ICE when equating TAIT and RPIT in canonical queries

[aliemjay]

Code

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=ec86c1b56e1fe08951cf0dbb67ea7d7b

#![feature(type_alias_impl_trait)]

type Opaque = impl Sized;

fn get_rpit() -> impl Clone {}

fn query(_: impl FnOnce() -> Opaque) {}

fn test() -> Opaque {
    query(get_rpit);
    get_rpit()
}

While trying to prove the predicate <get_rpit as FnOnce>::Output == Opaque, we end up equating get_rpit::opaque#0 == Opaque::opaque#0. Because inside canonical queries we know nothing about the defining scope (DefiningAnchor::Bubble), we register the hidden type as get_rpit::opaque := Opaque::opaque instead of Opaque::opaque := get_rpit::opaque.

Cc @oli-obk.

Meta

Nightly version: 1.69.0-nightly (2023-02-25 34e6673)

Error output

Backtrace

warning: type alias `Opaque` is never used
 --> src/lib.rs:3:6
  |
3 | type Opaque = impl Sized;
  |      ^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: function `get_rpit` is never used
 --> src/lib.rs:5:4
  |
5 | fn get_rpit() -> impl Clone {}
  |    ^^^^^^^^

warning: function `query` is never used
 --> src/lib.rs:7:4
  |
7 | fn query(_: impl FnOnce() -> Opaque) {}
  |    ^^^^^

warning: function `test` is never used
 --> src/lib.rs:9:4
  |
9 | fn test() -> Opaque {
  |    ^^^^

error: internal compiler error: no errors encountered even though `delay_span_bug` issued

error: internal compiler error: VecMap([(OpaqueTypeKey { def_id: DefId(0:8 ~ playground[4e8e]::get_rpit::{opaque#0}), substs: [] }, OpaqueTypeDecl { hidden_type: OpaqueHiddenType { span: no-location (#0), ty: Opaque }, origin: FnReturn(DefId(0:4 ~ playground[4e8e]::get_rpit)) })])
  |
  = note: delayed at    0: <rustc_errors::HandlerInner>::emit_diagnostic
             1: <rustc_errors::Handler>::delay_span_bug::<rustc_span::span_encoding::Span, &alloc::string::String>
             2: core::ptr::drop_in_place::<rustc_infer::infer::InferCtxt>
             3: <rustc_infer::infer::InferCtxtBuilder as rustc_trait_selection::infer::InferCtxtBuilderExt>::enter_canonical_trait_query::<rustc_middle::ty::ParamEnvAnd<rustc_middle::traits::query::type_op::ProvePredicate>, (), rustc_traits::type_op::type_op_prove_predicate::{closure#0}>
             4: rustc_query_system::query::plumbing::get_query::<rustc_query_impl::queries::type_op_prove_predicate, rustc_query_impl::plumbing::QueryCtxt, rustc_middle::dep_graph::dep_node::DepKind>
             5: <rustc_middle::ty::ParamEnvAnd<rustc_middle::traits::query::type_op::ProvePredicate> as rustc_trait_selection::traits::query::type_op::TypeOp>::fully_perform
             6: <rustc_borrowck::type_check::TypeChecker>::normalize_and_prove_instantiated_predicates
             7: <rustc_borrowck::type_check::TypeVerifier as rustc_middle::mir::visit::Visitor>::visit_constant
             8: <rustc_borrowck::type_check::TypeVerifier as rustc_middle::mir::visit::Visitor>::visit_body
             9: rustc_borrowck::type_check::type_check
            10: rustc_borrowck::nll::compute_regions
            11: rustc_borrowck::do_mir_borrowck
            12: rustc_borrowck::mir_borrowck
            13: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::queries::mir_borrowck, rustc_query_impl::plumbing::QueryCtxt>
            14: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::mir_borrowck
            15: <rustc_hir_analysis::collect::type_of::find_opaque_ty_constraints_for_tait::ConstraintLocator>::check
            16: <rustc_hir_analysis::collect::type_of::find_opaque_ty_constraints_for_tait::ConstraintLocator as rustc_hir::intravisit::Visitor>::visit_nested_item
            17: rustc_hir::intravisit::walk_mod::<rustc_hir_analysis::collect::type_of::find_opaque_ty_constraints_for_tait::ConstraintLocator>
            18: rustc_hir_analysis::collect::type_of::find_opaque_ty_constraints_for_tait
            19: rustc_hir_analysis::collect::type_of::type_of
            20: rustc_query_system::query::plumbing::get_query::<rustc_query_impl::queries::type_of, rustc_query_impl::plumbing::QueryCtxt, rustc_middle::dep_graph::dep_node::DepKind>
            21: rustc_hir_analysis::check::check::check_mod_item_types
            22: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::queries::check_mod_item_types, rustc_query_impl::plumbing::QueryCtxt>
            23: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::check_mod_item_types
            24: <rustc_middle::hir::map::Map>::for_each_module::<rustc_hir_analysis::check_crate::{closure#6}::{closure#0}>
            25: rustc_hir_analysis::check_crate
            26: rustc_interface::passes::analysis
            27: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::queries::analysis, rustc_query_impl::plumbing::QueryCtxt>
            28: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::analysis
            29: <rustc_middle::ty::context::GlobalCtxt>::enter::<rustc_driver_impl::run_compiler::{closure#1}::{closure#2}::{closure#4}, core::result::Result<(), rustc_errors::ErrorGuaranteed>>
            30: <rustc_interface::interface::Compiler>::enter::<rustc_driver_impl::run_compiler::{closure#1}::{closure#2}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_errors::ErrorGuaranteed>>
            31: rustc_span::with_source_map::<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_interface::interface::run_compiler<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#1}>::{closure#0}::{closure#0}>
            32: std::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#1}>::{closure#0}, core::result::Result<(), rustc_errors::ErrorGuaranteed>>::{closure#0}::{closure#0}, core::result::Result<(), rustc_errors::ErrorGuaranteed>>
            33: <<std::thread::Builder>::spawn_unchecked_<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#1}>::{closure#0}, core::result::Result<(), rustc_errors::ErrorGuaranteed>>::{closure#0}::{closure#0}, core::result::Result<(), rustc_errors::ErrorGuaranteed>>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
            34: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
                       at /rustc/34e6673a0473e90ef01a18eb575392c9e3859747/library/alloc/src/boxed.rs:1988:9
            35: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
                       at /rustc/34e6673a0473e90ef01a18eb575392c9e3859747/library/alloc/src/boxed.rs:1988:9
            36: std::sys::unix::thread::Thread::new::thread_start
                       at /rustc/34e6673a0473e90ef01a18eb575392c9e3859747/library/std/src/sys/unix/thread.rs:108:17
            37: start_thread
            38: clone
          

error: internal compiler error: broken MIR in DefId(0:6 ~ playground[4e8e]::test) (NoSolution): could not prove Binder(ProjectionPredicate(AliasTy { substs: [fn() -> impl std::clone::Clone {get_rpit}, ()], def_id: DefId(2:2923 ~ core[df15]::ops::function::FnOnce::Output) }, Term::Ty(Opaque)), [])
  |
  = note: delayed at    0: <rustc_errors::HandlerInner>::emit_diagnostic
             1: <rustc_errors::Handler>::delay_span_bug::<rustc_span::span_encoding::Span, &str>
             2: <rustc_borrowck::type_check::TypeChecker>::normalize_and_prove_instantiated_predicates
             3: <rustc_borrowck::type_check::TypeVerifier as rustc_middle::mir::visit::Visitor>::visit_constant
             4: <rustc_borrowck::type_check::TypeVerifier as rustc_middle::mir::visit::Visitor>::visit_body
             5: rustc_borrowck::type_check::type_check
             6: rustc_borrowck::nll::compute_regions
             7: rustc_borrowck::do_mir_borrowck
             8: rustc_borrowck::mir_borrowck
             9: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::queries::mir_borrowck, rustc_query_impl::plumbing::QueryCtxt>
            10: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::mir_borrowck
            11: <rustc_hir_analysis::collect::type_of::find_opaque_ty_constraints_for_tait::ConstraintLocator>::check
            12: <rustc_hir_analysis::collect::type_of::find_opaque_ty_constraints_for_tait::ConstraintLocator as rustc_hir::intravisit::Visitor>::visit_nested_item
            13: rustc_hir::intravisit::walk_mod::<rustc_hir_analysis::collect::type_of::find_opaque_ty_constraints_for_tait::ConstraintLocator>
            14: rustc_hir_analysis::collect::type_of::find_opaque_ty_constraints_for_tait
            15: rustc_hir_analysis::collect::type_of::type_of
            16: rustc_query_system::query::plumbing::get_query::<rustc_query_impl::queries::type_of, rustc_query_impl::plumbing::QueryCtxt, rustc_middle::dep_graph::dep_node::DepKind>
            17: rustc_hir_analysis::check::check::check_mod_item_types
            18: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::queries::check_mod_item_types, rustc_query_impl::plumbing::QueryCtxt>
            19: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::check_mod_item_types
            20: <rustc_middle::hir::map::Map>::for_each_module::<rustc_hir_analysis::check_crate::{closure#6}::{closure#0}>
            21: rustc_hir_analysis::check_crate
            22: rustc_interface::passes::analysis
            23: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::queries::analysis, rustc_query_impl::plumbing::QueryCtxt>
            24: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::analysis
            25: <rustc_middle::ty::context::GlobalCtxt>::enter::<rustc_driver_impl::run_compiler::{closure#1}::{closure#2}::{closure#4}, core::result::Result<(), rustc_errors::ErrorGuaranteed>>
            26: <rustc_interface::interface::Compiler>::enter::<rustc_driver_impl::run_compiler::{closure#1}::{closure#2}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_errors::ErrorGuaranteed>>
            27: rustc_span::with_source_map::<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_interface::interface::run_compiler<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#1}>::{closure#0}::{closure#0}>
            28: std::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#1}>::{closure#0}, core::result::Result<(), rustc_errors::ErrorGuaranteed>>::{closure#0}::{closure#0}, core::result::Result<(), rustc_errors::ErrorGuaranteed>>
            29: <<std::thread::Builder>::spawn_unchecked_<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#1}>::{closure#0}, core::result::Result<(), rustc_errors::ErrorGuaranteed>>::{closure#0}::{closure#0}, core::result::Result<(), rustc_errors::ErrorGuaranteed>>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
            30: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
                       at /rustc/34e6673a0473e90ef01a18eb575392c9e3859747/library/alloc/src/boxed.rs:1988:9
            31: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
                       at /rustc/34e6673a0473e90ef01a18eb575392c9e3859747/library/alloc/src/boxed.rs:1988:9
            32: std::sys::unix::thread::Thread::new::thread_start
                       at /rustc/34e6673a0473e90ef01a18eb575392c9e3859747/library/std/src/sys/unix/thread.rs:108:17
            33: start_thread
            34: clone
          

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 1.69.0-nightly (34e6673a0 2023-02-25) running on x86_64-unknown-linux-gnu

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
warning: `playground` (lib) generated 4 warnings
error: could not compile `playground`; 4 warnings emitted

@rustbot label F-type_alias_impl_trait T-types

[compiler-errors]

This would require some more design and stuff -- but I wonder if, when we relate an opaque with another opaque in a DefiningAnchor::Bubble scope, we should emit some other sort of opaque type constraint, like OpaqueEq(LHS, RHS), that isn't just LHS := RHS. Then when we instantiate it in some other infcx, we'll choose whether LHS := RHS or RHS := LHS depending on what the defining anchor of that one is.

[oli-obk]

yea, that was always the plan, but it seemed too niche to put the effort in for now