rust-lang / rust-lang/rust

[ICE]: `Attempting to cast non-castable types`

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C-bug F-checked_type_aliases I-ICE T-compiler WG-trait-system-refactor
Dominant language
Rust
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Description

snippet:

//@compile-flags: -Znext-solver=globally
#![feature(lazy_type_alias)]

const CONST_MULTI_FN: &'static fn() = &(STATIC_BAR as fn());

struct Foo<'a>;

type Bar<'b: 'a> = Foo<'a>;
static STATIC_BAR: Bar = {};

Version information

rustc 1.97.0-nightly (22cc6747b 2026-04-19)
binary: rustc
commit-hash: 22cc6747b18b3a6738e98b06676fd91f1756b33a
commit-date: 2026-04-19
host: x86_64-unknown-linux-gnu
release: 1.97.0-nightly
LLVM version: 22.1.2

Possibly related line of code:
https://github.com/rust-lang/rust/blob/22cc6747b18b3a6738e98b06676fd91f1756b33a/compiler/rustc_middle/src/ty/cast.rs#L87-L99

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc -Znext-solver=globally

Program output 

error[E0261]: use of undeclared lifetime name `'a`
 --> /tmp/icemaker_global_tempdir.zrAofuFD0Qi9/rustc_testrunner_tmpdir_reporting.ktKoLEMAjHEa/mvce.rs:7:14
  |
7 | type Bar<'b: 'a> = Foo<'a>;
  |              ^^ undeclared lifetime
  |
help: consider introducing lifetime `'a` here
  |
7 | type Bar<'a, 'b: 'a> = Foo<'a>;
  |          +++

error[E0261]: use of undeclared lifetime name `'a`
 --> /tmp/icemaker_global_tempdir.zrAofuFD0Qi9/rustc_testrunner_tmpdir_reporting.ktKoLEMAjHEa/mvce.rs:7:24
  |
7 | type Bar<'b: 'a> = Foo<'a>;
  |                        ^^ undeclared lifetime
  |
help: consider introducing lifetime `'a` here
  |
7 | type Bar<'a, 'b: 'a> = Foo<'a>;
  |          +++

warning: the feature `lazy_type_alias` is incomplete and may not be safe to use and/or cause compiler crashes
 --> /tmp/icemaker_global_tempdir.zrAofuFD0Qi9/rustc_testrunner_tmpdir_reporting.ktKoLEMAjHEa/mvce.rs:1:12
  |
1 | #![feature(lazy_type_alias)]
  |            ^^^^^^^^^^^^^^^
  |
  = note: see issue #112792 <https://github.com/rust-lang/rust/issues/112792> for more information
  = note: `#[warn(incomplete_features)]` on by default

error[E0601]: `main` function not found in crate `mvce`
 --> /tmp/icemaker_global_tempdir.zrAofuFD0Qi9/rustc_testrunner_tmpdir_reporting.ktKoLEMAjHEa/mvce.rs:8:29
  |
8 | static STATIC_BAR: Bar = {};
  |                             ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.zrAofuFD0Qi9/rustc_testrunner_tmpdir_reporting.ktKoLEMAjHEa/mvce.rs`

error[E0392]: lifetime parameter `'a` is never used
 --> /tmp/icemaker_global_tempdir.zrAofuFD0Qi9/rustc_testrunner_tmpdir_reporting.ktKoLEMAjHEa/mvce.rs:5:12
  |
5 | struct Foo<'a>;
  |            ^^ unused lifetime parameter
  |
  = help: consider removing `'a`, referring to it in a field, or using a marker such as `PhantomData`

error: internal compiler error: /rustc-dev/22cc6747b18b3a6738e98b06676fd91f1756b33a/compiler/rustc_middle/src/ty/cast.rs:93:13: Attempting to cast non-castable types Foo<'{erased}> and Binder { value: fn(), bound_vars: [] }


thread 'rustc' (400247) panicked at /rustc-dev/22cc6747b18b3a6738e98b06676fd91f1756b33a/compiler/rustc_middle/src/ty/cast.rs:93:13:
Box<dyn Any>
stack backtrace:
   0:     0x7f8401615b3b - <<std[33dac3ce30492785]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[bc88cc8ff231c3b1]::fmt::Display>::fmt
   1:     0x7f8401c4d008 - core[bc88cc8ff231c3b1]::fmt::write
   2:     0x7f840162cb36 - <std[33dac3ce30492785]::sys::stdio::unix::Stderr as std[33dac3ce30492785]::io::Write>::write_fmt
   3:     0x7f84015ebdb8 - std[33dac3ce30492785]::panicking::default_hook::{closure#0}
   4:     0x7f84016091a3 - std[33dac3ce30492785]::panicking::default_hook
   5:     0x7f84005cf42c - std[33dac3ce30492785]::panicking::update_hook::<alloc[301a6134ec5d1e94]::boxed::Box<rustc_driver_impl[757881b80e69d34]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f8401609482 - std[33dac3ce30492785]::panicking::panic_with_hook
   7:     0x7f84005fece1 - std[33dac3ce30492785]::panicking::begin_panic::<rustc_errors[1cf5f0732ede9803]::ExplicitBug>::{closure#0}
   8:     0x7f84005f79c6 - std[33dac3ce30492785]::sys::backtrace::__rust_end_short_backtrace::<std[33dac3ce30492785]::panicking::begin_panic<rustc_errors[1cf5f0732ede9803]::ExplicitBug>::{closure#0}, !>
   9:     0x7f84005f5658 - std[33dac3ce30492785]::panicking::begin_panic::<rustc_errors[1cf5f0732ede9803]::ExplicitBug>
  10:     0x7f840060a021 - <rustc_errors[1cf5f0732ede9803]::diagnostic::BugAbort as rustc_errors[1cf5f0732ede9803]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  11:     0x7f8400c06929 - rustc_middle[363116004d559c57]::util::bug::opt_span_bug_fmt::<rustc_span[5aa3195f2cc887f0]::span_encoding::Span>::{closure#0}
  12:     0x7f8400c06ab2 - rustc_middle[363116004d559c57]::ty::context::tls::with_opt::<rustc_middle[363116004d559c57]::util::bug::opt_span_bug_fmt<rustc_span[5aa3195f2cc887f0]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  13:     0x7f8400bf78bb - rustc_middle[363116004d559c57]::ty::context::tls::with_context_opt::<rustc_middle[363116004d559c57]::ty::context::tls::with_opt<rustc_middle[363116004d559c57]::util::bug::opt_span_bug_fmt<rustc_span[5aa3195f2cc887f0]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  14:     0x7f83fdd2f7e4 - rustc_middle[363116004d559c57]::util::bug::bug_fmt
  15:     0x7f84035389c1 - rustc_middle[363116004d559c57]::ty::cast::mir_cast_kind.cold
  16:     0x7f8401f630df - <rustc_mir_build[ea91d66cad601e93]::builder::Builder>::as_rvalue
  17:     0x7f8401f66d16 - <rustc_mir_build[ea91d66cad601e93]::builder::Builder>::expr_into_dest
  18:     0x7f8401f5d9ab - <rustc_mir_build[ea91d66cad601e93]::builder::Builder>::as_temp::{closure#0}
  19:     0x7f8401f54481 - <rustc_mir_build[ea91d66cad601e93]::builder::Builder>::expr_as_place
  20:     0x7f8401f53b0f - <rustc_mir_build[ea91d66cad601e93]::builder::Builder>::expr_as_place
  21:     0x7f8401f674bb - <rustc_mir_build[ea91d66cad601e93]::builder::Builder>::expr_into_dest
  22:     0x7f8401f5d3e4 - <rustc_mir_build[ea91d66cad601e93]::builder::Builder>::as_temp::{closure#0}
  23:     0x7f8401f54481 - <rustc_mir_build[ea91d66cad601e93]::builder::Builder>::expr_as_place
  24:     0x7f8401f5355e - <rustc_mir_build[ea91d66cad601e93]::builder::Builder>::expr_as_place
  25:     0x7f8401f674bb - <rustc_mir_build[ea91d66cad601e93]::builder::Builder>::expr_into_dest
  26:     0x7f8401f678d7 - <rustc_mir_build[ea91d66cad601e93]::builder::Builder>::expr_into_dest
  27:     0x7f84022d8240 - rustc_mir_build[ea91d66cad601e93]::builder::build_mir_inner_impl
  28:     0x7f8401c0a96a - rustc_mir_transform[df8373edb371aa52]::mir_built
  29:     0x7f8401d26026 - rustc_query_impl[aac33c90bd2db6e0]::execution::try_execute_query::<rustc_data_structures[fdbb4dd6aa2a74fe]::vec_cache::VecCache<rustc_span[5aa3195f2cc887f0]::def_id::LocalDefId, rustc_middle[363116004d559c57]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[363116004d559c57]::dep_graph::graph::DepNodeIndex>, false>
  30:     0x7f8401d25b43 - rustc_query_impl[aac33c90bd2db6e0]::query_impl::mir_built::execute_query_non_incr::__rust_end_short_backtrace
  31:     0x7f8401f043e1 - rustc_query_impl[aac33c90bd2db6e0]::query_impl::trivial_const::invoke_provider_fn::__rust_begin_short_backtrace
  32:     0x7f8401f03880 - rustc_query_impl[aac33c90bd2db6e0]::execution::try_execute_query::<rustc_middle[363116004d559c57]::query::caches::DefIdCache<rustc_middle[363116004d559c57]::query::erase::ErasedData<[u8; 32usize]>>, false>
  33:     0x7f8401f02ed0 - rustc_query_impl[aac33c90bd2db6e0]::query_impl::trivial_const::execute_query_non_incr::__rust_end_short_backtrace
  34:     0x7f8402582653 - rustc_const_eval[e3f13cd356a925f2]::const_eval::eval_queries::eval_to_const_value_raw_provider
  35:     0x7f8402582396 - rustc_query_impl[aac33c90bd2db6e0]::query_impl::eval_to_const_value_raw::invoke_provider_fn::__rust_begin_short_backtrace
  36:     0x7f84025841fe - rustc_query_impl[aac33c90bd2db6e0]::execution::try_execute_query::<rustc_middle[363116004d559c57]::query::caches::DefaultCache<rustc_middle[363116004d559c57]::ty::PseudoCanonicalInput<rustc_middle[363116004d559c57]::mir::interpret::GlobalId>, rustc_middle[363116004d559c57]::query::erase::ErasedData<[u8; 24usize]>>, false>
  37:     0x7f8402583dbf - rustc_query_impl[aac33c90bd2db6e0]::query_impl::eval_to_const_value_raw::execute_query_non_incr::__rust_end_short_backtrace
  38:     0x7f84022ca65c - <rustc_middle[363116004d559c57]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_hir_analysis[16357dcd3fcca287]::check_crate::{closure#1}>::{closure#0}
  39:     0x7f84022c9c5f - rustc_hir_analysis[16357dcd3fcca287]::check_crate
  40:     0x7f8402b29ea7 - rustc_interface[36b1173c6b6f3fe5]::passes::analysis
  41:     0x7f8402dc2809 - rustc_query_impl[aac33c90bd2db6e0]::execution::try_execute_query::<rustc_middle[363116004d559c57]::query::caches::SingleCache<rustc_middle[363116004d559c57]::query::erase::ErasedData<[u8; 0usize]>>, false>
  42:     0x7f8402dc2469 - rustc_query_impl[aac33c90bd2db6e0]::query_impl::analysis::execute_query_non_incr::__rust_end_short_backtrace
  43:     0x7f8402dab96d - rustc_interface[36b1173c6b6f3fe5]::interface::run_compiler::<(), rustc_driver_impl[757881b80e69d34]::run_compiler::{closure#0}>::{closure#1}
  44:     0x7f8402d851be - std[33dac3ce30492785]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[36b1173c6b6f3fe5]::util::run_in_thread_with_globals<rustc_interface[36b1173c6b6f3fe5]::util::run_in_thread_pool_with_globals<rustc_interface[36b1173c6b6f3fe5]::interface::run_compiler<(), rustc_driver_impl[757881b80e69d34]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  45:     0x7f8402d858ad - <std[33dac3ce30492785]::thread::lifecycle::spawn_unchecked<rustc_interface[36b1173c6b6f3fe5]::util::run_in_thread_with_globals<rustc_interface[36b1173c6b6f3fe5]::util::run_in_thread_pool_with_globals<rustc_interface[36b1173c6b6f3fe5]::interface::run_compiler<(), rustc_driver_impl[757881b80e69d34]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[bc88cc8ff231c3b1]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  46:     0x7f8402d8672c - <std[33dac3ce30492785]::sys::thread::unix::Thread>::new::thread_start
  47:     0x7f83fc8a597a - <unknown>
  48:     0x7f83fc9292bc - <unknown>
  49:                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 1.97.0-nightly (22cc6747b 2026-04-19) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z next-solver=globally -Z dump-mir-dir=dir

query stack during panic:
#0 [mir_built] building MIR for `CONST_MULTI_FN`
#1 [trivial_const] checking if `CONST_MULTI_FN` is a trivial const
#2 [eval_to_const_value_raw] simplifying constant for the type system `CONST_MULTI_FN`
#3 [analysis] running analysis passes on crate `mvce`
end of query stack
error: aborting due to 5 previous errors; 1 warning emitted

Some errors have detailed explanations: E0261, E0392, E0601.
For more information about an error, try `rustc --explain E0261`.

@rustbot label +F-lazy_type_alias +WG-trait-system-refactor

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by reproducing the ICE from the provided a.rs snippet with -Znext-solver=globally, then inspect compiler/rustc_middle/src/ty/cast.rs around lines 87-99. Trace the MIR-building path for CONST_MULTI_FN shown in the query stack and identify why the cast reaches the non-castable-types assertion. Done means the reproducer no longer panics and a regression test covers this case.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Active
Clarity
Mostly clear
Newbie friendliness
45/100

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