[ICE]: `encountered unexpected unsized field in layout of `

[matthiaskrgr]

snippet:

type Opaque2<'a> = dyn Sync + 'static;

fn test2() -> impl for<'a, 'b> Fn((&'a str, &'b str)) -> (Opaque2<'a>, Box<[u8]>) {
    move |_| loop {}
}

fn main() {}

Version information

rustc 1.100.0-nightly (5ceaf6608 2026-09-25)
binary: rustc
commit-hash: 5ceaf6608eb354c2f5bbb3b8d974caa367dac81c
commit-date: 2026-09-25
host: x86_64-unknown-linux-gnu
release: 1.100.0-nightly
LLVM version: 23.1.1

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc

Program output

warning: type alias `Opaque2` is never used
 --> /tmp/icemaker_global_tempdir.O1Kui7qc4fFZ/rustc_testrunner_tmpdir_reporting.I4KJqBrTyhgY/mvce.rs:1:6
  |
1 | type Opaque2<'a> = dyn Sync + 'static;
  |      ^^^^^^^
  |
  = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default

warning: function `test2` is never used
 --> /tmp/icemaker_global_tempdir.O1Kui7qc4fFZ/rustc_testrunner_tmpdir_reporting.I4KJqBrTyhgY/mvce.rs:3:4
  |
3 | fn test2() -> impl for<'a, 'b> Fn((&'a str, &'b str)) -> (Opaque...
  |    ^^^^^

warning: 2 warnings emitted

note: no errors encountered even though delayed bugs were created

note: those delayed bugs will now be shown as internal compiler errors

error: internal compiler error: encountered unexpected unsized field in layout of (dyn [Binder { value: AutoTrait(DefId(2:4558 ~ core[df69]::marker::Sync)), bound_vars: [] }] + '{erased}, std::boxed::Box<[u8], std::alloc::Global>): TyAndLayout {
                                    ty: dyn Sync,
                                    layout: Layout {
                                        size: Size(0 bytes),
                                        align: AbiAlign {
                                            abi: Align(1 bytes),
                                        },
                                        backend_repr: Memory {
                                            sized: false,
                                        },
                                        fields: Arbitrary {
                                            offsets: [],
                                            in_memory_order: [],
                                        },
                                        largest_niche: None,
                                        uninhabited: false,
                                        variants: Single {
                                            index: 0,
                                        },
                                        max_repr_align: None,
                                        unadjusted_abi_align: Align(1 bytes),
                                        randomization_seed: 0,
                                    },
                                }
  |
  = note: delayed at /rustc-dev/5ceaf6608eb354c2f5bbb3b8d974caa367dac81c/compiler/rustc_ty_utils/src/layout.rs:133:32
             0: <rustc_errors::DiagCtxtInner>::emit_diagnostic
             1: <rustc_errors::DiagCtxtHandle>::emit_diagnostic
             2: <rustc_errors::diagnostic::Diag>::emit_err
             3: <rustc_errors::DiagCtxtHandle>::delayed_bug::<alloc::string::String>
             4: rustc_ty_utils::layout::map_error
             5: rustc_ty_utils::layout::layout_of_uncached::{closure#3}
             6: rustc_ty_utils::layout::layout_of_uncached
             7: rustc_ty_utils::layout::layout_of
             8: rustc_query_impl::query_vtables::layout_of::invoke_provider_fn::__rust_begin_short_backtrace
             9: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::DefaultCache<rustc_middle::ty::PseudoCanonicalInput<rustc_middle::ty::Ty>, rustc_middle::query::erase::ErasedData<[u8; 16]>>, false>
            10: rustc_query_impl::query_vtables::layout_of::execute_query_non_incr::__rust_end_short_backtrace
            11: <rustc_mir_transform::known_panics_lint::KnownPanicsLint as rustc_mir_transform::pass_manager::MirLint>::run_lint
            12: rustc_mir_transform::run_analysis_to_runtime_passes
            13: rustc_mir_transform::mir_drops_elaborated_and_const_checked
            14: rustc_query_impl::execution::try_execute_query::<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::ErasedData<[u8; 8]>, rustc_middle::dep_graph::graph::DepNodeIndex>, false>
            15: rustc_query_impl::query_vtables::mir_drops_elaborated_and_const_checked::execute_query_non_incr::__rust_end_short_backtrace
            16: <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface::passes::run_required_analyses::{closure#2}::{closure#0}>::{closure#0}
            17: rustc_interface::passes::analysis
            18: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::SingleCache<rustc_middle::query::erase::ErasedData<[u8; 0]>>, false>
            19: rustc_query_impl::query_vtables::analysis::execute_query_non_incr::__rust_end_short_backtrace
            20: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#2}
            21: std::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
            22: <std::thread::lifecycle::spawn_unchecked<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
            23: <std::sys::thread::unix::Thread>::new::thread_start
            24: <unknown>
            25: <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 1.100.0-nightly (5ceaf6608 2026-09-25) running on x86_64-unknown-linux-gnu

query stack during panic:
end of query stack

[matthiaskrgr]

this compiled with nightly-2023-04-21

warning: type alias `Opaque2` is never used
 --> /tmp/crash.rs:1:6
  |
1 | type Opaque2<'a> = dyn Sync + 'static;
  |      ^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: function `test2` is never used
 --> /tmp/crash.rs:3:4
  |
3 | fn test2() -> impl for<'a, 'b> Fn((&'a str, &'b str)) -> (Opaque2<'a>, Box<[u8]>) {
  |    ^^^^^

warning: 2 warnings emitted

and ices with nightly-2023-04-22

warning: type alias `Opaque2` is never used
 --> /tmp/crash.rs:1:6
  |
1 | type Opaque2<'a> = dyn Sync + 'static;
  |      ^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: function `test2` is never used
 --> /tmp/crash.rs:3:4
  |
3 | fn test2() -> impl for<'a, 'b> Fn((&'a str, &'b str)) -> (Opaque2<'a>, Box<[u8]>) {
  |    ^^^^^

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

error: internal compiler error: univariant: field #2 comes after unsized field
  |
  = note: delayed at    0: <rustc_errors::HandlerInner>::emit_diagnostic
             1: <rustc_errors::Handler>::delay_span_bug::<rustc_span::span_encoding::Span, &str>
             2: <rustc_middle::ty::layout::LayoutCx<rustc_middle::ty::context::TyCtxt> as rustc_abi::layout::LayoutCalculator>::univariant
             3: rustc_ty_utils::layout::layout_of
             4: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::queries::layout_of, rustc_query_impl::plumbing::QueryCtxt>
             5: <rustc_mir_transform::const_prop::CanConstProp>::check
             6: <rustc_mir_transform::const_prop_lint::ConstProp as rustc_mir_transform::pass_manager::MirLint>::run_lint
             7: rustc_mir_transform::mir_drops_elaborated_and_const_checked
             8: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::queries::mir_drops_elaborated_and_const_checked, rustc_query_impl::plumbing::QueryCtxt>
             9: <rustc_session::session::Session>::time::<(), rustc_interface::passes::analysis::{closure#3}>
            10: rustc_interface::passes::analysis
            11: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::queries::analysis, rustc_query_impl::plumbing::QueryCtxt>
            12: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::analysis
            13: <rustc_middle::ty::context::GlobalCtxt>::enter::<rustc_driver_impl::run_compiler::{closure#1}::{closure#2}::{closure#4}, core::result::Result<(), rustc_span::ErrorGuaranteed>>
            14: <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_span::ErrorGuaranteed>>
            15: rustc_span::set_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}>
            16: <scoped_tls::ScopedKey<rustc_span::SessionGlobals>>::set::<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>>
            17: 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>>
            18: <<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}
            19: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
                       at /rustc/fec9adcdbc21469ef105162cc8cabf81c72d06be/library/alloc/src/boxed.rs:1962:9
            20: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
                       at /rustc/fec9adcdbc21469ef105162cc8cabf81c72d06be/library/alloc/src/boxed.rs:1962:9
            21: std::sys::unix::thread::Thread::new::thread_start
                       at /rustc/fec9adcdbc21469ef105162cc8cabf81c72d06be/library/std/src/sys/unix/thread.rs:108:17
            22: <unknown>
            23: <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: rustc 1.71.0-nightly (fec9adcdb 2023-04-21) running on x86_64-unknown-linux-gnu

query stack during panic:
end of query stack
error: aborting due to 2 previous errors; 2 warnings emitted

[cyrgani]

smaller:

fn test2() -> impl Fn() -> (str, u8) {
    || loop {}
}

[rustbot]

Assigning P-medium (discussion on Zulip).