ICE upcasting or calling a supertrait method, for trait object with unfulfilled bound on generic argument

[steffahn]

Original/first issue variant (using trait_upcasting)

This (first) example for an ICE is broken out from the review of #135318, the PR after which it’s first appearing:

#![feature(trait_upcasting)]

trait Supertrait<T> {
    fn method(&self) {}
}
impl<T> Supertrait<T> for () {}

trait WithAssoc {
    type Assoc;
}
trait Trait<P: WithAssoc>: Supertrait<P::Assoc> + Supertrait<()> {}

fn upcast<P>(x: &dyn Trait<P>) -> &dyn Supertrait<()> {
    x
}

fn main() {
    println!("{:p}", upcast::<()> as *const ());
}

(playground)

The relevant subtle design/implementation point about trait objects is that upcasting them is (apparently) supported fully generically, for any &dyn Trait<P> type, even when P: WithAssoc isn’t met, even though Trait itself as a trait does require P: WithAssoc. The new vtable-deduplication logic then wants to determine whether or not Supertrait<P::Assoc> and Supertrait<()> are the same&deduplicated; and the possibility that P::Assoc might be impossible to normalize during monomorphization never handled: with P = () above, which doesn’t implement WithAssoc at all, <() as WithAssoc>::Assoc can’t be normalized.

---

Expected behavior: builds successfully; this was also the behavior on nightly until #135318 was merged

Actual behavior (since #135318 was merged):

error: internal compiler error: compiler/rustc_middle/src/ty/normalize_erasing_regions.rs:169:13: Failed to normalize Alias(Projection, AliasTy { args: [()], def_id: DefId(0:9 ~ playground[2f00]::WithAssoc::Assoc), .. }) in typing_env=TypingEnv { typing_mode: PostAnalysis, param_env: ParamEnv { caller_bounds: [] } }, maybe try to call `try_normalize_erasing_regions` instead

(backtrace…)

thread 'rustc' panicked at compiler/rustc_middle/src/ty/normalize_erasing_regions.rs:169:13:
Box<dyn Any>
stack backtrace:
   0:     0x7aac496307e0 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::ha7a887e7d71c6e16
   1:     0x7aac49e147e6 - core::fmt::write::h8bce7c34d32c16d5
   2:     0x7aac4b0f8491 - std::io::Write::write_fmt::h61588d2e153c1fe1
   3:     0x7aac49630642 - std::sys::backtrace::BacktraceLock::print::h5e897e4367a4390b
   4:     0x7aac49632ac2 - std::panicking::default_hook::{{closure}}::hfb627b30254cb12b
   5:     0x7aac4963294a - std::panicking::default_hook::hc4630489198bafcd
   6:     0x7aac4878cf89 - std[a1eb87e154533a0e]::panicking::update_hook::<alloc[ad594d350f83b1d5]::boxed::Box<rustc_driver_impl[a791c9c5bd8cfc65]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x7aac49633603 - std::panicking::rust_panic_with_hook::h5ee98a90e6655b2f
   8:     0x7aac487c7c71 - std[a1eb87e154533a0e]::panicking::begin_panic::<rustc_errors[4c730af126acd4ca]::ExplicitBug>::{closure#0}
   9:     0x7aac487bcbe6 - std[a1eb87e154533a0e]::sys::backtrace::__rust_end_short_backtrace::<std[a1eb87e154533a0e]::panicking::begin_panic<rustc_errors[4c730af126acd4ca]::ExplicitBug>::{closure#0}, !>
  10:     0x7aac487bc99f - std[a1eb87e154533a0e]::panicking::begin_panic::<rustc_errors[4c730af126acd4ca]::ExplicitBug>
  11:     0x7aac487d1c11 - <rustc_errors[4c730af126acd4ca]::diagnostic::BugAbort as rustc_errors[4c730af126acd4ca]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  12:     0x7aac48daf9a3 - rustc_middle[99a44bd93e00f3c6]::util::bug::opt_span_bug_fmt::<rustc_span[a22240cbbe35b408]::span_encoding::Span>::{closure#0}
  13:     0x7aac48d9859a - rustc_middle[99a44bd93e00f3c6]::ty::context::tls::with_opt::<rustc_middle[99a44bd93e00f3c6]::util::bug::opt_span_bug_fmt<rustc_span[a22240cbbe35b408]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  14:     0x7aac48d9842b - rustc_middle[99a44bd93e00f3c6]::ty::context::tls::with_context_opt::<rustc_middle[99a44bd93e00f3c6]::ty::context::tls::with_opt<rustc_middle[99a44bd93e00f3c6]::util::bug::opt_span_bug_fmt<rustc_span[a22240cbbe35b408]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  15:     0x7aac46f522b0 - rustc_middle[99a44bd93e00f3c6]::util::bug::bug_fmt
  16:     0x7aac4a3cdbb4 - <rustc_middle[99a44bd93e00f3c6]::ty::normalize_erasing_regions::NormalizeAfterErasingRegionsFolder as rustc_type_ir[5dd542ab83ff2458]::fold::TypeFolder<rustc_middle[99a44bd93e00f3c6]::ty::context::TyCtxt>>::fold_ty
  17:     0x7aac493c5164 - <&rustc_middle[99a44bd93e00f3c6]::ty::list::RawList<(), rustc_middle[99a44bd93e00f3c6]::ty::generic_args::GenericArg> as rustc_type_ir[5dd542ab83ff2458]::fold::TypeFoldable<rustc_middle[99a44bd93e00f3c6]::ty::context::TyCtxt>>::try_fold_with::<rustc_middle[99a44bd93e00f3c6]::ty::normalize_erasing_regions::NormalizeAfterErasingRegionsFolder>
  18:     0x7aac4b9569af - <rustc_middle[99a44bd93e00f3c6]::ty::context::TyCtxt>::normalize_erasing_late_bound_regions::<rustc_type_ir[5dd542ab83ff2458]::predicate::TraitPredicate<rustc_middle[99a44bd93e00f3c6]::ty::context::TyCtxt>>.cold
  19:     0x7aac494d09dd - rustc_trait_selection[326587cd639b8c1b]::traits::vtable::supertrait_vtable_slot
  20:     0x7aac4917d766 - rustc_query_impl[7662b9ec157989e2]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[7662b9ec157989e2]::query_impl::supertrait_vtable_slot::dynamic_query::{closure#2}::{closure#0}, rustc_middle[99a44bd93e00f3c6]::query::erase::Erased<[u8; 16usize]>>
  21:     0x7aac49161dc9 - <rustc_query_impl[7662b9ec157989e2]::query_impl::supertrait_vtable_slot::dynamic_query::{closure#2} as core[4442fbc2d924c325]::ops::function::FnOnce<(rustc_middle[99a44bd93e00f3c6]::ty::context::TyCtxt, (rustc_middle[99a44bd93e00f3c6]::ty::Ty, rustc_middle[99a44bd93e00f3c6]::ty::Ty))>>::call_once
  22:     0x7aac490d0ce0 - rustc_query_system[68979f2bdbd6f64a]::query::plumbing::try_execute_query::<rustc_query_impl[7662b9ec157989e2]::DynamicConfig<rustc_query_system[68979f2bdbd6f64a]::query::caches::DefaultCache<(rustc_middle[99a44bd93e00f3c6]::ty::Ty, rustc_middle[99a44bd93e00f3c6]::ty::Ty), rustc_middle[99a44bd93e00f3c6]::query::erase::Erased<[u8; 16usize]>>, false, false, false>, rustc_query_impl[7662b9ec157989e2]::plumbing::QueryCtxt, false>
  23:     0x7aac4919c291 - rustc_query_impl[7662b9ec157989e2]::query_impl::supertrait_vtable_slot::get_query_non_incr::__rust_end_short_backtrace
  24:     0x7aac4a328991 - rustc_codegen_ssa[eabe536e63931777]::base::unsized_info::<rustc_codegen_llvm[a131874fd9aa18eb]::builder::GenericBuilder<rustc_codegen_llvm[a131874fd9aa18eb]::context::CodegenCx>>
  25:     0x7aac4a327d18 - rustc_codegen_ssa[eabe536e63931777]::base::unsize_ptr::<rustc_codegen_llvm[a131874fd9aa18eb]::builder::GenericBuilder<rustc_codegen_llvm[a131874fd9aa18eb]::context::CodegenCx>>
  26:     0x7aac4a3029c6 - <rustc_codegen_ssa[eabe536e63931777]::mir::FunctionCx<rustc_codegen_llvm[a131874fd9aa18eb]::builder::GenericBuilder<rustc_codegen_llvm[a131874fd9aa18eb]::context::CodegenCx>>>::codegen_rvalue_operand
  27:     0x7aac4a95d8c1 - rustc_codegen_ssa[eabe536e63931777]::mir::codegen_mir::<rustc_codegen_llvm[a131874fd9aa18eb]::builder::GenericBuilder<rustc_codegen_llvm[a131874fd9aa18eb]::context::CodegenCx>>
  28:     0x7aac4a9080bf - rustc_codegen_llvm[a131874fd9aa18eb]::base::compile_codegen_unit::module_codegen
  29:     0x7aac4adf3588 - <rustc_codegen_llvm[a131874fd9aa18eb]::LlvmCodegenBackend as rustc_codegen_ssa[eabe536e63931777]::traits::backend::ExtraBackendMethods>::compile_codegen_unit
  30:     0x7aac4adef817 - rustc_codegen_ssa[eabe536e63931777]::base::codegen_crate::<rustc_codegen_llvm[a131874fd9aa18eb]::LlvmCodegenBackend>
  31:     0x7aac4adeebaf - <rustc_codegen_llvm[a131874fd9aa18eb]::LlvmCodegenBackend as rustc_codegen_ssa[eabe536e63931777]::traits::backend::CodegenBackend>::codegen_crate
  32:     0x7aac4adf65f4 - <rustc_interface[71b698d67f628f7f]::queries::Linker>::codegen_and_build_linker
  33:     0x7aac4ad9c69f - rustc_interface[71b698d67f628f7f]::passes::create_and_enter_global_ctxt::<core[4442fbc2d924c325]::option::Option<rustc_interface[71b698d67f628f7f]::queries::Linker>, rustc_driver_impl[a791c9c5bd8cfc65]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  34:     0x7aac4ad7bcb3 - rustc_interface[71b698d67f628f7f]::interface::run_compiler::<(), rustc_driver_impl[a791c9c5bd8cfc65]::run_compiler::{closure#0}>::{closure#1}
  35:     0x7aac4ace8bb5 - std[a1eb87e154533a0e]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[71b698d67f628f7f]::util::run_in_thread_with_globals<rustc_interface[71b698d67f628f7f]::util::run_in_thread_pool_with_globals<rustc_interface[71b698d67f628f7f]::interface::run_compiler<(), rustc_driver_impl[a791c9c5bd8cfc65]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  36:     0x7aac4ace8899 - <<std[a1eb87e154533a0e]::thread::Builder>::spawn_unchecked_<rustc_interface[71b698d67f628f7f]::util::run_in_thread_with_globals<rustc_interface[71b698d67f628f7f]::util::run_in_thread_pool_with_globals<rustc_interface[71b698d67f628f7f]::interface::run_compiler<(), rustc_driver_impl[a791c9c5bd8cfc65]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[4442fbc2d924c325]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  37:     0x7aac4ace802b - std::sys::pal::unix::thread::Thread::new::thread_start::h83cea321e92f80f9
  38:     0x7aac4c5a0a94 - <unknown>
  39:     0x7aac4c62da34 - clone
  40:                0x0 - <unknown>

@compiler-errors had already opened a PR with a possible fix, and it includes (a variant of) this test case.

• Ensure that we never try to monomorphize the upcasting or vtable calls of impossible dyn types #136311

@rustbot label A-trait-objects F-trait_upcasting
(deliberately no “requires-nightly” label, see next section)

---

Second issue variant (regression without unstable features)

Looking at the PR, it seems @compiler-errors has also identified that this ICEs from a method call, not just upcasting (as what the original repro did):

trait Supertrait<T> {
    fn method(&self) {}
}
impl<T> Supertrait<T> for () {}

trait WithAssoc {
    type Assoc;
}
trait Trait<P: WithAssoc>: Supertrait<P::Assoc> + Supertrait<()> {}

fn call<P>(x: &dyn Trait<P>) {
    x.method();
}

fn main() {
    println!("{:p}", call::<()> as *const ());
}

(playground)

(behavior: same as above, w.r.t expected behavior, regression point, and [similar] ICE message)

error: internal compiler error: compiler/rustc_middle/src/ty/normalize_erasing_regions.rs:169:13: Failed to normalize Alias(Projection, AliasTy { args: [()], def_id: DefId(0:9 ~ playground[2f00]::WithAssoc::Assoc), .. }) in typing_env=TypingEnv { typing_mode: PostAnalysis, param_env: ParamEnv { caller_bounds: [] } }, maybe try to call `try_normalize_erasing_regions` instead

(backtrace…)

thread 'rustc' panicked at compiler/rustc_middle/src/ty/normalize_erasing_regions.rs:169:13:
Box<dyn Any>
stack backtrace:
   0:     0x751dd00307e0 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::ha7a887e7d71c6e16
   1:     0x751dd08147e6 - core::fmt::write::h8bce7c34d32c16d5
   2:     0x751dd1af8491 - std::io::Write::write_fmt::h61588d2e153c1fe1
   3:     0x751dd0030642 - std::sys::backtrace::BacktraceLock::print::h5e897e4367a4390b
   4:     0x751dd0032ac2 - std::panicking::default_hook::{{closure}}::hfb627b30254cb12b
   5:     0x751dd003294a - std::panicking::default_hook::hc4630489198bafcd
   6:     0x751dcf18cf89 - std[a1eb87e154533a0e]::panicking::update_hook::<alloc[ad594d350f83b1d5]::boxed::Box<rustc_driver_impl[a791c9c5bd8cfc65]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x751dd0033603 - std::panicking::rust_panic_with_hook::h5ee98a90e6655b2f
   8:     0x751dcf1c7c71 - std[a1eb87e154533a0e]::panicking::begin_panic::<rustc_errors[4c730af126acd4ca]::ExplicitBug>::{closure#0}
   9:     0x751dcf1bcbe6 - std[a1eb87e154533a0e]::sys::backtrace::__rust_end_short_backtrace::<std[a1eb87e154533a0e]::panicking::begin_panic<rustc_errors[4c730af126acd4ca]::ExplicitBug>::{closure#0}, !>
  10:     0x751dcf1bc99f - std[a1eb87e154533a0e]::panicking::begin_panic::<rustc_errors[4c730af126acd4ca]::ExplicitBug>
  11:     0x751dcf1d1c11 - <rustc_errors[4c730af126acd4ca]::diagnostic::BugAbort as rustc_errors[4c730af126acd4ca]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  12:     0x751dcf7af9a3 - rustc_middle[99a44bd93e00f3c6]::util::bug::opt_span_bug_fmt::<rustc_span[a22240cbbe35b408]::span_encoding::Span>::{closure#0}
  13:     0x751dcf79859a - rustc_middle[99a44bd93e00f3c6]::ty::context::tls::with_opt::<rustc_middle[99a44bd93e00f3c6]::util::bug::opt_span_bug_fmt<rustc_span[a22240cbbe35b408]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  14:     0x751dcf79842b - rustc_middle[99a44bd93e00f3c6]::ty::context::tls::with_context_opt::<rustc_middle[99a44bd93e00f3c6]::ty::context::tls::with_opt<rustc_middle[99a44bd93e00f3c6]::util::bug::opt_span_bug_fmt<rustc_span[a22240cbbe35b408]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  15:     0x751dcd9522b0 - rustc_middle[99a44bd93e00f3c6]::util::bug::bug_fmt
  16:     0x751dd0dcdbb4 - <rustc_middle[99a44bd93e00f3c6]::ty::normalize_erasing_regions::NormalizeAfterErasingRegionsFolder as rustc_type_ir[5dd542ab83ff2458]::fold::TypeFolder<rustc_middle[99a44bd93e00f3c6]::ty::context::TyCtxt>>::fold_ty
  17:     0x751dcfdc5164 - <&rustc_middle[99a44bd93e00f3c6]::ty::list::RawList<(), rustc_middle[99a44bd93e00f3c6]::ty::generic_args::GenericArg> as rustc_type_ir[5dd542ab83ff2458]::fold::TypeFoldable<rustc_middle[99a44bd93e00f3c6]::ty::context::TyCtxt>>::try_fold_with::<rustc_middle[99a44bd93e00f3c6]::ty::normalize_erasing_regions::NormalizeAfterErasingRegionsFolder>
  18:     0x751dd23569af - <rustc_middle[99a44bd93e00f3c6]::ty::context::TyCtxt>::normalize_erasing_late_bound_regions::<rustc_type_ir[5dd542ab83ff2458]::predicate::TraitPredicate<rustc_middle[99a44bd93e00f3c6]::ty::context::TyCtxt>>.cold
  19:     0x751dd15e56c4 - rustc_trait_selection[326587cd639b8c1b]::traits::vtable::first_method_vtable_slot
  20:     0x751dd15e52f6 - rustc_query_impl[7662b9ec157989e2]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[7662b9ec157989e2]::query_impl::first_method_vtable_slot::dynamic_query::{closure#2}::{closure#0}, rustc_middle[99a44bd93e00f3c6]::query::erase::Erased<[u8; 8usize]>>
  21:     0x751dd15e52db - <rustc_query_impl[7662b9ec157989e2]::query_impl::first_method_vtable_slot::dynamic_query::{closure#2} as core[4442fbc2d924c325]::ops::function::FnOnce<(rustc_middle[99a44bd93e00f3c6]::ty::context::TyCtxt, rustc_type_ir[5dd542ab83ff2458]::predicate::TraitRef<rustc_middle[99a44bd93e00f3c6]::ty::context::TyCtxt>)>>::call_once
  22:     0x751dd15ebacf - rustc_query_system[68979f2bdbd6f64a]::query::plumbing::try_execute_query::<rustc_query_impl[7662b9ec157989e2]::DynamicConfig<rustc_query_system[68979f2bdbd6f64a]::query::caches::DefaultCache<rustc_type_ir[5dd542ab83ff2458]::predicate::TraitRef<rustc_middle[99a44bd93e00f3c6]::ty::context::TyCtxt>, rustc_middle[99a44bd93e00f3c6]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[7662b9ec157989e2]::plumbing::QueryCtxt, false>
  23:     0x751dd15eb7ec - rustc_query_impl[7662b9ec157989e2]::query_impl::first_method_vtable_slot::get_query_non_incr::__rust_end_short_backtrace
  24:     0x751dce0504ad - rustc_ty_utils[6f69adb5a450b42]::instance::resolve_instance_raw
  25:     0x751dd0cdc8f2 - rustc_query_impl[7662b9ec157989e2]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[7662b9ec157989e2]::query_impl::resolve_instance_raw::dynamic_query::{closure#2}::{closure#0}, rustc_middle[99a44bd93e00f3c6]::query::erase::Erased<[u8; 32usize]>>
  26:     0x751dd0cdccae - rustc_query_system[68979f2bdbd6f64a]::query::plumbing::try_execute_query::<rustc_query_impl[7662b9ec157989e2]::DynamicConfig<rustc_query_system[68979f2bdbd6f64a]::query::caches::DefaultCache<rustc_middle[99a44bd93e00f3c6]::ty::PseudoCanonicalInput<(rustc_span[a22240cbbe35b408]::def_id::DefId, &rustc_middle[99a44bd93e00f3c6]::ty::list::RawList<(), rustc_middle[99a44bd93e00f3c6]::ty::generic_args::GenericArg>)>, rustc_middle[99a44bd93e00f3c6]::query::erase::Erased<[u8; 32usize]>>, false, false, false>, rustc_query_impl[7662b9ec157989e2]::plumbing::QueryCtxt, false>
  27:     0x751dd0cdc86e - rustc_query_impl[7662b9ec157989e2]::query_impl::resolve_instance_raw::get_query_non_incr::__rust_end_short_backtrace
  28:     0x751dd0cda903 - <rustc_middle[99a44bd93e00f3c6]::ty::instance::Instance>::expect_resolve
  29:     0x751dcd4aea0d - rustc_monomorphize[86e20e627c2db3b3]::collector::items_of_instance
  30:     0x751dd131e090 - rustc_query_impl[7662b9ec157989e2]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[7662b9ec157989e2]::query_impl::items_of_instance::dynamic_query::{closure#2}::{closure#0}, rustc_middle[99a44bd93e00f3c6]::query::erase::Erased<[u8; 32usize]>>
  31:     0x751dd131d8f4 - rustc_query_system[68979f2bdbd6f64a]::query::plumbing::try_execute_query::<rustc_query_impl[7662b9ec157989e2]::DynamicConfig<rustc_query_system[68979f2bdbd6f64a]::query::caches::DefaultCache<(rustc_middle[99a44bd93e00f3c6]::ty::instance::Instance, rustc_middle[99a44bd93e00f3c6]::mir::mono::CollectionMode), rustc_middle[99a44bd93e00f3c6]::query::erase::Erased<[u8; 32usize]>>, false, false, false>, rustc_query_impl[7662b9ec157989e2]::plumbing::QueryCtxt, false>
  32:     0x751dd131d52c - rustc_query_impl[7662b9ec157989e2]::query_impl::items_of_instance::get_query_non_incr::__rust_end_short_backtrace
  33:     0x751dd1319f44 - rustc_monomorphize[86e20e627c2db3b3]::collector::collect_items_rec::{closure#0}
  34:     0x751dcd495516 - rustc_monomorphize[86e20e627c2db3b3]::collector::collect_items_rec
  35:     0x751dcd495e7a - rustc_monomorphize[86e20e627c2db3b3]::collector::collect_items_rec
  36:     0x751dcda449bc - rustc_monomorphize[86e20e627c2db3b3]::partitioning::collect_and_partition_mono_items
  37:     0x751dd17d3a16 - rustc_query_impl[7662b9ec157989e2]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[7662b9ec157989e2]::query_impl::collect_and_partition_mono_items::dynamic_query::{closure#2}::{closure#0}, rustc_middle[99a44bd93e00f3c6]::query::erase::Erased<[u8; 40usize]>>
  38:     0x751dd17d39ef - <rustc_query_impl[7662b9ec157989e2]::query_impl::collect_and_partition_mono_items::dynamic_query::{closure#2} as core[4442fbc2d924c325]::ops::function::FnOnce<(rustc_middle[99a44bd93e00f3c6]::ty::context::TyCtxt, ())>>::call_once
  39:     0x751dd17d355a - rustc_query_system[68979f2bdbd6f64a]::query::plumbing::try_execute_query::<rustc_query_impl[7662b9ec157989e2]::DynamicConfig<rustc_query_system[68979f2bdbd6f64a]::query::caches::SingleCache<rustc_middle[99a44bd93e00f3c6]::query::erase::Erased<[u8; 40usize]>>, false, false, false>, rustc_query_impl[7662b9ec157989e2]::plumbing::QueryCtxt, false>
  40:     0x751dd17d32fc - rustc_query_impl[7662b9ec157989e2]::query_impl::collect_and_partition_mono_items::get_query_non_incr::__rust_end_short_backtrace
  41:     0x751dd17ef10a - rustc_codegen_ssa[eabe536e63931777]::base::codegen_crate::<rustc_codegen_llvm[a131874fd9aa18eb]::LlvmCodegenBackend>
  42:     0x751dd17eebaf - <rustc_codegen_llvm[a131874fd9aa18eb]::LlvmCodegenBackend as rustc_codegen_ssa[eabe536e63931777]::traits::backend::CodegenBackend>::codegen_crate
  43:     0x751dd17f65f4 - <rustc_interface[71b698d67f628f7f]::queries::Linker>::codegen_and_build_linker
  44:     0x751dd179c69f - rustc_interface[71b698d67f628f7f]::passes::create_and_enter_global_ctxt::<core[4442fbc2d924c325]::option::Option<rustc_interface[71b698d67f628f7f]::queries::Linker>, rustc_driver_impl[a791c9c5bd8cfc65]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  45:     0x751dd177bcb3 - rustc_interface[71b698d67f628f7f]::interface::run_compiler::<(), rustc_driver_impl[a791c9c5bd8cfc65]::run_compiler::{closure#0}>::{closure#1}
  46:     0x751dd16e8bb5 - std[a1eb87e154533a0e]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[71b698d67f628f7f]::util::run_in_thread_with_globals<rustc_interface[71b698d67f628f7f]::util::run_in_thread_pool_with_globals<rustc_interface[71b698d67f628f7f]::interface::run_compiler<(), rustc_driver_impl[a791c9c5bd8cfc65]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  47:     0x751dd16e8899 - <<std[a1eb87e154533a0e]::thread::Builder>::spawn_unchecked_<rustc_interface[71b698d67f628f7f]::util::run_in_thread_with_globals<rustc_interface[71b698d67f628f7f]::util::run_in_thread_pool_with_globals<rustc_interface[71b698d67f628f7f]::interface::run_compiler<(), rustc_driver_impl[a791c9c5bd8cfc65]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[4442fbc2d924c325]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  48:     0x751dd16e802b - std::sys::pal::unix::thread::Thread::new::thread_start::h83cea321e92f80f9
  49:     0x751dd2ecfa94 - <unknown>
  50:     0x751dd2f5ca34 - clone
  51:                0x0 - <unknown>

This means we do have an ICE regression on nightly without unstable features, which I don’t see tracked anywhere else yet (let's track this so it can't be forgotten, especially with the next beta cut coming up soon-ish).

The relevant subtle design/implementation point about trait objects is that a &dyn Trait<P> trait object will still be considered to implement all its supertraits, including Supertrait<()> in particular (so you can call its supertrait methods), even when P: WithAssoc doesn’t hold. For illustration, this variant of fn call also compiles:

fn call<P>(x: &dyn Trait<P>) {
    trt_check(x);
}

fn trt_check(x: &(impl Supertrait<()> + ?Sized)) {
    x.method();
}

(and it can be used in the same way to now ICE)

@rustbot label regression-from-stable-to-nightly

[compiler-errors]

I don't expect this to exist in practice and they both should be fixed by the PR I put up

[steffahn]

Closing with #136311 merged