollup-d8l7y1o, r=Dylan-DPC
commit[5] 2022-07-15: Auto merge of #99013 - RalfJung:dont-poison-my-places, r=oli-obk
commit[6] 2022-07-15: Auto merge of #99278 - Dylan-DPC:rollup-fcln6st, r=Dylan-DPC
commit[7] 2022-07-15: Auto merge of #99046 - nnethercote:final-derive-output-improvements, r=Mark-Simulacrum
commit[8] 2022-07-15: Auto merge of #99015 - lcnr:fn-ctxt-no-more, r=compiler-errors