ICE: Creating trait object of wrapper struct of a async closure with non-generic arugment and generic arguments with multiple allowed arities

@lcnr

edit(@lcnr): minimized regression test added in #163806

//@ revisions: old next
//@ ignore-compare-mode-next-solver (explicit revisions)
//@[next] compile-flags: -Znext-solver
//@[old] failure-status: 101
//@[old] dont-check-compiler-stderr
//@[old] known-bug: #147719
//@ edition: 2024

// A variant of `old-solver-only-leak-check-on-ambig.rs` which
// relies on us deferring the inference of async closure upvars.
// `Fut` is higher ranked as it ends up capturing the function
// argument.

struct Wrap<F>(F);
trait NotImplemented {}
trait NodeImpl {}
impl<T: NotImplemented> NodeImpl for T {}
impl<F, Fut> NodeImpl for Wrap<F>
where
    F: Fn(&()) -> Fut,
{
}

fn node_impl<T: NodeImpl>(_: T) {}
fn main() {
    node_impl(Wrap(async |_: &()| ()));
    //[next]~^ ERROR: the trait bound
}

Code

use std::marker::PhantomData;

struct RuntimeContext {}

pub trait NodeImpl {}

struct Wrap<F, P>(F, PhantomData<P>);

impl<F, P> Wrap<F, P> {
    fn new(func: F) -> Self {
        Self(func, PhantomData)
    }
}

trait Arg {}
impl Arg for i128 {}

impl<F, Fut, A> NodeImpl for Wrap<F, A>
where
    F: Fn(&RuntimeContext, A) -> Fut,
    Fut: Future<Output = i128>,
    A: Arg,
{
}

impl<F, Fut, A, B> NodeImpl for Wrap<F, (A, B)>
where
    F: Fn(&RuntimeContext, A, B) -> Fut,
    Fut: Future<Output = i128>,
    A: Arg,
    B: Arg,
{
}

pub fn trigger_ice() {
    let x: Box<dyn NodeImpl> = Box::new(Wrap::<_, (i128, i128)>::new(
        async |_context: &RuntimeContext, x: i128, y: i128| 10,
    ));
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=52a3b870e6d97bb25cbb3279074618e5

Error output

note: no errors encountered even though delayed bugs were created

note: those delayed bugs will now be shown as internal compiler errors

error: internal compiler error: error performing operation: fully_perform
  --> src/lib.rs:36:32
   |
36 |       let x: Box<dyn NodeImpl> = Box::new(Wrap::<_, (i128, i128)>::new(
   |  ________________________________^
37 | |         async |_context: &RuntimeContext, x: i128, y: i128| 10,
38 | |     ));
   | |______^
   |
note: delayed at /rustc-dev/4b94758d2ba7d0ef71ccf5fde29ce4bc5d6fe2a4/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs:95:25
  --> src/lib.rs:36:32
   |
36 |       let x: Box<dyn NodeImpl> = Box::new(Wrap::<_, (i128, i128)>::new(
   |  ________________________________^
37 | |         async |_context: &RuntimeContext, x: i128, y: i128| 10,
38 | |     ));
   | |______^

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: please attach the file at `/playground/rustc-ice-2025-10-15T09_32_10-46.txt` to your bug report

note: compiler flags: --crate-type lib -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2

note: some of the compiler flags provided by cargo are hidden

Backtrace

0: <rustc_errors::DiagCtxtInner>::emit_diagnostic
         1: <rustc_errors::DiagCtxtHandle>::emit_diagnostic
         2: <rustc_span::ErrorGuaranteed as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
         3: <rustc_errors::DiagCtxtHandle>::span_delayed_bug::<rustc_span::span_encoding::Span, alloc::string::String>
         4: <rustc_borrowck::type_check::TypeChecker>::prove_trait_ref
         5: <rustc_borrowck::type_check::TypeChecker as rustc_middle::mir::visit::Visitor>::visit_rvalue
         6: <rustc_borrowck::type_check::TypeChecker as rustc_middle::mir::visit::Visitor>::visit_body
         7: rustc_borrowck::type_check::type_check
         8: <rustc_borrowck::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
         9: rustc_borrowck::mir_borrowck
        10: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::mir_borrowck::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
        11: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::Erased<[u8; 8]>, rustc_query_system::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
        12: rustc_query_impl::query_impl::mir_borrowck::get_query_non_incr::__rust_end_short_backtrace
        13: <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface::passes::run_required_analyses::{closure#1}::{closure#0}>::{closure#0}
        14: rustc_interface::passes::analysis
        15: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 0]>>
        16: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::SingleCache<rustc_middle::query::erase::Erased<[u8; 0]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
        17: rustc_query_impl::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
        18: <rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core::ops::function::FnOnce<(&rustc_session::session::Session, rustc_middle::ty::context::CurrentGcx, alloc::sync::Arc<rustc_data_structures::jobserver::Proxy>, &std::sync::once_lock::OnceLock<rustc_middle::ty::context::GlobalCtxt>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_middle::arena::Arena>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_hir::Arena>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
        19: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
        20: std::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
        21: <<std::thread::Builder>::spawn_unchecked_<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
        22: std::sys::thread::unix::Thread::new::thread_start
        23: <unknown>
        24: clone

a bit smaller:

use std::marker::PhantomData;

pub trait NodeImpl {}

struct Wrap<F, P>(F, PhantomData<P>);

impl<F, P> Wrap<F, P> {
    fn new(_: F) -> Self {
        loop {}
    }
}

trait Arg {}

impl<F, A> NodeImpl for Wrap<F, A> where A: Arg {}

impl<F, Fut, A> NodeImpl for Wrap<F, (A, )>
where
    F: Fn(&(), A) -> Fut,
{
}

pub fn trigger_ice() {
    let _: &dyn NodeImpl = &Wrap::<_, (i128,)>::new(
        async |_: &(), _: i128| (),
    );
}

auto-reduced

trait NodeImpl {}
struct Wrap<F, P>(F, P);
impl<F, P> Wrap<F, P> {
    fn new(_: F) -> Self {
        loop {}
    }
}
trait Arg {}
impl<F, A> NodeImpl for Wrap<F, A> where A: Arg {}
impl<F, Fut, A> NodeImpl for Wrap<F, (A,)> where F: Fn(&(), A) -> Fut {}
fn trigger_ice() {
    let _: &dyn NodeImpl = &Wrap::<_, (i128,)>::new(async |_: &(), i128| 0);
}

Rollup merge of rust-lang#163806 - lcnr:crashes-migration-cont, r=lqd

Unrolled build for #163806 in rollup 163957

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ICE: Creating trait object of wrapper struct of a async closure with non-generic arugment and generic arguments with multiple allowed arities #147719

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ICE: Creating trait object of wrapper struct of a async closure with non-generic arugment and generic arguments with multiple allowed arities #147719

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cyrgani commented on Oct 15, 2025

matthiaskrgr commented on Oct 15, 2025

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