[ICE]: has escaping bound vars, so it cannot be wrapped in a dummy binder

@rustbot

auto-reduced (treereduce-rust):

//@compile-flags: -Znext-solver=globally
trait FooMut {
    fn bar<I>(&self, _: I)
    where
        for<'b> &'b I: Iterator<Item = &'b ()>;

    fn bar<I>(&self, _: I)
    where
        I: Iterator,
    {
        let collection = vec![_I].iter().map(|x| ());
        self.bar(collection);
    }
}

original:

trait FooMut {
    fn bar<I>(&self, _: I)
    where
        for<'b> &'b I: Iterator<Item = &'b ()>;

    fn bar<I>(&self, _: I)
    where
        I: Iterator,
    {
        let collection = vec![_I].iter().map(|x| ());
        self.bar(collection);
    }
}

fn main() {}

Version information

rustc 1.98.0-nightly (4429659e4 2026-06-22)
binary: rustc
commit-hash: 4429659e4745016bd3f26a4a421843edc7fbc422
commit-date: 2026-06-22
host: x86_64-unknown-linux-gnu
release: 1.98.0-nightly
LLVM version: 22.1.7

Possibly related line of code:

rust/compiler/rustc_type_ir/src/relate/solver_relating.rs

Lines 381 to 393 in 4429659

    
           fn register_goals(&mut self, obligations: impl IntoIterator<Item = Goal<I, I::Predicate>>) { 
        
               self.goals.extend(obligations); 
        
           } 
        
           fn register_alias_relate_predicate(&mut self, a: I::Ty, b: I::Ty) { 
        
               self.register_predicates([ty::Binder::dummy(match self.ambient_variance { 
        
                   ty::Covariant => ty::PredicateKind::AliasRelate( 
        
                       a.into(), 
        
                       b.into(), 
        
                       ty::AliasRelationDirection::Subtype, 
        
                   ), 
        
                   // a :> b is b <: a

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc -Znext-solver=globally

Program output

error[E0428]: the name `bar` is defined multiple times
  --> /tmp/icemaker_global_tempdir.izummaQbYnwZ/rustc_testrunner_tmpdir_reporting.ofzxHrCribxf/mvce.rs:6:5
   |
 2 | /     fn bar<I>(&self, _: I)
 3 | |     where
 4 | |         for<'b> &'b I: Iterator<Item = &'b ()>;
   | |_______________________________________________- previous definition of the value `bar` here
 5 |
 6 | /     fn bar<I>(&self, _: I)
 7 | |     where
 8 | |         I: Iterator,
...  |
11 | |         self.bar(collection);
12 | |     }
   | |_____^ `bar` redefined here
   |
   = note: `bar` must be defined only once in the value namespace of this trait

error[E0425]: cannot find value `_I` in this scope
  --> /tmp/icemaker_global_tempdir.izummaQbYnwZ/rustc_testrunner_tmpdir_reporting.ofzxHrCribxf/mvce.rs:10:31
   |
10 |         let collection = vec![_I].iter().map(|x| ());
   |                               ^^ not found in this scope

error[E0601]: `main` function not found in crate `mvce`
  --> /tmp/icemaker_global_tempdir.izummaQbYnwZ/rustc_testrunner_tmpdir_reporting.ofzxHrCribxf/mvce.rs:13:2
   |
13 | }
   |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.izummaQbYnwZ/rustc_testrunner_tmpdir_reporting.ofzxHrCribxf/mvce.rs`

error[E0277]: `&'b _` is not an iterator
  --> /tmp/icemaker_global_tempdir.izummaQbYnwZ/rustc_testrunner_tmpdir_reporting.ofzxHrCribxf/mvce.rs:11:18
   |
11 |         self.bar(collection);
   |              --- ^^^^^^^^^^ `&'b _` is not an iterator
   |              |
   |              required by a bound introduced by this call
   |
   = help: the trait `for<'b> Iterator` is not implemented for `&'b _`
note: required by a bound in `FooMut::bar`
  --> /tmp/icemaker_global_tempdir.izummaQbYnwZ/rustc_testrunner_tmpdir_reporting.ofzxHrCribxf/mvce.rs:4:24
   |
 2 |     fn bar<I>(&self, _: I)
   |        --- required by a bound in this associated function
 3 |     where
 4 |         for<'b> &'b I: Iterator<Item = &'b ()>;
   |                        ^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `FooMut::bar`


thread 'rustc' (3118313) panicked at /rustc-dev/4429659e4745016bd3f26a4a421843edc7fbc422/compiler/rustc_type_ir/src/relate/solver_relating.rs:387:35:
`AliasRelate(Term::Ty(&'^0.Named(DefId(0:6 ~ mvce[b5bc]::FooMut::bar::'b)) ()), Equate, Term::Ty(Alias(Alias { kind: Projection { def_id: DefId(2:10209 ~ core[1e5d]::iter::traits::iterator::Iterator::Item) }, args: [{type error}], .. })))` has escaping bound vars, so it cannot be wrapped in a dummy binder.
stack backtrace:
   0:     0x7fc2b3822629 - <<std[3d46086344a7ae7a]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[1e5dff68517d21c3]::fmt::Display>::fmt
   1:     0x7fc2b3e0ed1e - core[1e5dff68517d21c3]::fmt::write
   2:     0x7fc2b3838cc6 - <std[3d46086344a7ae7a]::sys::stdio::unix::Stderr as std[3d46086344a7ae7a]::io::Write>::write_fmt
   3:     0x7fc2b37f78ee - std[3d46086344a7ae7a]::panicking::default_hook::{closure#0}
   4:     0x7fc2b3815313 - std[3d46086344a7ae7a]::panicking::default_hook
   5:     0x7fc2b2870661 - std[3d46086344a7ae7a]::panicking::update_hook::<alloc[b6e8defdd65683e4]::boxed::Box<rustc_driver_impl[e98da4d991f149da]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7fc2b38155f2 - std[3d46086344a7ae7a]::panicking::panic_with_hook
   7:     0x7fc2b37f79b2 - std[3d46086344a7ae7a]::panicking::panic_handler::{closure#0}
   8:     0x7fc2b37ee679 - std[3d46086344a7ae7a]::sys::backtrace::__rust_end_short_backtrace::<std[3d46086344a7ae7a]::panicking::panic_handler::{closure#0}, !>
   9:     0x7fc2b37f941d - __rustc[9ba924033e29ab74]::rust_begin_unwind
  10:     0x7fc2b06f713c - core[1e5dff68517d21c3]::panicking::panic_fmt
  11:     0x7fc2b3ed25aa - <rustc_type_ir[ddd872802aba9539]::relate::solver_relating::SolverRelating<rustc_infer[8178364f2f6d9ffd]::infer::InferCtxt, rustc_middle[cff461d045763160]::ty::context::TyCtxt> as rustc_type_ir[ddd872802aba9539]::relate::TypeRelation<rustc_middle[cff461d045763160]::ty::context::TyCtxt>>::tys
  12:     0x7fc2b4bfaeaf - <rustc_infer[8178364f2f6d9ffd]::infer::InferCtxt as rustc_type_ir[ddd872802aba9539]::relate::solver_relating::RelateExt>::relate::<rustc_middle[cff461d045763160]::ty::Ty>
  13:     0x7fc2b45636ba - <rustc_trait_selection[2a52d8c4a2c04eed]::traits::engine::ObligationCtxt>::eq::<rustc_middle[cff461d045763160]::ty::Ty>
  14:     0x7fc2b3548ec9 - <rustc_trait_selection[2a52d8c4a2c04eed]::error_reporting::TypeErrCtxt>::point_at_chain::<rustc_span[c0fd46f789e8d999]::ErrorGuaranteed>
  15:     0x7fc2b3569718 - <rustc_trait_selection[2a52d8c4a2c04eed]::error_reporting::TypeErrCtxt>::note_function_argument_obligation::<rustc_span[c0fd46f789e8d999]::ErrorGuaranteed>
  16:     0x7fc2b355e93f - <rustc_trait_selection[2a52d8c4a2c04eed]::error_reporting::TypeErrCtxt>::note_obligation_cause_code::<rustc_span[c0fd46f789e8d999]::ErrorGuaranteed, rustc_middle[cff461d045763160]::ty::predicate::Predicate>
  17:     0x7fc2b3650b06 - <rustc_trait_selection[2a52d8c4a2c04eed]::error_reporting::TypeErrCtxt>::note_obligation_cause
  18:     0x7fc2b21bc4cf - <rustc_trait_selection[2a52d8c4a2c04eed]::error_reporting::TypeErrCtxt>::report_fulfillment_errors
  19:     0x7fc2b4723065 - <rustc_hir_typeck[469cc3357cf82113]::fn_ctxt::FnCtxt>::check_expr_method_call
  20:     0x7fc2b4127f40 - <rustc_hir_typeck[469cc3357cf82113]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  21:     0x7fc2b40f990c - <rustc_hir_typeck[469cc3357cf82113]::fn_ctxt::FnCtxt>::check_expr_block
  22:     0x7fc2b4127f6d - <rustc_hir_typeck[469cc3357cf82113]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  23:     0x7fc2b40ee42c - rustc_hir_typeck[469cc3357cf82113]::check::check_fn
  24:     0x7fc2b42cfc56 - rustc_hir_typeck[469cc3357cf82113]::typeck_with_inspect::{closure#0}
  25:     0x7fc2b42ccc70 - rustc_query_impl[f15f39df5e5f8d31]::query_impl::typeck_root::invoke_provider_fn::__rust_begin_short_backtrace
  26:     0x7fc2b42cc0a9 - rustc_query_impl[f15f39df5e5f8d31]::execution::try_execute_query::<rustc_data_structures[69d529ec17ad3052]::vec_cache::VecCache<rustc_span[c0fd46f789e8d999]::def_id::LocalDefId, rustc_middle[cff461d045763160]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[cff461d045763160]::dep_graph::graph::DepNodeIndex>, false>
  27:     0x7fc2b42cba43 - rustc_query_impl[f15f39df5e5f8d31]::query_impl::typeck_root::execute_query_non_incr::__rust_end_short_backtrace
  28:     0x7fc2b42c646d - <rustc_middle[cff461d045763160]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_hir_analysis[cb7c76efa507416e]::check_crate::{closure#1}>::{closure#0}
  29:     0x7fc2b42c5e61 - rustc_hir_analysis[cb7c76efa507416e]::check_crate
  30:     0x7fc2b42c81b4 - rustc_interface[3222c6a6230bc2a3]::passes::analysis
  31:     0x7fc2b4f06b07 - rustc_query_impl[f15f39df5e5f8d31]::execution::try_execute_query::<rustc_middle[cff461d045763160]::query::caches::SingleCache<rustc_middle[cff461d045763160]::query::erase::ErasedData<[u8; 0usize]>>, false>
  32:     0x7fc2b4f06769 - rustc_query_impl[f15f39df5e5f8d31]::query_impl::analysis::execute_query_non_incr::__rust_end_short_backtrace
  33:     0x7fc2b4f5d13d - rustc_interface[3222c6a6230bc2a3]::interface::run_compiler::<(), rustc_driver_impl[e98da4d991f149da]::run_compiler::{closure#0}>::{closure#1}
  34:     0x7fc2b4f18b7a - std[3d46086344a7ae7a]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[3222c6a6230bc2a3]::util::run_in_thread_with_globals<rustc_interface[3222c6a6230bc2a3]::util::run_in_thread_pool_with_globals<rustc_interface[3222c6a6230bc2a3]::interface::run_compiler<(), rustc_driver_impl[e98da4d991f149da]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  35:     0x7fc2b4f1892d - <std[3d46086344a7ae7a]::thread::lifecycle::spawn_unchecked<rustc_interface[3222c6a6230bc2a3]::util::run_in_thread_with_globals<rustc_interface[3222c6a6230bc2a3]::util::run_in_thread_pool_with_globals<rustc_interface[3222c6a6230bc2a3]::interface::run_compiler<(), rustc_driver_impl[e98da4d991f149da]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[1e5dff68517d21c3]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  36:     0x7fc2b4f1cc92 - <std[3d46086344a7ae7a]::sys::thread::unix::Thread>::new::thread_start
  37:     0x7fc2aea981b9 - <unknown>
  38:     0x7fc2aeb1d21c - <unknown>
  39:                0x0 - <unknown>

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: please make sure that you have updated to the latest nightly

note: rustc 1.98.0-nightly (4429659e4 2026-06-22) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z next-solver=globally -Z dump-mir-dir=dir

query stack during panic:
#0 [typeck_root] type-checking `FooMut::bar`
#1 [analysis] running analysis passes on crate `mvce`
end of query stack
error: aborting due to 4 previous errors

Some errors have detailed explanations: E0277, E0425, E0428, E0601.
For more information about an error, try `rustc --explain E0277`.

@rustbot label +WG-trait-system-refactor

since #156619

Rollup merge of rust-lang#158536 - ShoyuVanilla:issue-158317, r=lcnr

Unrolled build for #158536

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[ICE]: `has escaping bound vars, so it cannot be wrapped in a dummy binder` #158317

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	fn register_goals(&mut self, obligations: impl IntoIterator<Item = Goal<I, I::Predicate>>) {
	self.goals.extend(obligations);
	}

	fn register_alias_relate_predicate(&mut self, a: I::Ty, b: I::Ty) {
	self.register_predicates([ty::Binder::dummy(match self.ambient_variance {
	ty::Covariant => ty::PredicateKind::AliasRelate(
	a.into(),
	b.into(),
	ty::AliasRelationDirection::Subtype,
	),
	// a :> b is b <: a

[ICE]: has escaping bound vars, so it cannot be wrapped in a dummy binder #158317

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matthiaskrgr commented on Jun 23, 2026

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[ICE]: `has escaping bound vars, so it cannot be wrapped in a dummy binder` #158317