[ICE]: could not replace AliasTerm

[gj]

Code

pub trait Helper { type Out; }
impl<T: Send> Helper for Option<T> { type Out = (); }

pub trait Callback: Fn(<Option<String> as Helper>::Out) {
    fn method(&self);
}

pub fn trigger(f: &dyn Callback) {
    f.method();
}

Meta

rustc --version --verbose:

rustc 1.97.0 (2d8144b78 2026-07-07)
binary: rustc
commit-hash: 2d8144b7880597b6e6d3dfd63a9a9efae3f533d3
commit-date: 2026-07-07
host: aarch64-apple-darwin
release: 1.97.0
LLVM version: 22.1.6

rustc 1.97.1 (8bab26f4f 2026-07-14)
binary: rustc
commit-hash: 8bab26f4f68e0e26f0bb7960be334d5b520ea452
commit-date: 2026-07-14
host: aarch64-apple-darwin
release: 1.97.1
LLVM version: 22.1.6

rustc 1.99.0-nightly (da80ed070 2026-07-14)
binary: rustc
commit-hash: da80ed0708a09dc096c184345d6eb42cbcd50a1e
commit-date: 2026-07-14
host: aarch64-apple-darwin
release: 1.99.0-nightly
LLVM version: 22.1.8

Panics on 1.97.0 stable, 1.97.1 stable, and nightly; compiles fine on 1.96.1 stable.

Error output

❯ RUST_BACKTRACE=1 rustc --edition 2024 --crate-type lib repro.rs

thread 'rustc' (23080133) panicked at /rustc-dev/2d8144b7880597b6e6d3dfd63a9a9efae3f533d3/compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs:1005:13:
could not replace AliasTerm { args: [dyn [Binder { value: Trait(Callback), bound_vars: [] }, Binder { value: Projection(ExistentialProjection { def_id: DefId(2:4448 ~ core[eea8]::ops::function::FnOnce::Output), args: [((),)], term: Term::Ty(()), .. }), bound_vars: [] }] + '!0, (Alias(AliasTy { args: [std::option::Option<std::string::String>], kind: Projection { def_id: DefId(0:4 ~ repro[1d8e]::Helper::Out) }, .. }),)], kind: ProjectionTy { def_id: DefId(2:4448 ~ core[eea8]::ops::function::FnOnce::Output) }, .. } with term from from dyn [Binder { value: Trait(Callback), bound_vars: [] }, Binder { value: Projection(ExistentialProjection { def_id: DefId(2:4448 ~ core[eea8]::ops::function::FnOnce::Output), args: [((),)], term: Term::Ty(()), .. }), bound_vars: [] }] + '!0

Backtrace

❯ RUST_BACKTRACE=1 rustc --edition 2024 --crate-type lib repro.rs

thread 'rustc' (23080133) panicked at /rustc-dev/2d8144b7880597b6e6d3dfd63a9a9efae3f533d3/compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs:1005:13:
could not replace AliasTerm { args: [dyn [Binder { value: Trait(Callback), bound_vars: [] }, Binder { value: Projection(ExistentialProjection { def_id: DefId(2:4448 ~ core[eea8]::ops::function::FnOnce::Output), args: [((),)], term: Term::Ty(()), .. }), bound_vars: [] }] + '!0, (Alias(AliasTy { args: [std::option::Option<std::string::String>], kind: Projection { def_id: DefId(0:4 ~ repro[1d8e]::Helper::Out) }, .. }),)], kind: ProjectionTy { def_id: DefId(2:4448 ~ core[eea8]::ops::function::FnOnce::Output) }, .. } with term from from dyn [Binder { value: Trait(Callback), bound_vars: [] }, Binder { value: Projection(ExistentialProjection { def_id: DefId(2:4448 ~ core[eea8]::ops::function::FnOnce::Output), args: [((),)], term: Term::Ty(()), .. }), bound_vars: [] }] + '!0
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: <rustc_next_trait_solver::solve::assembly::structural_traits::ReplaceProjectionWith<rustc_middle::ty::context::TyCtxt, rustc_trait_selection::solve::delegate::SolverDelegate>>::try_eagerly_replace_alias
   3: <rustc_middle::ty::generic_args::GenericArg as rustc_type_ir::fold::TypeFoldable<rustc_middle::ty::context::TyCtxt>>::try_fold_with::<rustc_next_trait_solver::solve::assembly::structural_traits::ReplaceProjectionWith<rustc_middle::ty::context::TyCtxt, rustc_trait_selection::solve::delegate::SolverDelegate>>
   4: <&rustc_middle::ty::list::RawList<(), rustc_middle::ty::generic_args::GenericArg> as rustc_type_ir::fold::TypeFoldable<rustc_middle::ty::context::TyCtxt>>::try_fold_with::<rustc_next_trait_solver::solve::assembly::structural_traits::ReplaceProjectionWith<rustc_middle::ty::context::TyCtxt, rustc_trait_selection::solve::delegate::SolverDelegate>>
   5: rustc_next_trait_solver::solve::assembly::structural_traits::predicates_for_object_candidate::<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>
   6: <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>>::match_assumption::<<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>>::probe_and_consider_object_bound_candidate::{closure#0}>
   7: <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>>::probe_and_match_goal_against_assumption::<<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>>::probe_and_consider_object_bound_candidate::{closure#0}>
   8: <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::compute_trait_goal
   9: <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}
  10: <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>>::evaluate_goal_in_task
  11: <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>>::evaluate_goal::{closure#0}
  12: <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::evaluate_goal_raw
  13: <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::evaluate_goal
  14: <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::evaluate_added_goals_step
  15: <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::{closure#0}
  16: <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>>::match_assumption::<<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>>::probe_and_consider_object_bound_candidate::{closure#0}>
  17: <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>>::probe_and_match_goal_against_assumption::<<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>>::probe_and_consider_object_bound_candidate::{closure#0}>
  18: <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::compute_trait_goal
  19: <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}
  20: <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>>::evaluate_goal_in_task
  21: <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>>::evaluate_goal::{closure#0}
  22: <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::evaluate_goal_raw
  23: <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::evaluate_goal
  24: <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::evaluate_added_goals_step
  25: <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::{closure#0}
  26: <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>>::match_assumption::<<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>>::probe_and_consider_object_bound_candidate::{closure#0}>
  27: <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>>::probe_and_match_goal_against_assumption::<<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>>::probe_and_consider_object_bound_candidate::{closure#0}>
  28: <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::compute_trait_goal
  29: <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}
  30: <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>>::evaluate_goal_in_task
  31: <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>>::evaluate_goal::{closure#0}
  32: <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::evaluate_goal_raw
  33: <rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<rustc_trait_selection::solve::delegate::SolverDelegate, rustc_middle::ty::context::TyCtxt>>::evaluate_goal
  34: <rustc_trait_selection::solve::delegate::SolverDelegate as rustc_next_trait_solver::solve::eval_ctxt::SolverDelegateEvalExt>::evaluate_root_goal
  35: <rustc_trait_selection::solve::fulfill::FulfillmentCtxt<rustc_infer::traits::engine::ScrubbedTraitError> as rustc_infer::traits::engine::TraitEngine<rustc_infer::traits::engine::ScrubbedTraitError>>::try_evaluate_obligations
  36: rustc_trait_selection::traits::impossible_predicates
  37: rustc_trait_selection::traits::instantiate_and_check_impossible_predicates
      [... omitted 1 frame ...]
  38: rustc_trait_selection::traits::vtable::first_method_vtable_slot
  39: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::DefaultCache<rustc_type_ir::predicate::TraitRef<rustc_middle::ty::context::TyCtxt>, rustc_middle::query::erase::ErasedData<[u8; 8]>>, false>
  40: rustc_ty_utils::instance::resolve_instance_raw
      [... omitted 1 frame ...]
  41: <rustc_middle::ty::instance::Instance>::try_resolve
  42: <rustc_mir_transform::check_call_recursion::CallRecursion as rustc_mir_transform::check_call_recursion::TerminatorClassifier>::is_recursive_terminator
  43: <rustc_mir_transform::check_call_recursion::CheckCallRecursion as rustc_mir_transform::pass_manager::MirLint>::run_lint
  44: rustc_mir_transform::pass_manager::run_passes_inner
  45: rustc_mir_transform::mir_built
  46: rustc_query_impl::execution::try_execute_query::<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::ErasedData<[u8; 8]>, rustc_middle::dep_graph::graph::DepNodeIndex>, false>
  47: rustc_mir_build::check_unsafety::check_unsafety
  48: rustc_query_impl::execution::try_execute_query::<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::ErasedData<[u8; 0]>, rustc_middle::dep_graph::graph::DepNodeIndex>, false>
  49: <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface::passes::run_required_analyses::{closure#2}::{closure#0}>::{closure#0}
  50: rustc_interface::passes::analysis
  51: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::SingleCache<rustc_middle::query::erase::ErasedData<[u8; 0]>>, false>
  52: rustc_interface::passes::create_and_enter_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>
  53: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: the compiler unexpectedly panicked. This is a bug

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: rustc 1.97.0 (2d8144b78 2026-07-07) running on aarch64-apple-darwin

note: compiler flags: --crate-type lib

query stack during panic:
#0 [instantiate_and_check_impossible_predicates] checking impossible instantiated predicates: `Callback`
#1 [first_method_vtable_slot] finding the slot within the vtable of `dyn Callback<Output = ()>` for the implementation of `Callback`
#2 [resolve_instance_raw] resolving instance `<dyn Callback<Output = ()> as Callback>::method`
#3 [mir_built] building MIR for `trigger`
#4 [check_unsafety] unsafety-checking `trigger`
#5 [analysis] running analysis passes on crate `repro`
end of query stack

[theemathas]

Bisection result:

searched nightlies: from nightly-2026-04-10 to nightly-2026-05-22
regressed nightly: nightly-2026-05-07
searched commit range: e95e732...365c0e1
regressed commit: 365c0e1

bisected with cargo-bisect-rustc v0.6.11

Host triple: aarch64-apple-darwin
Reproduce with:

cargo bisect-rustc --start=1.96.0 --end=1.97.0

---

Regressed in #155443. cc @jdonszelmann, @lcnr

[theemathas]

My attempt at "simplifying" the reproducer:

pub trait Helper {
    type Out;
}
// Note: This Send here has to be an auto trait
impl<T: Send> Helper for T {
    type Out = ();
}

pub trait Super<T> {
    type Assoc;
    fn method(&self);
}

pub trait Sub: Super<<String as Helper>::Out, Assoc = ()> {}

pub fn trigger(f: &dyn Sub) {
    f.method();
}