ICE: ` Failed to normalize Alias(Free, AliasTy {`

[matthiaskrgr]

auto-reduced (treereduce-rust):

trait Trait<T> {}
type Alias<P: Trait> = impl Trait<U>;

pub enum UninhabitedVariants {
    Tuple(Alias),
}

fn uwu(x: UninhabitedVariants) {
    match x {}
}

original:

// regression test for #127353

#![feature(WithEmplacableForFn)]
trait Trait<T> {}
type Alias<P: Trait> = impl Trait<U>;

pub enum UninhabitedVariants {
    Tuple(Alias),
    //~^ ERROR missing lifetime specifier
    //~| ERROR missing generics
}

#[define_opaque(Alias)]
fn uwu(x: UninhabitedVariants) {
    match x {}
    //~^ ERROR non-exhaustive patterns
}

fn main() {}

Version information

rustc 1.89.0-nightly (5df0f729f 2025-05-21)
binary: rustc
commit-hash: 5df0f729f5355cb3cd91f6bcff13566ae96dc220
commit-date: 2025-05-21
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.5

Possibly related line of code:

rust/compiler/rustc_middle/src/ty/normalize_erasing_regions.rs

Lines 165 to 177 in 5df0f72

	fn normalize_generic_arg_after_erasing_regions(
	&self,
	arg: ty::GenericArg<'tcx>,
	) -> ty::GenericArg<'tcx> {
	let arg = self.typing_env.as_query_input(arg);
	self.tcx.try_normalize_generic_arg_after_erasing_regions(arg).unwrap_or_else(|_| {
	bug!(
	"Failed to normalize {:?} in typing_env={:?}, \
	maybe try to call `try_normalize_erasing_regions` instead",
	arg.value,
	self.typing_env,
	)
	})

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

Program output

error[E0412]: cannot find type `U` in this scope
 --> /tmp/icemaker_global_tempdir.xe1wnEfpxqbR/rustc_testrunner_tmpdir_reporting.uicvEyZQMqkA/mvce.rs:2:35
  |
2 | type Alias<P: Trait> = impl Trait<U>;
  |            -                      ^ help: a type parameter with a similar name exists: `P`
  |            |
  |            similarly named type parameter `P` defined here

error[E0658]: `impl Trait` in type aliases is unstable
 --> /tmp/icemaker_global_tempdir.xe1wnEfpxqbR/rustc_testrunner_tmpdir_reporting.uicvEyZQMqkA/mvce.rs:2:24
  |
2 | type Alias<P: Trait> = impl Trait<U>;
  |                        ^^^^^^^^^^^^^
  |
  = note: see issue #63063 <https://github.com/rust-lang/rust/issues/63063> for more information
  = help: add `#![feature(type_alias_impl_trait)]` to the crate attributes to enable
  = note: this compiler was built on 2025-05-21; consider upgrading it if it is out of date

error[E0601]: `main` function not found in crate `mvce`
  --> /tmp/icemaker_global_tempdir.xe1wnEfpxqbR/rustc_testrunner_tmpdir_reporting.uicvEyZQMqkA/mvce.rs:10:2
   |
10 | }
   |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.xe1wnEfpxqbR/rustc_testrunner_tmpdir_reporting.uicvEyZQMqkA/mvce.rs`

error[E0107]: missing generics for trait `Trait`
 --> /tmp/icemaker_global_tempdir.xe1wnEfpxqbR/rustc_testrunner_tmpdir_reporting.uicvEyZQMqkA/mvce.rs:2:15
  |
2 | type Alias<P: Trait> = impl Trait<U>;
  |               ^^^^^ expected 1 generic argument
  |
note: trait defined here, with 1 generic parameter: `T`
 --> /tmp/icemaker_global_tempdir.xe1wnEfpxqbR/rustc_testrunner_tmpdir_reporting.uicvEyZQMqkA/mvce.rs:1:7
  |
1 | trait Trait<T> {}
  |       ^^^^^ -
help: add missing generic argument
  |
2 | type Alias<P: Trait<T>> = impl Trait<U>;
  |                    +++

error[E0107]: missing generics for type alias `Alias`
 --> /tmp/icemaker_global_tempdir.xe1wnEfpxqbR/rustc_testrunner_tmpdir_reporting.uicvEyZQMqkA/mvce.rs:5:11
  |
5 |     Tuple(Alias),
  |           ^^^^^ expected 1 generic argument
  |
note: type alias defined here, with 1 generic parameter: `P`
 --> /tmp/icemaker_global_tempdir.xe1wnEfpxqbR/rustc_testrunner_tmpdir_reporting.uicvEyZQMqkA/mvce.rs:2:6
  |
2 | type Alias<P: Trait> = impl Trait<U>;
  |      ^^^^^ -
help: add missing generic argument
  |
5 |     Tuple(Alias<P>),
  |                +++

error: unconstrained opaque type
 --> /tmp/icemaker_global_tempdir.xe1wnEfpxqbR/rustc_testrunner_tmpdir_reporting.uicvEyZQMqkA/mvce.rs:2:24
  |
2 | type Alias<P: Trait> = impl Trait<U>;
  |                        ^^^^^^^^^^^^^
  |
  = note: `Alias` must be used in combination with a concrete type within the same crate

error: internal compiler error: compiler/rustc_middle/src/ty/normalize_erasing_regions.rs:171:13: Failed to normalize Alias(Free, AliasTy { args: [{type error}], def_id: DefId(0:5 ~ mvce[5ed8]::Alias), .. }) in typing_env=TypingEnv { typing_mode: Analysis { defining_opaque_types_and_generators: [] }, param_env: ParamEnv { caller_bounds: [] } }, maybe try to call `try_normalize_erasing_regions` instead


thread 'rustc' panicked at compiler/rustc_middle/src/ty/normalize_erasing_regions.rs:171:13:
Box<dyn Any>
stack backtrace:
   0:     0x785763574943 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::hc8215a9c66aef2ea
   1:     0x785763c05a77 - core::fmt::write::h3f8319699ee957ae
   2:     0x78576507c8d1 - std::io::Write::write_fmt::h7f7ae6e4cf0613ba
   3:     0x7857635747a2 - std::sys::backtrace::BacktraceLock::print::h772c5bb915649b23
   4:     0x78576357839a - std::panicking::default_hook::{{closure}}::h5a4bb9d04f4fb636
   5:     0x785763577f1f - std::panicking::default_hook::h7a3399257665eec0
   6:     0x7857625a0003 - std[87abf682fc70726]::panicking::update_hook::<alloc[be5c6767dc16e40]::boxed::Box<rustc_driver_impl[67fa6dff91713239]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x785763578c13 - std::panicking::rust_panic_with_hook::h1d16204f4298f006
   8:     0x7857625dc471 - std[87abf682fc70726]::panicking::begin_panic::<rustc_errors[c4c2187cf855cad6]::ExplicitBug>::{closure#0}
   9:     0x7857625d04a6 - std[87abf682fc70726]::sys::backtrace::__rust_end_short_backtrace::<std[87abf682fc70726]::panicking::begin_panic<rustc_errors[c4c2187cf855cad6]::ExplicitBug>::{closure#0}, !>
  10:     0x7857625d0333 - std[87abf682fc70726]::panicking::begin_panic::<rustc_errors[c4c2187cf855cad6]::ExplicitBug>
  11:     0x7857625e6be1 - <rustc_errors[c4c2187cf855cad6]::diagnostic::BugAbort as rustc_errors[c4c2187cf855cad6]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  12:     0x785762c5611a - rustc_middle[361c9b966945ad5f]::util::bug::opt_span_bug_fmt::<rustc_span[644369ff50095af5]::span_encoding::Span>::{closure#0}
  13:     0x785762c2f67a - rustc_middle[361c9b966945ad5f]::ty::context::tls::with_opt::<rustc_middle[361c9b966945ad5f]::util::bug::opt_span_bug_fmt<rustc_span[644369ff50095af5]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  14:     0x785762c2f4eb - rustc_middle[361c9b966945ad5f]::ty::context::tls::with_context_opt::<rustc_middle[361c9b966945ad5f]::ty::context::tls::with_opt<rustc_middle[361c9b966945ad5f]::util::bug::opt_span_bug_fmt<rustc_span[644369ff50095af5]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  15:     0x78575fe8a2b0 - rustc_middle[361c9b966945ad5f]::util::bug::bug_fmt
  16:     0x78576440cc58 - <rustc_middle[361c9b966945ad5f]::ty::normalize_erasing_regions::NormalizeAfterErasingRegionsFolder as rustc_type_ir[a70c36853e4e3da4]::fold::TypeFolder<rustc_middle[361c9b966945ad5f]::ty::context::TyCtxt>>::fold_ty
  17:     0x785764c3a8bb - <rustc_pattern_analysis[1ef3719a35dbe2f]::rustc::RustcPatCtxt as rustc_pattern_analysis[1ef3719a35dbe2f]::PatCx>::ctor_sub_tys
  18:     0x785764c3a0fb - <rustc_pattern_analysis[1ef3719a35dbe2f]::pat::WitnessPat<rustc_pattern_analysis[1ef3719a35dbe2f]::rustc::RustcPatCtxt>>::wild_from_ctor
  19:     0x785763ff7eea - <rustc_pattern_analysis[1ef3719a35dbe2f]::usefulness::WitnessMatrix<rustc_pattern_analysis[1ef3719a35dbe2f]::rustc::RustcPatCtxt>>::apply_constructor
  20:     0x785763ffaa95 - rustc_pattern_analysis[1ef3719a35dbe2f]::usefulness::compute_exhaustiveness_and_usefulness::<rustc_pattern_analysis[1ef3719a35dbe2f]::rustc::RustcPatCtxt>::{closure#0}
  21:     0x7857640a6782 - rustc_pattern_analysis[1ef3719a35dbe2f]::rustc::analyze_match
  22:     0x78576444322d - <rustc_mir_build[e66f7be2fd208f8d]::thir::pattern::check_match::MatchVisitor as rustc_middle[361c9b966945ad5f]::thir::visit::Visitor>::visit_expr
  23:     0x7857644437af - <rustc_mir_build[e66f7be2fd208f8d]::thir::pattern::check_match::MatchVisitor as rustc_middle[361c9b966945ad5f]::thir::visit::Visitor>::visit_expr
  24:     0x785764442e0d - <rustc_mir_build[e66f7be2fd208f8d]::thir::pattern::check_match::MatchVisitor as rustc_middle[361c9b966945ad5f]::thir::visit::Visitor>::visit_expr
  25:     0x7857644437af - <rustc_mir_build[e66f7be2fd208f8d]::thir::pattern::check_match::MatchVisitor as rustc_middle[361c9b966945ad5f]::thir::visit::Visitor>::visit_expr
  26:     0x785764442e0d - <rustc_mir_build[e66f7be2fd208f8d]::thir::pattern::check_match::MatchVisitor as rustc_middle[361c9b966945ad5f]::thir::visit::Visitor>::visit_expr
  27:     0x785764445c70 - rustc_mir_build[e66f7be2fd208f8d]::thir::pattern::check_match::check_match
  28:     0x78576444589b - rustc_query_impl[6163178075f45bcf]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[6163178075f45bcf]::query_impl::check_match::dynamic_query::{closure#2}::{closure#0}, rustc_middle[361c9b966945ad5f]::query::erase::Erased<[u8; 1usize]>>
  29:     0x785764467825 - rustc_query_system[c023df386c88c8e7]::query::plumbing::try_execute_query::<rustc_query_impl[6163178075f45bcf]::DynamicConfig<rustc_data_structures[79350e7213dd8471]::vec_cache::VecCache<rustc_span[644369ff50095af5]::def_id::LocalDefId, rustc_middle[361c9b966945ad5f]::query::erase::Erased<[u8; 1usize]>, rustc_query_system[c023df386c88c8e7]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[6163178075f45bcf]::plumbing::QueryCtxt, false>
  30:     0x785764467461 - rustc_query_impl[6163178075f45bcf]::query_impl::check_match::get_query_non_incr::__rust_end_short_backtrace
  31:     0x7857640140ca - rustc_mir_build[e66f7be2fd208f8d]::builder::build_mir
  32:     0x785763c09db5 - rustc_mir_transform[4cfe12e99b605988]::mir_built
  33:     0x785763c09d87 - rustc_query_impl[6163178075f45bcf]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[6163178075f45bcf]::query_impl::mir_built::dynamic_query::{closure#2}::{closure#0}, rustc_middle[361c9b966945ad5f]::query::erase::Erased<[u8; 8usize]>>
  34:     0x785763dca3a6 - rustc_query_system[c023df386c88c8e7]::query::plumbing::try_execute_query::<rustc_query_impl[6163178075f45bcf]::DynamicConfig<rustc_data_structures[79350e7213dd8471]::vec_cache::VecCache<rustc_span[644369ff50095af5]::def_id::LocalDefId, rustc_middle[361c9b966945ad5f]::query::erase::Erased<[u8; 8usize]>, rustc_query_system[c023df386c88c8e7]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[6163178075f45bcf]::plumbing::QueryCtxt, false>
  35:     0x785763dc9ed2 - rustc_query_impl[6163178075f45bcf]::query_impl::mir_built::get_query_non_incr::__rust_end_short_backtrace
  36:     0x785764578b43 - rustc_mir_build[e66f7be2fd208f8d]::check_unsafety::check_unsafety
  37:     0x785764578937 - rustc_query_impl[6163178075f45bcf]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[6163178075f45bcf]::query_impl::check_unsafety::dynamic_query::{closure#2}::{closure#0}, rustc_middle[361c9b966945ad5f]::query::erase::Erased<[u8; 0usize]>>
  38:     0x785764577f45 - rustc_query_system[c023df386c88c8e7]::query::plumbing::try_execute_query::<rustc_query_impl[6163178075f45bcf]::DynamicConfig<rustc_data_structures[79350e7213dd8471]::vec_cache::VecCache<rustc_span[644369ff50095af5]::def_id::LocalDefId, rustc_middle[361c9b966945ad5f]::query::erase::Erased<[u8; 0usize]>, rustc_query_system[c023df386c88c8e7]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[6163178075f45bcf]::plumbing::QueryCtxt, false>
  39:     0x785764577c13 - rustc_query_impl[6163178075f45bcf]::query_impl::check_unsafety::get_query_non_incr::__rust_end_short_backtrace
  40:     0x785763dc8888 - <rustc_middle[361c9b966945ad5f]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface[77530f27f9e4bbc4]::passes::run_required_analyses::{closure#2}::{closure#0}>::{closure#0}
  41:     0x785763dc6bb2 - rustc_interface[77530f27f9e4bbc4]::passes::run_required_analyses
  42:     0x785764b0ca9e - rustc_interface[77530f27f9e4bbc4]::passes::analysis
  43:     0x785764b0ca75 - rustc_query_impl[6163178075f45bcf]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[6163178075f45bcf]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[361c9b966945ad5f]::query::erase::Erased<[u8; 0usize]>>
  44:     0x785764b0eb7a - rustc_query_system[c023df386c88c8e7]::query::plumbing::try_execute_query::<rustc_query_impl[6163178075f45bcf]::DynamicConfig<rustc_query_system[c023df386c88c8e7]::query::caches::SingleCache<rustc_middle[361c9b966945ad5f]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[6163178075f45bcf]::plumbing::QueryCtxt, false>
  45:     0x785764b0e84f - rustc_query_impl[6163178075f45bcf]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  46:     0x785764d5cd87 - rustc_interface[77530f27f9e4bbc4]::passes::create_and_enter_global_ctxt::<core[5f9a00d7f27d7732]::option::Option<rustc_interface[77530f27f9e4bbc4]::queries::Linker>, rustc_driver_impl[67fa6dff91713239]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  47:     0x785764c9d14f - rustc_interface[77530f27f9e4bbc4]::interface::run_compiler::<(), rustc_driver_impl[67fa6dff91713239]::run_compiler::{closure#0}>::{closure#1}
  48:     0x785764c57246 - std[87abf682fc70726]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[77530f27f9e4bbc4]::util::run_in_thread_with_globals<rustc_interface[77530f27f9e4bbc4]::util::run_in_thread_pool_with_globals<rustc_interface[77530f27f9e4bbc4]::interface::run_compiler<(), rustc_driver_impl[67fa6dff91713239]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  49:     0x785764c56eeb - <<std[87abf682fc70726]::thread::Builder>::spawn_unchecked_<rustc_interface[77530f27f9e4bbc4]::util::run_in_thread_with_globals<rustc_interface[77530f27f9e4bbc4]::util::run_in_thread_pool_with_globals<rustc_interface[77530f27f9e4bbc4]::interface::run_compiler<(), rustc_driver_impl[67fa6dff91713239]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[5f9a00d7f27d7732]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  50:     0x785764c589fd - std::sys::pal::unix::thread::Thread::new::thread_start::hcb0cd5abf7f32abf
  51:     0x78575eaa57eb - <unknown>
  52:     0x78575eb2918c - <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: rustc 1.89.0-nightly (5df0f729f 2025-05-21) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [check_match] match-checking `uwu`
#1 [mir_built] building MIR for `uwu`
#2 [check_unsafety] unsafety-checking `uwu`
#3 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 7 previous errors

Some errors have detailed explanations: E0107, E0412, E0601, E0658.
For more information about an error, try `rustc --explain E0107`.

@rustbot label +F-WithEmplacableForFn

[rustbot]

Unknown labels: F-WithEmplacableForFn

[matthiaskrgr]

trait a {}
type b<c: a> = impl a;
enum d {
    e(b),
}
fn f(g: d) {
    match g {}
}

[matthiaskrgr]

also crashes on stable

[compiler-errors]

This may have been said before, but lowercasing all the identifier names doesn't really simplify the MCVE in a meaningful way, and on the contrary actually makes it harder to reason about since although capitalization in Rust is not semantic it does carry a lot of pragmatic weight.

Furthermore, it anonymizes the fact that this test was originally reduced from a regression test for #127353, which is useful for whoever is responsible for fixing and filing a new test for this bug.

An idea: It would be very cool to change whatever minimizer you're using to have capitalization-sensitive name reduction, or just skip sharing the "identifier reduced" version of the code at all unless the identifiers are really that misleading or complicated.

[matthiaskrgr]

I like it because it reduces the mvce into something like a Logical Form, different reproducers may still boil down to the same LF and this helps us spot and avoid duplicates.

All the info (var names, comments etc) is still contained in the topmost report though, you could look at that, too.

[matthiaskrgr]

trait Trait;
type Alias<P : Trait> = impl;
enum UninhabitedVariants { Tuple(Alias) } fn uwu(x : UninhabitedVariants) {
  match x {}
}

creduce --timeout 10 --remove-pass pass_clang rename-fun --remove-pass pass_clang rename-param --remove-pass pass_clang rename-var --remove-pass pass_clang rename-class --remove-pass pass_clang rename-cxx-method --remove-pass pass_clex rename-toks script.sh ./code.rs