ICE: no errors encountered even though delay_span_bug issued, expected ReFree to map to ReEarlyBound

[jswrenn]

Code

(Playground)

#![feature(associated_type_bounds, return_position_impl_trait_in_trait)]
#![allow(incomplete_features)]

trait Iterator {
    type Item;
}

trait IntoIterator {
    fn into_iterator(&self) -> impl Iterator<Item:> + '_;
}

struct IntoIterFn<F, I>
{
    f: F,
    _marker: core::marker::PhantomData<I>,
}

impl<F, I> IntoIterator for IntoIterFn<F, I>
where
    F: Fn() -> I,
    I: Iterator,
{
    fn into_iterator(&self) -> impl Iterator<Item: '_> {
        (self.f)()
    }
}

Meta

Version:

rustc 1.69.0-nightly (7281249a1 2023-02-27)

Error output

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

error: internal compiler error: expected ReFree to map to ReEarlyBound
  --> src/lib.rs:23:32
   |
23 |     fn into_iterator(&self) -> impl Iterator<Item: '_> {
   |                                ^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: delayed at    0: <rustc_errors::HandlerInner>::emit_diagnostic
              1: <rustc_session::session::Session>::delay_span_bug::<rustc_span::span_encoding::Span, &str>
              2: rustc_hir_analysis::check::compare_impl_item::collect_return_position_impl_trait_in_trait_tys::{closure#0}::{closure#2}
              3: rustc_middle::ty::util::fold_list::<rustc_middle::ty::fold::RegionFolder, rustc_middle::ty::subst::GenericArg, <&rustc_middle::ty::list::List<rustc_middle::ty::subst::GenericArg> as rustc_type_ir::fold::TypeFoldable<rustc_middle::ty::context::TyCtxt>>::try_fold_with<rustc_middle::ty::fold::RegionFolder>::{closure#0}>
              4: <rustc_middle::ty::Ty as rustc_type_ir::fold::TypeSuperFoldable<rustc_middle::ty::context::TyCtxt>>::super_fold_with::<rustc_middle::ty::fold::RegionFolder>
              5: <rustc_middle::ty::context::TyCtxt>::fold_regions::<rustc_middle::ty::Ty, rustc_hir_analysis::check::compare_impl_item::collect_return_position_impl_trait_in_trait_tys::{closure#0}::{closure#2}>
              6: rustc_hir_analysis::check::compare_impl_item::collect_return_position_impl_trait_in_trait_tys
              7: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::queries::collect_return_position_impl_trait_in_trait_tys, rustc_query_impl::plumbing::QueryCtxt>
              8: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::collect_return_position_impl_trait_in_trait_tys
              9: <rustc_middle::ty::context::TyCtxt>::bound_return_position_impl_trait_in_trait_tys
             10: rustc_trait_selection::traits::project::opt_normalize_projection_type
             11: <rustc_trait_selection::traits::project::AssocTypeNormalizer as rustc_type_ir::fold::TypeFolder<rustc_middle::ty::context::TyCtxt>>::fold_ty
             12: <&rustc_middle::ty::list::List<rustc_middle::ty::Ty> as rustc_type_ir::fold::TypeFoldable<rustc_middle::ty::context::TyCtxt>>::try_fold_with::<rustc_trait_selection::traits::project::AssocTypeNormalizer>
             13: <rustc_infer::infer::at::At as rustc_trait_selection::traits::project::NormalizeExt>::normalize::<rustc_middle::ty::sty::FnSig>
             14: rustc_hir_analysis::check::compare_impl_item::compare_method_predicate_entailment
             15: rustc_hir_analysis::check::compare_impl_item::compare_impl_method
             16: rustc_hir_analysis::check::check::check_impl_items_against_trait
             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_session::session::Session>::time::<(), rustc_hir_analysis::check_crate::{closure#6}>
             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_span::ErrorGuaranteed>>
             26: rustc_span::with_source_map::<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_interface::interface::run_compiler<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#1}>::{closure#0}::{closure#0}>
             27: 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_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#1}>::{closure#0}, core::result::Result<(), rustc_span::ErrorGuaranteed>>::{closure#0}::{closure#0}, core::result::Result<(), rustc_span::ErrorGuaranteed>>
             28: <<std::thread::Builder>::spawn_unchecked_<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#1}>::{closure#0}, core::result::Result<(), rustc_span::ErrorGuaranteed>>::{closure#0}::{closure#0}, core::result::Result<(), rustc_span::ErrorGuaranteed>>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
             29: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
                        at /rustc/7281249a19a9755e9d889ee251ec323629caadab/library/alloc/src/boxed.rs:1988:9
             30: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
                        at /rustc/7281249a19a9755e9d889ee251ec323629caadab/library/alloc/src/boxed.rs:1988:9
             31: std::sys::unix::thread::Thread::new::thread_start
                        at /rustc/7281249a19a9755e9d889ee251ec323629caadab/library/std/src/sys/unix/thread.rs:108:17
             32: start_thread
             33: 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 (7281249a1 2023-02-27) 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

[compiler-errors]

This is also not related to associated type bounds:

#![feature(return_position_impl_trait_in_trait)]

trait Foo {
    fn bar(&self) -> impl Iterator<Item = impl Sized> + '_;
}

impl Foo for () {
    fn bar(&self) -> impl Iterator + '_ {
        vec![()].into_iter()
    }
}

This was fixed by #105255, which was subsequently reverted in #106759. The fix here is to make opaque types inherit their parent's generics, which can probably be done if #106729 lands.

[compiler-errors]

Triage: This no longer iCEs, now it errors with the following message

error: return type captures more lifetimes than trait definition
 --> src/lib.rs:8:22
  |
8 |     fn bar(&self) -> impl Iterator + '_ {
  |            -         ^^^^^^^^^^^^^^^^^^
  |            |
  |            this lifetime was captured
  |
note: hidden type must only reference lifetimes captured by this impl trait
 --> src/lib.rs:4:43
  |
4 |     fn bar(&self) -> impl Iterator<Item = impl Sized> + '_;
  |                                           ^^^^^^^^^^
  = note: hidden type inferred to be `<impl Iterator + '_ as Iterator>::Item`