[ICE]: `it should not be possible to encounter unnormalized aliases in borrowck`

[matthiaskrgr]

Code

from tests/ui/coroutine/issue-58888.rs

//@ compile-flags: -Znext-solver=globally

#![feature(coroutines, coroutine_trait)]

use std::ops::Coroutine;

struct Database;

impl Database {
    fn get_connection(&self) -> impl Iterator/*<Item = ()>*/ {
        Some(()).into_iter()
    }

    fn check_connection(&self) -> impl Coroutine/*<Yield = (), Return = ()>*/  {
        #[coroutine]
        move || {
            let iter = self.get_connection();
            for i in iter {
                yield i
            }
        }
    }
}

fn main() {
    let _ = Database.check_connection();
}

Meta

rustc --version --verbose:

rustc 1.99.0-nightly (84b36a78a 2026-08-06)
binary: rustc
commit-hash: 84b36a78a28a63f134171c670be1932ffa2485f8
commit-date: 2026-08-06
host: x86_64-unknown-linux-gnu
release: 1.99.0-nightly
LLVM version: 23.1.0

Error output

<output>

Backtrace

error: internal compiler error: /rustc-dev/84b36a78a28a63f134171c670be1932ffa2485f8/compiler/rustc_borrowck/src/type_check/relate_tys.rs:389:17: it should not be possible to encounter unnormalized aliases in borrowck
  --> coroutine/issue-58888.rs:20:17
   |
20 |                 yield i
   |                 ^^^^^^^


thread 'rustc' (2706795) panicked at /rustc-dev/84b36a78a28a63f134171c670be1932ffa2485f8/compiler/rustc_borrowck/src/type_check/relate_tys.rs:389:17:
Box<dyn Any>
stack backtrace:
   0:     0x7fa1a52f2956 - <<std[6cb999a3dbb652d9]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[cdb7ffe4be53d62]::fmt::Display>::fmt
   1:     0x7fa1a5a1118f - core[cdb7ffe4be53d62]::fmt::write
   2:     0x7fa1a530787c - <std[6cb999a3dbb652d9]::sys::stdio::unix::Stderr as core[cdb7ffe4be53d62]::io::write::Write>::write_fmt
   3:     0x7fa1a52c573a - std[6cb999a3dbb652d9]::panicking::default_hook::{closure#0}
   4:     0x7fa1a52e64d3 - std[6cb999a3dbb652d9]::panicking::default_hook
   5:     0x7fa1a4241caa - std[6cb999a3dbb652d9]::panicking::update_hook::<alloc[1d52d4102791da9a]::boxed::Box<rustc_driver_impl[3bc4d727b6aa2dc4]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7fa1a52e6972 - std[6cb999a3dbb652d9]::panicking::panic_with_hook
   7:     0x7fa1a426f804 - std[6cb999a3dbb652d9]::panicking::begin_panic::<rustc_errors[ff393dc04c4b72ae]::ExplicitBug>::{closure#0}
   8:     0x7fa1a42683e6 - std[6cb999a3dbb652d9]::sys::backtrace::__rust_end_short_backtrace::<std[6cb999a3dbb652d9]::panicking::begin_panic<rustc_errors[ff393dc04c4b72ae]::ExplicitBug>::{closure#0}, !>
   9:     0x7fa1a426828b - std[6cb999a3dbb652d9]::panicking::begin_panic::<rustc_errors[ff393dc04c4b72ae]::ExplicitBug>
  10:     0x7fa1a427b0f1 - <rustc_errors[ff393dc04c4b72ae]::diagnostic::BugAbort as rustc_errors[ff393dc04c4b72ae]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  11:     0x7fa1a47f8cac - <rustc_errors[ff393dc04c4b72ae]::DiagCtxtHandle>::span_bug::<rustc_span[78ea2eb5514a41c4]::span_encoding::Span, alloc[1d52d4102791da9a]::string::String>
  12:     0x7fa1a48295c6 - rustc_middle[72de90f4b57d7b8b]::util::bug::opt_span_bug_fmt::<rustc_span[78ea2eb5514a41c4]::span_encoding::Span>::{closure#0}
  13:     0x7fa1a48299e2 - rustc_middle[72de90f4b57d7b8b]::ty::context::tls::with_opt::<rustc_middle[72de90f4b57d7b8b]::util::bug::opt_span_bug_fmt<rustc_span[78ea2eb5514a41c4]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  14:     0x7fa1a4814330 - rustc_middle[72de90f4b57d7b8b]::ty::context::tls::with_context_opt::<rustc_middle[72de90f4b57d7b8b]::ty::context::tls::with_opt<rustc_middle[72de90f4b57d7b8b]::util::bug::opt_span_bug_fmt<rustc_span[78ea2eb5514a41c4]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  15:     0x7fa1a20b6278 - rustc_middle[72de90f4b57d7b8b]::util::bug::span_bug_fmt::<rustc_span[78ea2eb5514a41c4]::span_encoding::Span>
  16:     0x7fa1a5f082a5 - <rustc_borrowck[de8aafb42a22f6c5]::type_check::TypeChecker>::relate_types
  17:     0x7fa1a714cca8 - <rustc_borrowck[de8aafb42a22f6c5]::type_check::TypeChecker as rustc_middle[72de90f4b57d7b8b]::mir::visit::Visitor>::visit_body
  18:     0x7fa1a1c573cb - rustc_borrowck[de8aafb42a22f6c5]::type_check::type_check
  19:     0x7fa1a6789f9b - <rustc_borrowck[de8aafb42a22f6c5]::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
  20:     0x7fa1a67822d5 - rustc_borrowck[de8aafb42a22f6c5]::mir_borrowck
  21:     0x7fa1a6781fe5 - rustc_query_impl[528f4f3081ff05f8]::query_impl::mir_borrowck::invoke_provider_fn::__rust_begin_short_backtrace
  22:     0x7fa1a5f86eb5 - rustc_query_impl[528f4f3081ff05f8]::execution::try_execute_query::<rustc_data_structures[1c35ab8e0d4fbb02]::vec_cache::VecCache<rustc_span[78ea2eb5514a41c4]::def_id::LocalDefId, rustc_middle[72de90f4b57d7b8b]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[72de90f4b57d7b8b]::dep_graph::graph::DepNodeIndex>, false>
  23:     0x7fa1a5f893c3 - rustc_query_impl[528f4f3081ff05f8]::query_impl::mir_borrowck::execute_query_non_incr::__rust_end_short_backtrace
  24:     0x7fa1a6919cbd - rustc_hir_analysis[18be3156a7e28758]::collect::type_of::opaque::find_opaque_ty_constraints_for_rpit
  25:     0x7fa1a6919986 - rustc_hir_analysis[18be3156a7e28758]::collect::type_of::type_of_opaque
  26:     0x7fa1a5a2713d - rustc_query_impl[528f4f3081ff05f8]::execution::try_execute_query::<rustc_middle[72de90f4b57d7b8b]::query::caches::DefIdCache<rustc_middle[72de90f4b57d7b8b]::query::erase::ErasedData<[u8; 8usize]>>, false>
  27:     0x7fa1a6cae7d0 - rustc_query_impl[528f4f3081ff05f8]::query_impl::type_of_opaque::execute_query_non_incr::__rust_end_short_backtrace
  28:     0x7fa1a5e5e83c - rustc_hir_analysis[18be3156a7e28758]::collect::type_of::type_of
  29:     0x7fa1a5e64c66 - rustc_query_impl[528f4f3081ff05f8]::query_impl::type_of::invoke_provider_fn::__rust_begin_short_backtrace
  30:     0x7fa1a5a2713d - rustc_query_impl[528f4f3081ff05f8]::execution::try_execute_query::<rustc_middle[72de90f4b57d7b8b]::query::caches::DefIdCache<rustc_middle[72de90f4b57d7b8b]::query::erase::ErasedData<[u8; 8usize]>>, false>
  31:     0x7fa1a5a26cd0 - rustc_query_impl[528f4f3081ff05f8]::query_impl::type_of::execute_query_non_incr::__rust_end_short_backtrace
  32:     0x7fa1a691a1a9 - rustc_hir_analysis[18be3156a7e28758]::check::check::check_opaque
  33:     0x7fa1a67625ed - rustc_hir_analysis[18be3156a7e28758]::check::check::check_item_type
  34:     0x7fa1a675926e - rustc_hir_analysis[18be3156a7e28758]::check::wfcheck::check_well_formed
  35:     0x7fa1a675924f - rustc_query_impl[528f4f3081ff05f8]::query_impl::check_well_formed::invoke_provider_fn::__rust_begin_short_backtrace
  36:     0x7fa1a67588bd - rustc_query_impl[528f4f3081ff05f8]::execution::try_execute_query::<rustc_data_structures[1c35ab8e0d4fbb02]::vec_cache::VecCache<rustc_span[78ea2eb5514a41c4]::def_id::LocalDefId, rustc_middle[72de90f4b57d7b8b]::query::erase::ErasedData<[u8; 1usize]>, rustc_middle[72de90f4b57d7b8b]::dep_graph::graph::DepNodeIndex>, false>
  37:     0x7fa1a6758647 - rustc_query_impl[528f4f3081ff05f8]::query_impl::check_well_formed::execute_query_non_incr::__rust_end_short_backtrace
  38:     0x7fa1a674e166 - <rustc_middle[72de90f4b57d7b8b]::hir::ModuleItems>::par_opaques::<rustc_hir_analysis[18be3156a7e28758]::check::wfcheck::check_type_wf::{closure#5}>
  39:     0x7fa1a6755bb1 - rustc_hir_analysis[18be3156a7e28758]::check::wfcheck::check_type_wf
  40:     0x7fa1a6755757 - rustc_query_impl[528f4f3081ff05f8]::query_impl::check_type_wf::invoke_provider_fn::__rust_begin_short_backtrace
  41:     0x7fa1a6c481c9 - rustc_query_impl[528f4f3081ff05f8]::execution::try_execute_query::<rustc_middle[72de90f4b57d7b8b]::query::caches::SingleCache<rustc_middle[72de90f4b57d7b8b]::query::erase::ErasedData<[u8; 1usize]>>, false>
  42:     0x7fa1a6c47fa9 - rustc_query_impl[528f4f3081ff05f8]::query_impl::check_type_wf::execute_query_non_incr::__rust_end_short_backtrace
  43:     0x7fa1a625e0e4 - rustc_hir_analysis[18be3156a7e28758]::check_crate
  44:     0x7fa1a5f883ba - rustc_interface[1b7d442e2e02cb08]::passes::analysis
  45:     0x7fa1a6c4668a - rustc_query_impl[528f4f3081ff05f8]::execution::try_execute_query::<rustc_middle[72de90f4b57d7b8b]::query::caches::SingleCache<rustc_middle[72de90f4b57d7b8b]::query::erase::ErasedData<[u8; 0usize]>>, false>
  46:     0x7fa1a6c462e9 - rustc_query_impl[528f4f3081ff05f8]::query_impl::analysis::execute_query_non_incr::__rust_end_short_backtrace
  47:     0x7fa1a6cf4777 - rustc_interface[1b7d442e2e02cb08]::interface::run_compiler::<(), rustc_driver_impl[3bc4d727b6aa2dc4]::run_compiler::{closure#0}>::{closure#2}
  48:     0x7fa1a6d1b424 - std[6cb999a3dbb652d9]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[1b7d442e2e02cb08]::util::run_in_thread_with_globals<rustc_interface[1b7d442e2e02cb08]::util::run_in_thread_pool_with_globals<rustc_interface[1b7d442e2e02cb08]::interface::run_compiler<(), rustc_driver_impl[3bc4d727b6aa2dc4]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  49:     0x7fa1a6d1b1ed - <std[6cb999a3dbb652d9]::thread::lifecycle::spawn_unchecked<rustc_interface[1b7d442e2e02cb08]::util::run_in_thread_with_globals<rustc_interface[1b7d442e2e02cb08]::util::run_in_thread_pool_with_globals<rustc_interface[1b7d442e2e02cb08]::interface::run_compiler<(), rustc_driver_impl[3bc4d727b6aa2dc4]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[cdb7ffe4be53d62]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  50:     0x7fa1a6d237c0 - <std[6cb999a3dbb652d9]::sys::thread::unix::Thread>::new::thread_start
  51:     0x7fa1a0097739 - <unknown>
  52:     0x7fa1a011bedc - <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 `/home/matthias/vcs/github/rust_misc_stuff/tests/ui/rustc-ice-2026-08-06T18_06_35-2706793.txt` to your bug report

note: rustc 1.99.0-nightly (84b36a78a 2026-08-06) running on x86_64-unknown-linux-gnu

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

query stack during panic:
#0 [mir_borrowck] borrow-checking `<impl at coroutine/issue-58888.rs:10:1: 10:14>::check_connection`
#1 [type_of_opaque] computing type of opaque `<impl at coroutine/issue-58888.rs:10:1: 10:14>::check_connection::{opaque#0}`
#2 [type_of] computing type of `<impl at coroutine/issue-58888.rs:10:1: 10:14>::check_connection::{opaque#0}`
#3 [check_well_formed] checking that `<impl at coroutine/issue-58888.rs:10:1: 10:14>::check_connection::{opaque#0}` is well-formed
#4 [check_type_wf] checking that types are well-formed
#5 [analysis] running analysis passes on crate `issue_58888`
end of query stack
error: aborting due to 1 previous error