rust-lang / rust-lang/rust

ICE: failed to normalize in `build_adt_ctor` with TAIT

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C-bug F-type_alias_impl_trait I-ICE S-bug-has-test T-compiler
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Rust
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Description

Code

playground link

#![feature(type_alias_impl_trait)]

use std::marker::PhantomData;

trait Project1 {
    type Assoc1;
}

impl<T> Project1 for T {
    type Assoc1 = ();
}

trait Project2 {
    type Assoc2;
}

impl<T: Project1<Assoc1 = ()>> Project2 for PhantomData<T> {
    type Assoc2 = ();
}

type Alias<T> = impl Project2;

fn constrain<T>() -> Alias<T> {
    PhantomData::<T>
}

struct AdtConstructor<T: Project1>(<Alias<T> as Project2>::Assoc2);

Here, the hidden type of Alias<T> is PhantomData<T> and normalizing <PhantomData<T> as Project2>::Assoc2 requires PhantomData<T>: Project2, which in turn requires T: Project1<Assoc1 = ()>. This is satisfied by the blanket impl<T> Project1 for T, but due to the T: Project1 bound, we do not normalize <T as Project1>::Assoc1 via the impl and so PhantomData<T>: Project2 does not hold and we get a normalization error.

Meta

rustc --version --verbose:

Build using the Nightly version: 1.86.0-nightly

(2025-02-13 a567209daab72b7ea59e)
Error output
error: internal compiler error: compiler/rustc_middle/src/ty/normalize_erasing_regions.rs:169:13: Failed to normalize Alias(Projection, AliasTy { args: [Alias(Weak, AliasTy { args: [T/#0], def_id: DefId(0:14 ~ playground[da1e]::Alias), .. })], def_id: DefId(0:10 ~ playground[da1e]::Project2::Assoc2), .. }) in typing_env=TypingEnv { typing_mode: PostAnalysis, param_env: ParamEnv { caller_bounds: [Binder { value: TraitPredicate(<T as Project1>, polarity:Positive), bound_vars: [] }, Binder { value: TraitPredicate(<T as std::marker::Sized>, polarity:Positive), bound_vars: [] }] } }, maybe try to call `try_normalize_erasing_regions` instead
Backtrace 

thread 'rustc' panicked at compiler/rustc_middle/src/ty/normalize_erasing_regions.rs:169:13:
Box<dyn Any>
stack backtrace:
   0:     0x7017b2440480 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::hacb3576dbc1be98e
   1:     0x7017b2c289bc - core::fmt::write::hc16e8be59a3a9fc5
   2:     0x7017b3f30f91 - std::io::Write::write_fmt::h8811cb6436262be0
   3:     0x7017b24402e2 - std::sys::backtrace::BacktraceLock::print::h0d789c81c6c2ec20
   4:     0x7017b2442762 - std::panicking::default_hook::{{closure}}::h493936f4b23e5565
   5:     0x7017b24425ea - std::panicking::default_hook::he1a5dbaa09c20e90
   6:     0x7017b15a7f79 - std[d810b76c7c5d023f]::panicking::update_hook::<alloc[c651bf31249c3228]::boxed::Box<rustc_driver_impl[d81ea691c08f90cf]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x7017b24432e3 - std::panicking::rust_panic_with_hook::hfbf782e335191cee
   8:     0x7017b15e2e21 - std[d810b76c7c5d023f]::panicking::begin_panic::<rustc_errors[d9109ad345debeea]::ExplicitBug>::{closure#0}
   9:     0x7017b15d7d96 - std[d810b76c7c5d023f]::sys::backtrace::__rust_end_short_backtrace::<std[d810b76c7c5d023f]::panicking::begin_panic<rustc_errors[d9109ad345debeea]::ExplicitBug>::{closure#0}, !>
  10:     0x7017b15d7d7d - std[d810b76c7c5d023f]::panicking::begin_panic::<rustc_errors[d9109ad345debeea]::ExplicitBug>
  11:     0x7017b15ecdf1 - <rustc_errors[d9109ad345debeea]::diagnostic::BugAbort as rustc_errors[d9109ad345debeea]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  12:     0x7017b1bc3bd3 - rustc_middle[4cc25c41f6c964b6]::util::bug::opt_span_bug_fmt::<rustc_span[b2d7632da7a4062]::span_encoding::Span>::{closure#0}
  13:     0x7017b1baca1a - rustc_middle[4cc25c41f6c964b6]::ty::context::tls::with_opt::<rustc_middle[4cc25c41f6c964b6]::util::bug::opt_span_bug_fmt<rustc_span[b2d7632da7a4062]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  14:     0x7017b1bac8ab - rustc_middle[4cc25c41f6c964b6]::ty::context::tls::with_context_opt::<rustc_middle[4cc25c41f6c964b6]::ty::context::tls::with_opt<rustc_middle[4cc25c41f6c964b6]::util::bug::opt_span_bug_fmt<rustc_span[b2d7632da7a4062]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  15:     0x7017af908d20 - rustc_middle[4cc25c41f6c964b6]::util::bug::bug_fmt
  16:     0x7017b3073998 - <rustc_middle[4cc25c41f6c964b6]::ty::normalize_erasing_regions::NormalizeAfterErasingRegionsFolder as rustc_type_ir[ef0ec1dfcf4e1bc9]::fold::TypeFolder<rustc_middle[4cc25c41f6c964b6]::ty::context::TyCtxt>>::fold_ty
  17:     0x7017b356c3c3 - rustc_mir_transform[15a268890479543d]::shim::build_adt_ctor
  18:     0x7017b3235286 - rustc_mir_transform[15a268890479543d]::optimized_mir
  19:     0x7017b3234a0d - rustc_query_impl[9cdc7c18babae614]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[9cdc7c18babae614]::query_impl::optimized_mir::dynamic_query::{closure#2}::{closure#0}, rustc_middle[4cc25c41f6c964b6]::query::erase::Erased<[u8; 8usize]>>
  20:     0x7017b2e6ee4c - rustc_query_system[40979d04b54c932c]::query::plumbing::try_execute_query::<rustc_query_impl[9cdc7c18babae614]::DynamicConfig<rustc_query_system[40979d04b54c932c]::query::caches::DefIdCache<rustc_middle[4cc25c41f6c964b6]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[9cdc7c18babae614]::plumbing::QueryCtxt, false>
  21:     0x7017b2e6e25f - rustc_query_impl[9cdc7c18babae614]::query_impl::optimized_mir::get_query_non_incr::__rust_end_short_backtrace
  22:     0x7017b37d0b69 - rustc_middle[4cc25c41f6c964b6]::query::plumbing::query_get_at::<rustc_query_system[40979d04b54c932c]::query::caches::DefIdCache<rustc_middle[4cc25c41f6c964b6]::query::erase::Erased<[u8; 8usize]>>>
  23:     0x7017b37d7f2a - <rustc_metadata[b4caab51ca970add]::rmeta::encoder::EncodeContext>::encode_crate_root
  24:     0x7017b3c0450e - rustc_metadata[b4caab51ca970add]::rmeta::encoder::encode_metadata
  25:     0x7017b3c125bc - rustc_metadata[b4caab51ca970add]::fs::encode_and_write_metadata
  26:     0x7017b3c1148e - <rustc_interface[1654fa466b78c029]::queries::Linker>::codegen_and_build_linker
  27:     0x7017b3c07ddd - rustc_interface[1654fa466b78c029]::passes::create_and_enter_global_ctxt::<core[79df64e2219b6595]::option::Option<rustc_interface[1654fa466b78c029]::queries::Linker>, rustc_driver_impl[d81ea691c08f90cf]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  28:     0x7017b3b578a6 - rustc_interface[1654fa466b78c029]::interface::run_compiler::<(), rustc_driver_impl[d81ea691c08f90cf]::run_compiler::{closure#0}>::{closure#1}
  29:     0x7017b3ac8cc4 - std[d810b76c7c5d023f]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[1654fa466b78c029]::util::run_in_thread_with_globals<rustc_interface[1654fa466b78c029]::util::run_in_thread_pool_with_globals<rustc_interface[1654fa466b78c029]::interface::run_compiler<(), rustc_driver_impl[d81ea691c08f90cf]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  30:     0x7017b3ac8999 - <<std[d810b76c7c5d023f]::thread::Builder>::spawn_unchecked_<rustc_interface[1654fa466b78c029]::util::run_in_thread_with_globals<rustc_interface[1654fa466b78c029]::util::run_in_thread_pool_with_globals<rustc_interface[1654fa466b78c029]::interface::run_compiler<(), rustc_driver_impl[d81ea691c08f90cf]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[79df64e2219b6595]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  31:     0x7017b3ac812f - std::sys::pal::unix::thread::Thread::new::thread_start::ha84bd1be5e559000
  32:     0x7017b51dfa94 - <unknown>
  33:     0x7017b526ca34 - clone
  34:                0x0 - <unknown>

Caused by this normalize call here: https://github.com/rust-lang/rust/blob/d8810e3e2dab96778d20dd6d746ff95465515509/compiler/rustc_mir_transform/src/shim.rs#L978

@rustbot label F-type_alias_impl_trait

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 compiling the linked playground reproducer with the nightly toolchain, then inspect compiler/rustc_mir_transform/src/shim.rs around line 978 and the build_adt_ctor entry shown in the backtrace. Done means the TAIT example no longer triggers an internal compiler error or normalization failure, with regression coverage for the reproducer.

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
38/100

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