[ICE]: Found unstable fingerprints for evaluate_obligation
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Description
rustc-ice-2026-04-05T19_13_50-219556.txt
I hit this bug when working with the tower library. I was writing tests that called services, where the services were ServiceFns, which implement the Service trait using an async closure. I moved calls to .ready().await?.call().await? into a helper function, which triggered the ICE during the incremental compilation.
This is as far as I've been able to minimize it:
Code
use std::{
pin::Pin,
task::{Context, Poll},
};
trait MyTrait {}
struct Wrapper<T>(T);
impl<F, T: Fn() -> F> MyTrait for T {}
impl<T: MyTrait> Future for Wrapper<T> {
type Output = ();
fn poll(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Self::Output> {
Poll::Ready(())
}
}
fn foo<T: MyTrait>(_: T) {}
fn main() {
let obj = async || {};
foo(obj);
// adding/removing this after first build causes ICE
async { Wrapper(obj).await };
}
Meta
rustc --version --verbose:
rustc 1.96.0-nightly (fb27476aa 2026-03-28)
binary: rustc
commit-hash: fb27476aaf1012f1f6ace6306f9b990e0d989c31
commit-date: 2026-03-28
host: x86_64-unknown-linux-gnu
release: 1.96.0-nightly
LLVM version: 22.1.2
I also tested this on the latest stable, which also has the bug.
Error output
thread 'rustc' (215962) panicked at /rustc-dev/fb27476aaf1012f1f6ace6306f9b990e0d989c31/compiler/rustc_middle/src/verify_ich.rs:80:9:
Found unstable fingerprints for evaluate_obligation(d10b6c7c46031ea5-69cd772e0bff9012): Ok(EvaluatedToOk)
Backtrace
0: __rustc::rust_begin_unwind
1: core::panicking::panic_fmt
2: rustc_middle::verify_ich::incremental_verify_ich_failed
3: rustc_middle::verify_ich::incremental_verify_ich::<rustc_middle::query::erase::ErasedData<[u8; 2]>>
4: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::DefaultCache<rustc_type_ir::canonical::CanonicalQueryInput<rustc_middle::ty::context::TyCtxt, rustc_middle::ty::ParamEnvAnd<rustc_middle::ty::predicate::Predicate>>, rustc_middle::query::erase::ErasedData<[u8; 2]>>, true>
5: <rustc_trait_selection::traits::fulfill::FulfillProcessor as rustc_data_structures::obligation_forest::ObligationProcessor>::process_obligation
6: <rustc_data_structures::obligation_forest::ObligationForest<rustc_trait_selection::traits::fulfill::PendingPredicateObligation>>::process_obligations::<rustc_trait_selection::traits::fulfill::FulfillProcessor>
7: <rustc_trait_selection::traits::engine::ObligationCtxt>::make_canonicalized_query_response::<()>
8: rustc_traits::type_op::type_op_prove_predicate
9: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::DefaultCache<rustc_type_ir::canonical::CanonicalQueryInput<rustc_middle::ty::context::TyCtxt, rustc_middle::ty::ParamEnvAnd<rustc_middle::traits::query::type_op::ProvePredicate>>, rustc_middle::query::erase::ErasedData<[u8; 8]>>, true>
10: <rustc_borrowck::type_check::TypeChecker>::normalize_and_prove_instantiated_predicates
11: <rustc_borrowck::type_check::TypeChecker as rustc_middle::mir::visit::Visitor>::visit_operand
12: <rustc_borrowck::type_check::TypeChecker as rustc_middle::mir::visit::Visitor>::visit_body
13: rustc_borrowck::type_check::type_check
14: <rustc_borrowck::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
15: rustc_borrowck::mir_borrowck
[... omitted 1 frame ...]
16: <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface::passes::run_required_analyses::{closure#2}::{closure#0}>::{closure#0}
17: rustc_interface::passes::analysis
18: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::SingleCache<rustc_middle::query::erase::ErasedData<[u8; 0]>>, true>
19: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by reproducing the minimized example with incremental compilation, then inspect the panic at compiler/rustc_middle/src/verify_ich.rs:80 and trace the reported path through rustc_trait_selection and rustc_borrowck. Done means adding or removing the async expression after the first build no longer triggers the unstable fingerprint ICE.
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