ICE: `cannot convert '{erased} to a region vid`

[matthiaskrgr]

auto-reduced (treereduce-rust):

//@compile-flags: --edition=2024 
fn invalid_future() -> impl Future {
    create_complex_future()
}

fn create_complex_future() -> impl Future<Output = impl ReturnsSend> {
    async { &|| async { invalid_future().await } }
}

fn coerce_impl_trait() -> impl Future<Output = impl Send> {
    create_complex_future()
}

trait ReturnsSend {}

impl<F, R> ReturnsSend for F
where
    F: Fn() -> R,
    R: Send,
{
}

fn main() {}

original:

//@ edition: 2021

use std::future::Future;

fn invalid_future() -> impl Future {
    create_complex_future()
}
//~^ ERROR `()` is not a future

fn create_complex_future() -> impl Future<Output = impl ReturnsSend> {
    async { &|| async { invalid_future().await } }
}

fn coerce_impl_trait() -> impl Future<Output = impl Send> {
    create_complex_future()
}

trait ReturnsSend {}

impl<F, R> ReturnsSend for F
where
    F: Fn() -> R,
    R: Send,
{
}

fn main() {}

Version information

rustc 1.92.0-nightly (e9385f9ee 2025-09-24)
binary: rustc
commit-hash: e9385f9eea0221ef295a188d49d16f8f5189abf1
commit-date: 2025-09-24
host: x86_64-unknown-linux-gnu
release: 1.92.0-nightly
LLVM version: 21.1.2

Possibly related line of code:

rust/compiler/rustc_borrowck/src/universal_regions.rs

Lines 924 to 936 in e9385f9

	// errors are being emitted and 2) it leaves the happy path unaffected.
	self.fr_static
	}
	_ => *self
	.indices
	.get(&r)
	.unwrap_or_else(|| bug!("cannot convert `{:?}` to a region vid", r)),
	}
	}
	/// Replaces all free regions in `value` with region vids, as
	/// returned by `to_region_vid`.
	fn fold_to_region_vids<T>(&self, tcx: TyCtxt<'tcx>, value: T) -> T

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc --edition=2024

Program output

error[E0720]: cannot resolve opaque type
 --> /tmp/icemaker_global_tempdir.aUMoW0s2aYlu/rustc_testrunner_tmpdir_reporting.gv3XOaBfqwDY/mvce.rs:1:24
  |
1 | fn invalid_future() -> impl Future {
  |                        ^^^^^^^^^^^ recursive opaque type
2 |     create_complex_future()
  |     ----------------------- returning here with type `impl Future<Output = impl ReturnsSend>`
...
5 | fn create_complex_future() -> impl Future<Output = impl ReturnsSend> {
  |                               -------------------------------------- returning this opaque type `impl Future<Output = impl ReturnsSend>`

error[E0720]: cannot resolve opaque type
 --> /tmp/icemaker_global_tempdir.aUMoW0s2aYlu/rustc_testrunner_tmpdir_reporting.gv3XOaBfqwDY/mvce.rs:5:31
  |
1 | fn invalid_future() -> impl Future {
  |                        ----------- returning this type `{async block@/tmp/icemaker_global_tempdir.aUMoW0s2aYlu/rustc_testrunner_tmpdir_reporting.gv3XOaBfqwDY/mvce.rs:6:5: 6:10}`
...
5 | fn create_complex_future() -> impl Future<Output = impl ReturnsSend> {
  |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ recursive opaque type
6 |     async { &|| async { invalid_future().await } }
  |     ---------------------------------------------- returning here with type `{async block@/tmp/icemaker_global_tempdir.aUMoW0s2aYlu/rustc_testrunner_tmpdir_reporting.gv3XOaBfqwDY/mvce.rs:6:5: 6:10}`

error: future cannot be sent between threads safely
 --> /tmp/icemaker_global_tempdir.aUMoW0s2aYlu/rustc_testrunner_tmpdir_reporting.gv3XOaBfqwDY/mvce.rs:5:52
  |
5 | fn create_complex_future() -> impl Future<Output = impl ReturnsSend> {
  |                                                    ^^^^^^^^^^^^^^^^ future created by async block is not `Send`
  |
  = note: cannot satisfy `impl Future<Output = impl ReturnsSend>: Send`
note: future is not `Send` as it awaits another future which is not `Send`
 --> /tmp/icemaker_global_tempdir.aUMoW0s2aYlu/rustc_testrunner_tmpdir_reporting.gv3XOaBfqwDY/mvce.rs:6:25
  |
6 |     async { &|| async { invalid_future().await } }
  |                         ^^^^^^^^^^^^^^^^ await occurs here on type `impl Future`, which is not `Send`
note: required by a bound in an opaque type
 --> /tmp/icemaker_global_tempdir.aUMoW0s2aYlu/rustc_testrunner_tmpdir_reporting.gv3XOaBfqwDY/mvce.rs:5:57
  |
5 | fn create_complex_future() -> impl Future<Output = impl ReturnsSend> {
  |                                                         ^^^^^^^^^^^

error: internal compiler error: compiler/rustc_borrowck/src/universal_regions.rs:930:36: cannot convert `'{erased}` to a region vid


thread 'rustc' (814462) panicked at compiler/rustc_borrowck/src/universal_regions.rs:930:36:
Box<dyn Any>
stack backtrace:
   0:     0x7ff7b07ca183 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h4a7ebf3c97b63e54
   1:     0x7ff7b0e02004 - core::fmt::write::h5e5759cada453e98
   2:     0x7ff7b077ea01 - std::io::Write::write_fmt::h503716d2664a2999
   3:     0x7ff7b078fef2 - std::sys::backtrace::BacktraceLock::print::haede8464af6f6593
   4:     0x7ff7b0795e59 - std::panicking::default_hook::{{closure}}::hd1d459f3747276de
   5:     0x7ff7b0795983 - std::panicking::default_hook::h4615f5c23d4bd826
   6:     0x7ff7af7ba5d7 - std[4d5cf81698346de3]::panicking::update_hook::<alloc[62720e50f6bf8d96]::boxed::Box<rustc_driver_impl[743c5953b67fc26f]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x7ff7b079627f - std::panicking::panic_with_hook::h290022751dfbc99c
   8:     0x7ff7af7f6861 - std[4d5cf81698346de3]::panicking::begin_panic::<rustc_errors[1d45ad03724e6281]::ExplicitBug>::{closure#0}
   9:     0x7ff7af7eac36 - std[4d5cf81698346de3]::sys::backtrace::__rust_end_short_backtrace::<std[4d5cf81698346de3]::panicking::begin_panic<rustc_errors[1d45ad03724e6281]::ExplicitBug>::{closure#0}, !>
  10:     0x7ff7af7e7b19 - std[4d5cf81698346de3]::panicking::begin_panic::<rustc_errors[1d45ad03724e6281]::ExplicitBug>
  11:     0x7ff7af801f51 - <rustc_errors[1d45ad03724e6281]::diagnostic::BugAbort as rustc_errors[1d45ad03724e6281]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  12:     0x7ff7afd946ca - rustc_middle[e8cb4ff3ac94f2d4]::util::bug::opt_span_bug_fmt::<rustc_span[80b09b3132128d8d]::span_encoding::Span>::{closure#0}
  13:     0x7ff7afd9488a - rustc_middle[e8cb4ff3ac94f2d4]::ty::context::tls::with_opt::<rustc_middle[e8cb4ff3ac94f2d4]::util::bug::opt_span_bug_fmt<rustc_span[80b09b3132128d8d]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  14:     0x7ff7afd8345b - rustc_middle[e8cb4ff3ac94f2d4]::ty::context::tls::with_context_opt::<rustc_middle[e8cb4ff3ac94f2d4]::ty::context::tls::with_opt<rustc_middle[e8cb4ff3ac94f2d4]::util::bug::opt_span_bug_fmt<rustc_span[80b09b3132128d8d]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  15:     0x7ff7ade7bbd0 - rustc_middle[e8cb4ff3ac94f2d4]::util::bug::bug_fmt
  16:     0x7ff7b175fa36 - <rustc_borrowck[6eb8f1ad86d728a7]::type_check::constraint_conversion::ConstraintConversion>::convert_all
  17:     0x7ff7b148b26d - <rustc_borrowck[6eb8f1ad86d728a7]::type_check::TypeChecker>::fully_perform_op::<(), rustc_borrowck[6eb8f1ad86d728a7]::type_check::InstantiateOpaqueType>
  18:     0x7ff7b1d7ea35 - <rustc_borrowck[6eb8f1ad86d728a7]::type_check::relate_tys::NllTypeRelating>::relate_opaques
  19:     0x7ff7b1e41b37 - <rustc_borrowck[6eb8f1ad86d728a7]::type_check::TypeChecker as rustc_middle[e8cb4ff3ac94f2d4]::mir::visit::Visitor>::visit_body
  20:     0x7ff7b1df6503 - rustc_borrowck[6eb8f1ad86d728a7]::type_check::type_check
  21:     0x7ff7b1ffaa81 - <rustc_borrowck[6eb8f1ad86d728a7]::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
  22:     0x7ff7b1ff3057 - rustc_borrowck[6eb8f1ad86d728a7]::mir_borrowck
  23:     0x7ff7b1ff2e1d - rustc_query_impl[c5c9f822c73cefb7]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[c5c9f822c73cefb7]::query_impl::mir_borrowck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[e8cb4ff3ac94f2d4]::query::erase::Erased<[u8; 8usize]>>
  24:     0x7ff7b0fd1644 - rustc_query_system[b5e6460b078a577c]::query::plumbing::try_execute_query::<rustc_query_impl[c5c9f822c73cefb7]::DynamicConfig<rustc_data_structures[a8767abe109b0a96]::vec_cache::VecCache<rustc_span[80b09b3132128d8d]::def_id::LocalDefId, rustc_middle[e8cb4ff3ac94f2d4]::query::erase::Erased<[u8; 8usize]>, rustc_query_system[b5e6460b078a577c]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[c5c9f822c73cefb7]::plumbing::QueryCtxt, false>
  25:     0x7ff7b0fd5f89 - rustc_query_impl[c5c9f822c73cefb7]::query_impl::mir_borrowck::get_query_non_incr::__rust_end_short_backtrace
  26:     0x7ff7b1e7a011 - rustc_hir_analysis[7f4f1816d2196b28]::collect::type_of::opaque::find_opaque_ty_constraints_for_rpit
  27:     0x7ff7b1e79d8c - rustc_hir_analysis[7f4f1816d2196b28]::collect::type_of::type_of_opaque
  28:     0x7ff7b1e79c91 - rustc_query_impl[c5c9f822c73cefb7]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[c5c9f822c73cefb7]::query_impl::type_of_opaque::dynamic_query::{closure#2}::{closure#0}, rustc_middle[e8cb4ff3ac94f2d4]::query::erase::Erased<[u8; 8usize]>>
  29:     0x7ff7b0e1af3a - rustc_query_system[b5e6460b078a577c]::query::plumbing::try_execute_query::<rustc_query_impl[c5c9f822c73cefb7]::DynamicConfig<rustc_query_system[b5e6460b078a577c]::query::caches::DefIdCache<rustc_middle[e8cb4ff3ac94f2d4]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[c5c9f822c73cefb7]::plumbing::QueryCtxt, false>
  30:     0x7ff7b2054d9b - rustc_query_impl[c5c9f822c73cefb7]::query_impl::type_of_opaque::get_query_non_incr::__rust_end_short_backtrace
  31:     0x7ff7b173f8d8 - rustc_hir_analysis[7f4f1816d2196b28]::collect::type_of::type_of
  32:     0x7ff7b0e1c2a6 - rustc_query_impl[c5c9f822c73cefb7]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[c5c9f822c73cefb7]::query_impl::type_of::dynamic_query::{closure#2}::{closure#0}, rustc_middle[e8cb4ff3ac94f2d4]::query::erase::Erased<[u8; 8usize]>>
  33:     0x7ff7b0e1af3a - rustc_query_system[b5e6460b078a577c]::query::plumbing::try_execute_query::<rustc_query_impl[c5c9f822c73cefb7]::DynamicConfig<rustc_query_system[b5e6460b078a577c]::query::caches::DefIdCache<rustc_middle[e8cb4ff3ac94f2d4]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[c5c9f822c73cefb7]::plumbing::QueryCtxt, false>
  34:     0x7ff7b0e1aadd - rustc_query_impl[c5c9f822c73cefb7]::query_impl::type_of::get_query_non_incr::__rust_end_short_backtrace
  35:     0x7ff7b1e7a6ec - rustc_hir_analysis[7f4f1816d2196b28]::check::check::check_opaque
  36:     0x7ff7b1701984 - rustc_hir_analysis[7f4f1816d2196b28]::check::check::check_item_type
  37:     0x7ff7b16f9c39 - rustc_hir_analysis[7f4f1816d2196b28]::check::wfcheck::check_well_formed
  38:     0x7ff7b16f9c1b - rustc_query_impl[c5c9f822c73cefb7]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[c5c9f822c73cefb7]::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle[e8cb4ff3ac94f2d4]::query::erase::Erased<[u8; 1usize]>>
  39:     0x7ff7b16f9492 - rustc_query_system[b5e6460b078a577c]::query::plumbing::try_execute_query::<rustc_query_impl[c5c9f822c73cefb7]::DynamicConfig<rustc_data_structures[a8767abe109b0a96]::vec_cache::VecCache<rustc_span[80b09b3132128d8d]::def_id::LocalDefId, rustc_middle[e8cb4ff3ac94f2d4]::query::erase::Erased<[u8; 1usize]>, rustc_query_system[b5e6460b078a577c]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[c5c9f822c73cefb7]::plumbing::QueryCtxt, false>
  40:     0x7ff7b16f8f96 - rustc_query_impl[c5c9f822c73cefb7]::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
  41:     0x7ff7b16f6615 - rustc_hir_analysis[7f4f1816d2196b28]::check::wfcheck::check_type_wf
  42:     0x7ff7b16f64b5 - rustc_query_impl[c5c9f822c73cefb7]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[c5c9f822c73cefb7]::query_impl::check_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle[e8cb4ff3ac94f2d4]::query::erase::Erased<[u8; 1usize]>>
  43:     0x7ff7b1dc6288 - rustc_query_system[b5e6460b078a577c]::query::plumbing::try_execute_query::<rustc_query_impl[c5c9f822c73cefb7]::DynamicConfig<rustc_query_system[b5e6460b078a577c]::query::caches::SingleCache<rustc_middle[e8cb4ff3ac94f2d4]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[c5c9f822c73cefb7]::plumbing::QueryCtxt, false>
  44:     0x7ff7b1dc604e - rustc_query_impl[c5c9f822c73cefb7]::query_impl::check_type_wf::get_query_non_incr::__rust_end_short_backtrace
  45:     0x7ff7b18e32dc - rustc_hir_analysis[7f4f1816d2196b28]::check_crate
  46:     0x7ff7b0fd2d30 - rustc_interface[e80a289d81ff7a8]::passes::analysis
  47:     0x7ff7b0fd29ed - rustc_query_impl[c5c9f822c73cefb7]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[c5c9f822c73cefb7]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[e8cb4ff3ac94f2d4]::query::erase::Erased<[u8; 0usize]>>
  48:     0x7ff7b1dcc666 - rustc_query_system[b5e6460b078a577c]::query::plumbing::try_execute_query::<rustc_query_impl[c5c9f822c73cefb7]::DynamicConfig<rustc_query_system[b5e6460b078a577c]::query::caches::SingleCache<rustc_middle[e8cb4ff3ac94f2d4]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[c5c9f822c73cefb7]::plumbing::QueryCtxt, false>
  49:     0x7ff7b1dcc2bc - rustc_query_impl[c5c9f822c73cefb7]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  50:     0x7ff7b2093e0f - rustc_interface[e80a289d81ff7a8]::passes::create_and_enter_global_ctxt::<core[aa10cf8c11ee53a9]::option::Option<rustc_interface[e80a289d81ff7a8]::queries::Linker>, rustc_driver_impl[743c5953b67fc26f]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  51:     0x7ff7b20303fe - rustc_interface[e80a289d81ff7a8]::interface::run_compiler::<(), rustc_driver_impl[743c5953b67fc26f]::run_compiler::{closure#0}>::{closure#1}
  52:     0x7ff7b1f67e78 - std[4d5cf81698346de3]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[e80a289d81ff7a8]::util::run_in_thread_with_globals<rustc_interface[e80a289d81ff7a8]::util::run_in_thread_pool_with_globals<rustc_interface[e80a289d81ff7a8]::interface::run_compiler<(), rustc_driver_impl[743c5953b67fc26f]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  53:     0x7ff7b1f67b48 - <<std[4d5cf81698346de3]::thread::Builder>::spawn_unchecked_<rustc_interface[e80a289d81ff7a8]::util::run_in_thread_with_globals<rustc_interface[e80a289d81ff7a8]::util::run_in_thread_pool_with_globals<rustc_interface[e80a289d81ff7a8]::interface::run_compiler<(), rustc_driver_impl[743c5953b67fc26f]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[aa10cf8c11ee53a9]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  54:     0x7ff7b1f6dc4d - std::sys::thread::unix::Thread::new::thread_start::hbfb8ed69a8a2081e
  55:     0x7ff7ab8969cb - <unknown>
  56:     0x7ff7ab91aa0c - <unknown>
  57:                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: rustc 1.92.0-nightly (e9385f9ee 2025-09-24) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z dump-mir-dir=dir

query stack during panic:
#0 [mir_borrowck] borrow-checking `coerce_impl_trait`
#1 [type_of_opaque] computing type of opaque `coerce_impl_trait::{opaque#0}`
#2 [type_of] computing type of `coerce_impl_trait::{opaque#0}`
#3 [check_well_formed] checking that `coerce_impl_trait::{opaque#0}` is well-formed
#4 [check_type_wf] checking that types are well-formed
#5 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 5 previous errors

Some errors have detailed explanations: E0601, E0720.
For more information about an error, try `rustc --explain E0601`.

[matthiaskrgr]

Regression in nightly-2023-09-24

  commit[0] 2023-09-22: Auto merge of #116077 - matthiaskrgr:rollup-2y1buzg, r=matthiaskrgr
  commit[1] 2023-09-23: Auto merge of #115695 - tmiasko:compiletest-supported-sanitizers, r=oli-obk
  commit[2] 2023-09-23: Auto merge of #116066 - weihanglo:update-cargo, r=weihanglo
  commit[3] 2023-09-23: Auto merge of #116052 - oli-obk:ceci_nest_pas_une_query, r=WaffleLapkin
  commit[4] 2023-09-23: Auto merge of #116071 - estebank:issue-115905, r=compiler-errors
  commit[5] 2023-09-23: Auto merge of #116045 - notriddle:notriddle/issue-83556, r=cjgillot
  commit[6] 2023-09-23: Auto merge of #116081 - compiler-errors:closure-captures-sized, r=cjgillot
  commit[7] 2023-09-23: Auto merge of #116047 - a-lafrance:I80836-codegen-test, r=Mark-Simulacrum
  commit[8] 2023-09-23: Auto merge of #107421 - cjgillot:drop-tracking-mir, r=oli-obk

could be #116081 maybe?