rust-lang / rust-lang/rust

ICE: `When translating generic parameters from DefId, the expected specialization failed to hold`

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C-bug F-specialization fixed-by-next-solver I-ICE S-bug-has-test S-has-mcve T-compiler
Dominant language
Rust
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Description

auto-reduced (treereduce-rust):

#![feature(specialization)]

trait Spec {
    type Assoc;
}

default impl<T, U> Spec for T
where
    T: IntoIterator<Item = U>,
{
    type Assoc = U;
}

impl<T> Spec for [T; 0] {}

fn main() {
    let x: <[_; 0] as Spec>::Assoc = 1;
}

original:

//@ revisions: current next
//@ ignore-compare-mode-next-solver (explicit revisions)
//@[next] compile-flags: -Znext-solver
//@[next] check-pass
//@[current] known-bug: unknown
//@[current] failure-status: 101
//~^ ERROR defaults for generic parameters are not allowed in `for<...>` binders

// Tests that rebasing from the concrete impl to the default impl also processes the
// `[u32; 0]: IntoIterator<Item = ?U>` predicate to constrain the `?U` impl arg.
// This test also makes sure that we don't do anything weird when rebasing the args
// is ambiguous.

#![feature(specialization)]
//[next]~^ WARN the feature `specialization` is incomplete

trait Spec {
    type Assoc;
}

default impl<T, U> Spec for T where T: IntoIterator<Item = U> {
    type Assoc = U;
}

impl<T> Spec for [T; 0] {}

fn main() {
    let x: <[_; 0] as Spec>::Assoc = 1;
}

Version information

rustc 1.84.0-nightly (ef972a346 2024-11-02)
binary: rustc
commit-hash: ef972a346668ed4234d1a43ed4ad7ca4e9c58d51
commit-date: 2024-11-02
host: x86_64-unknown-linux-gnu
release: 1.84.0-nightly
LLVM version: 19.1.1

Possibly related line of code:
https://github.com/rust-lang/rust/blob/ef972a346668ed4234d1a43ed4ad7ca4e9c58d51/compiler/rustc_trait_selection/src/traits/specialize/mod.rs#L121-L133

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

Program output 

warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
 --> /tmp/icemaker_global_tempdir.jxbDO1GMyC8v/rustc_testrunner_tmpdir_reporting.X0a6Q55Ph3Zn/mvce.rs:1:12
  |
1 | #![feature(specialization)]
  |            ^^^^^^^^^^^^^^
  |
  = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
  = help: consider using `min_specialization` instead, which is more stable and complete
  = note: `#[warn(incomplete_features)]` on by default

error: internal compiler error: compiler/rustc_trait_selection/src/traits/specialize/mod.rs:127:21: When translating generic parameters from DefId(0:9 ~ mvce[4f49]::{impl#1}) to DefId(0:5 ~ mvce[4f49]::{impl#0}), the expected specialization failed to hold

thread 'rustc' panicked at compiler/rustc_trait_selection/src/traits/specialize/mod.rs:127:21:
Box<dyn Any>
stack backtrace:
   0:     0x7737134584aa - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::hce429173f4835cf6
   1:     0x773713c040ca - core::fmt::write::hd474fbdfe63c8008
   2:     0x77371507b011 - std::io::Write::write_fmt::ha6b1be51a74d6c82
   3:     0x773713458302 - std::sys::backtrace::BacktraceLock::print::h9a37e75f7192ac21
   4:     0x77371345a806 - std::panicking::default_hook::{{closure}}::hd9ac8cf20a0b7897
   5:     0x77371345a650 - std::panicking::default_hook::hdde16dcfa738c4aa
   6:     0x7737124ddb49 - std[85c38ff02e03f0e6]::panicking::update_hook::<alloc[39d8aefd6e9d2d6]::boxed::Box<rustc_driver_impl[8ea1370c87ac4c52]::install_ice_hook::{closure#0}>>::{closure#0}
   7:     0x77371345af18 - std::panicking::rust_panic_with_hook::h6eef9cc88a2d4e79
   8:     0x773712517231 - std[85c38ff02e03f0e6]::panicking::begin_panic::<rustc_errors[a3358f633eebb703]::ExplicitBug>::{closure#0}
   9:     0x77371250a206 - std[85c38ff02e03f0e6]::sys::backtrace::__rust_end_short_backtrace::<std[85c38ff02e03f0e6]::panicking::begin_panic<rustc_errors[a3358f633eebb703]::ExplicitBug>::{closure#0}, !>
  10:     0x773712505829 - std[85c38ff02e03f0e6]::panicking::begin_panic::<rustc_errors[a3358f633eebb703]::ExplicitBug>
  11:     0x773712520e01 - <rustc_errors[a3358f633eebb703]::diagnostic::BugAbort as rustc_errors[a3358f633eebb703]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  12:     0x773712b9a363 - rustc_middle[b16c40c3cb132ea9]::util::bug::opt_span_bug_fmt::<rustc_span[94bdfa138cad1541]::span_encoding::Span>::{closure#0}
  13:     0x773712b8095a - rustc_middle[b16c40c3cb132ea9]::ty::context::tls::with_opt::<rustc_middle[b16c40c3cb132ea9]::util::bug::opt_span_bug_fmt<rustc_span[94bdfa138cad1541]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  14:     0x773712b807eb - rustc_middle[b16c40c3cb132ea9]::ty::context::tls::with_context_opt::<rustc_middle[b16c40c3cb132ea9]::ty::context::tls::with_opt<rustc_middle[b16c40c3cb132ea9]::util::bug::opt_span_bug_fmt<rustc_span[94bdfa138cad1541]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  15:     0x773710ccf260 - rustc_middle[b16c40c3cb132ea9]::util::bug::bug_fmt
  16:     0x773713c65283 - rustc_trait_selection[7d6948a7a05ac95]::traits::specialize::translate_args_with_cause::<rustc_trait_selection[7d6948a7a05ac95]::traits::specialize::translate_args::{closure#0}>
  17:     0x77371469a013 - rustc_trait_selection[7d6948a7a05ac95]::traits::project::opt_normalize_projection_term
  18:     0x7737146931df - <rustc_trait_selection[7d6948a7a05ac95]::traits::normalize::AssocTypeNormalizer as rustc_type_ir[45f01c5603667543]::fold::TypeFolder<rustc_middle[b16c40c3cb132ea9]::ty::context::TyCtxt>>::fold_ty
  19:     0x773713f06b53 - <rustc_hir_typeck[643602d1ca0da9fa]::fn_ctxt::FnCtxt>::normalize::<rustc_middle[b16c40c3cb132ea9]::ty::Ty>
  20:     0x7737145c13fb - <dyn rustc_hir_analysis[614b5c894f29c678]::hir_ty_lowering::HirTyLowerer>::lower_ty
  21:     0x7737147b4fa9 - <rustc_hir_typeck[643602d1ca0da9fa]::gather_locals::GatherLocalsVisitor>::declare
  22:     0x773713f2336d - <rustc_hir_typeck[643602d1ca0da9fa]::gather_locals::GatherLocalsVisitor as rustc_hir[885137f5f5b6a9f]::intravisit::Visitor>::visit_expr
  23:     0x773713f24f19 - rustc_hir_typeck[643602d1ca0da9fa]::check::check_fn
  24:     0x773713f1b2f5 - rustc_hir_typeck[643602d1ca0da9fa]::typeck
  25:     0x773713f1ac93 - rustc_query_impl[c0318c643aee4f05]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[c0318c643aee4f05]::query_impl::typeck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b16c40c3cb132ea9]::query::erase::Erased<[u8; 8usize]>>
  26:     0x773714321d81 - rustc_query_system[708ef94f36d60214]::query::plumbing::try_execute_query::<rustc_query_impl[c0318c643aee4f05]::DynamicConfig<rustc_query_system[708ef94f36d60214]::query::caches::VecCache<rustc_span[94bdfa138cad1541]::def_id::LocalDefId, rustc_middle[b16c40c3cb132ea9]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[c0318c643aee4f05]::plumbing::QueryCtxt, false>
  27:     0x77371432024d - rustc_query_impl[c0318c643aee4f05]::query_impl::typeck::get_query_non_incr::__rust_end_short_backtrace
  28:     0x77371431fec7 - <rustc_middle[b16c40c3cb132ea9]::hir::map::Map>::par_body_owners::<rustc_hir_analysis[614b5c894f29c678]::check_crate::{closure#4}>::{closure#0}
  29:     0x77371431de99 - rustc_hir_analysis[614b5c894f29c678]::check_crate
  30:     0x7737140e384a - rustc_interface[46946d42a40348f7]::passes::run_required_analyses
  31:     0x7737147d451e - rustc_interface[46946d42a40348f7]::passes::analysis
  32:     0x7737147d44ef - rustc_query_impl[c0318c643aee4f05]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[c0318c643aee4f05]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b16c40c3cb132ea9]::query::erase::Erased<[u8; 1usize]>>
  33:     0x773714ba1bae - rustc_query_system[708ef94f36d60214]::query::plumbing::try_execute_query::<rustc_query_impl[c0318c643aee4f05]::DynamicConfig<rustc_query_system[708ef94f36d60214]::query::caches::SingleCache<rustc_middle[b16c40c3cb132ea9]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[c0318c643aee4f05]::plumbing::QueryCtxt, false>
  34:     0x773714ba188e - rustc_query_impl[c0318c643aee4f05]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  35:     0x773714a65292 - rustc_interface[46946d42a40348f7]::interface::run_compiler::<core[d9ceb9bc2a384707]::result::Result<(), rustc_span[94bdfa138cad1541]::ErrorGuaranteed>, rustc_driver_impl[8ea1370c87ac4c52]::run_compiler::{closure#0}>::{closure#1}
  36:     0x773714af0590 - std[85c38ff02e03f0e6]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[46946d42a40348f7]::util::run_in_thread_with_globals<rustc_interface[46946d42a40348f7]::util::run_in_thread_pool_with_globals<rustc_interface[46946d42a40348f7]::interface::run_compiler<core[d9ceb9bc2a384707]::result::Result<(), rustc_span[94bdfa138cad1541]::ErrorGuaranteed>, rustc_driver_impl[8ea1370c87ac4c52]::run_compiler::{closure#0}>::{closure#1}, core[d9ceb9bc2a384707]::result::Result<(), rustc_span[94bdfa138cad1541]::ErrorGuaranteed>>::{closure#0}, core[d9ceb9bc2a384707]::result::Result<(), rustc_span[94bdfa138cad1541]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[d9ceb9bc2a384707]::result::Result<(), rustc_span[94bdfa138cad1541]::ErrorGuaranteed>>
  37:     0x773714af09ab - <<std[85c38ff02e03f0e6]::thread::Builder>::spawn_unchecked_<rustc_interface[46946d42a40348f7]::util::run_in_thread_with_globals<rustc_interface[46946d42a40348f7]::util::run_in_thread_pool_with_globals<rustc_interface[46946d42a40348f7]::interface::run_compiler<core[d9ceb9bc2a384707]::result::Result<(), rustc_span[94bdfa138cad1541]::ErrorGuaranteed>, rustc_driver_impl[8ea1370c87ac4c52]::run_compiler::{closure#0}>::{closure#1}, core[d9ceb9bc2a384707]::result::Result<(), rustc_span[94bdfa138cad1541]::ErrorGuaranteed>>::{closure#0}, core[d9ceb9bc2a384707]::result::Result<(), rustc_span[94bdfa138cad1541]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[d9ceb9bc2a384707]::result::Result<(), rustc_span[94bdfa138cad1541]::ErrorGuaranteed>>::{closure#1} as core[d9ceb9bc2a384707]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  38:     0x773714af1479 - std::sys::pal::unix::thread::Thread::new::thread_start::h00969398ad9e9953
  39:     0x77371631139d - <unknown>
  40:     0x77371639649c - <unknown>
  41:                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 1.84.0-nightly (ef972a346 2024-11-02) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [typeck] type-checking `main`
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 1 previous error; 1 warning emitted

@rustbot label +F-specialization

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 with the reduced reproducer and run it using the reported specialization feature and compiler flags. Read compiler/rustc_trait_selection/src/traits/specialize/mod.rs around lines 121-133, then compare the current and next-solver outcomes; done means the reproducer no longer triggers an ICE while preserving the expected check-pass behavior.

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
Needs clarification
Newbie friendliness
28/100

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