ICE: failed to normalize in `build_adt_ctor` with TAIT

[lukas-code]

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:

rust/compiler/rustc_mir_transform/src/shim.rs

Line 978 in d8810e3

let sig = tcx.normalize_erasing_regions(typing_env, sig);

@rustbot label F-type_alias_impl_trait