rust-lang / rust-lang/rust

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

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C-bug F-type_alias_impl_trait I-ICE T-compiler
Dominant language
Rust
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Description

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:
https://github.com/rust-lang/rust/blob/5df0f729f5355cb3cd91f6bcff13566ae96dc220/compiler/rustc_middle/src/ty/normalize_erasing_regions.rs#L165-L177

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  = note: this compiler was built on 2025-05-21; consider upgrading it if it is out of date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

Contributor guide

Open the contributing guide

First steps

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  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

Compile the reduced reproducer with the reported nightly, then inspect compiler/rustc_middle/src/ty/normalize_erasing_regions.rs around line 171 and the rustc_pattern_analysis stack frames. Trace how match exhaustiveness analysis reaches normalization for the invalid Alias type. Done means the reproducer emits diagnostics without an internal compiler error.

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
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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