rust-lang / rust-lang/rust

Hang in next-solver: exponential blowup due to rerunning on changed provisional result

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A-trait-system C-bug needs-triage T-compiler WG-trait-system-refactor
Dominant language
Rust
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Description

I tried this code: (auto-reduced)

trait Trait {}
trait Noop {
    type Assoc: ?Sized + ;
}
impl<T: ?Sized> Noop for T
where
    <NoopNewtype<T> as Noop>::Assoc: Noop,
{
    type Assoc = T;
}
struct NoopNewtype<T: ?Sized + Noop>(T::Assoc);
fn coerce_newtype<>() -> &NoopNewtype< Trait + '_> {
    x
}
impl<T> Noop for NoopNewtype<T> {}
fn main(){}
Release channel Result
Current Stable error
Current Nightly (default solver) error
Current Nightly (+ -Znext-solver=globally) Hang
Meta

rustc --version --verbose:

rustc 1.95.0-nightly (d222ddc4d 2026-01-23)
binary: rustc
commit-hash: d222ddc4d90743dfc1e53b610be8fc9d95893d2c
commit-date: 2026-01-23
host: x86_64-unknown-linux-gnu
release: 1.95.0-nightly
LLVM version: 21.1.8
Backtrace 

0  0x00007ffff775d39c in _RNCNCNvMs8_NtCs2qe7WCxsRC0_13rustc_type_ir12search_graphINtB9_11SearchGraphINtNtNtCsi7ETX5LXJIo_23rustc_next_trait_solver5solve12search_graph19SearchGraphDelegateNtNtNtCsa5muQJGXFHf_21rustc_trait_selection5solve8delegate14SolverDelegateENtNtNtCs4jqnZnYemQt_12rustc_middle2ty7context6TyCtxtE32rebase_provisional_cache_entries00B2G_.cold () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#0  0x00007ffff775d39c in _RNCNCNvMs8_NtCs2qe7WCxsRC0_13rustc_type_ir12search_graphINtB9_11SearchGraphINtNtNtCsi7ETX5LXJIo_23rustc_next_trait_solver5solve12search_graph19SearchGraphDelegateNtNtNtCsa5muQJGXFHf_21rustc_trait_selection5solve8delegate14SolverDelegateENtNtNtCs4jqnZnYemQt_12rustc_middle2ty7context6TyCtxtE32rebase_provisional_cache_entries00B2G_.cold () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#1  0x00007ffff6cd87a6 in <rustc_type_ir::search_graph::SearchGraph<rustc_next_trait_solver::solve::search_graph::SearchGraphDelegate<rustc_trait_selection::solve::delegate::SolverDelegate>, rustc_middle::ty::context::TyCtxt>>::rebase_provisional_cache_entries () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#2  0x00007ffff619e73e in _RNvMs0_NtNtCsi7ETX5LXJIo_23rustc_next_trait_solver5solve9eval_ctxtINtB5_8EvalCtxtNtNtNtCsa5muQJGXFHf_21rustc_trait_selection5solve8delegate14SolverDelegateNtNtNtCs4jqnZnYemQt_12rustc_middle2ty7context6TyCtxtE17evaluate_goal_rawB1n_.llvm.3737103394224460173 () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#3  0x00007ffff61a62a5 in <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::try_evaluate_added_goals () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#4  0x00007ffff6486fbe in <rustc_next_trait_solver::solve::search_graph::SearchGraphDelegate<rustc_trait_selection::solve::delegate::SolverDelegate> as rustc_type_ir::search_graph::Delegate>::compute_goal::{closure#0} () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#5  0x00007ffff619b763 in _RNvMs0_NtNtCsi7ETX5LXJIo_23rustc_next_trait_solver5solve9eval_ctxtINtB5_8EvalCtxtNtNtNtCsa5muQJGXFHf_21rustc_trait_selection5solve8delegate14SolverDelegateNtNtNtCs4jqnZnYemQt_12rustc_middle2ty7context6TyCtxtE17evaluate_goal_rawB1n_.llvm.3737103394224460173 () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#6  0x00007ffff61a5ef4 in <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::try_evaluate_added_goals () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#7  0x00007ffff61bf0cf in <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::evaluate_added_goals_and_make_canonical_response () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#8  0x00007ffff6484492 in <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::assemble_impl_candidates::<rustc_type_ir::predicate::TraitPredicate<rustc_middle::ty::context::TyCtxt>>::{closure#0} () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#9  0x00007ffff648e0de in <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::compute_trait_goal () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#10 0x00007ffff64878f8 in <rustc_next_trait_solver::solve::search_graph::SearchGraphDelegate<rustc_trait_selection::solve::delegate::SolverDelegate> as rustc_type_ir::search_graph::Delegate>::compute_goal::{closure#0} () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#11 0x00007ffff619b763 in _RNvMs0_NtNtCsi7ETX5LXJIo_23rustc_next_trait_solver5solve9eval_ctxtINtB5_8EvalCtxtNtNtNtCsa5muQJGXFHf_21rustc_trait_selection5solve8delegate14SolverDelegateNtNtNtCs4jqnZnYemQt_12rustc_middle2ty7context6TyCtxtE17evaluate_goal_rawB1n_.llvm.3737103394224460173 () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#12 0x00007ffff61a62a5 in <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::try_evaluate_added_goals () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#13 0x00007ffff6486fbe in <rustc_next_trait_solver::solve::search_graph::SearchGraphDelegate<rustc_trait_selection::solve::delegate::SolverDelegate> as rustc_type_ir::search_graph::Delegate>::compute_goal::{closure#0} () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#14 0x00007ffff619b763 in _RNvMs0_NtNtCsi7ETX5LXJIo_23rustc_next_trait_solver5solve9eval_ctxtINtB5_8EvalCtxtNtNtNtCsa5muQJGXFHf_21rustc_trait_selection5solve8delegate14SolverDelegateNtNtNtCs4jqnZnYemQt_12rustc_middle2ty7context6TyCtxtE17evaluate_goal_rawB1n_.llvm.3737103394224460173 () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#15 0x00007ffff61a5ef4 in <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::try_evaluate_added_goals () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#16 0x00007ffff61bf0cf in <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::evaluate_added_goals_and_make_canonical_response () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#17 0x00007ffff6484492 in <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::assemble_impl_candidates::<rustc_type_ir::predicate::TraitPredicate<rustc_middle::ty::context::TyCtxt>>::{closure#0} () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#18 0x00007ffff648e0de in <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::compute_trait_goal () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#19 0x00007ffff6486b0f in <rustc_next_trait_solver::solve::search_graph::SearchGraphDelegate<rustc_trait_selection::solve::delegate::SolverDelegate> as rustc_type_ir::search_graph::Delegate>::compute_goal::{closure#0} () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#20 0x00007ffff619b763 in _RNvMs0_NtNtCsi7ETX5LXJIo_23rustc_next_trait_solver5solve9eval_ctxtINtB5_8EvalCtxtNtNtNtCsa5muQJGXFHf_21rustc_trait_selection5solve8delegate14SolverDelegateNtNtNtCs4jqnZnYemQt_12rustc_middle2ty7context6TyCtxtE17evaluate_goal_rawB1n_.llvm.3737103394224460173 () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#21 0x00007ffff61a5ef4 in <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::try_evaluate_added_goals () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#22 0x00007ffff61bf0cf in <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::evaluate_added_goals_and_make_canonical_response () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#23 0x00007ffff648711a in <rustc_next_trait_solver::solve::search_graph::SearchGraphDelegate<rustc_trait_selection::solve::delegate::SolverDelegate> as rustc_type_ir::search_graph::Delegate>::compute_goal::{closure#0} () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#24 0x00007ffff619b763 in _RNvMs0_NtNtCsi7ETX5LXJIo_23rustc_next_trait_solver5solve9eval_ctxtINtB5_8EvalCtxtNtNtNtCsa5muQJGXFHf_21rustc_trait_selection5solve8delegate14SolverDelegateNtNtNtCs4jqnZnYemQt_12rustc_middle2ty7context6TyCtxtE17evaluate_goal_rawB1n_.llvm.3737103394224460173 () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#25 0x00007ffff61a5ef4 in <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::try_evaluate_added_goals () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#26 0x00007ffff61bf0cf in <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::evaluate_added_goals_and_make_canonical_response () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#27 0x00007ffff648a69c in <rustc_type_ir::predicate::TraitPredicate<rustc_middle::ty::context::TyCtxt> as rustc_next_trait_solver::solve::assembly::GoalKind<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::consider_builtin_sizedness_candidates () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#28 0x00007ffff649792e in <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::assemble_builtin_impl_candidates::<rustc_type_ir::predicate::TraitPredicate<rustc_middle::ty::context::TyCtxt>> () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so
#29 0x00007ffff648d01b in <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::compute_trait_goal () from /home/laix/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-e5a7829eabab3f13.so

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 running the minimized Rust program with nightly and -Znext-solver=globally, comparing it with the default solver. Trace the next-solver entry points named in the backtrace, especially SearchGraph::rebase_provisional_cache_entries and evaluate_goal_raw; done means the reproducer no longer hangs or exhibits exponential blowup.

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
Quiet
Clarity
Needs clarification
Newbie friendliness
38/100

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