[ICE]: Found unstable fingerprints for evaluate_obligation

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rustc --version --verbose:

rustc 1.95.0 (59807616e 2026-04-14)
binary: rustc
commit-hash: 59807616e1fa2540724bfbac14d7976d7e4a3860
commit-date: 2026-04-14
host: x86_64-unknown-linux-gnu
release: 1.95.0
LLVM version: 22.1.2

Error output

error: internal compiler error: encountered incremental compilation error with evaluate_obligation(5aab3bc36898b930-42973506f0649e50)
  |
  = note: please follow the instructions below to create a bug report with the provided information
  = note: for incremental compilation bugs, having a reproduction is vital
  = note: an ideal reproduction consists of the code before and some patch that then triggers the bug when applied and compiled again
  = note: as a workaround, you can run `cargo clean -p libclient` or `cargo clean` to allow your project to compile


thread 'rustc' (1004613) panicked at /rustc-dev/59807616e1fa2540724bfbac14d7976d7e4a3860/compiler/rustc_middle/src/verify_ich.rs:80:9:
Found unstable fingerprints for evaluate_obligation(5aab3bc36898b930-42973506f0649e50): Err(Canonical)
stack backtrace:
   0:     0x7d985679314b - <<std[e28293b1aa0f68bd]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[c1f1a4ba060b9bfa]::fmt::Display>::fmt
   1:     0x7d9856e1f408 - core[c1f1a4ba060b9bfa]::fmt::write
   2:     0x7d98583683b6 - <std[e28293b1aa0f68bd]::sys::stdio::unix::Stderr as std[e28293b1aa0f68bd]::io::Write>::write_fmt
   3:     0x7d9856769238 - std[e28293b1aa0f68bd]::panicking::default_hook::{closure#0}
   4:     0x7d98567865b3 - std[e28293b1aa0f68bd]::panicking::default_hook
   5:     0x7d985575cc30 - std[e28293b1aa0f68bd]::panicking::update_hook::<alloc[fdfd2bd8633a6659]::boxed::Box<rustc_driver_impl[e4ca9dd5dd6cf2a8]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7d9856786892 - std[e28293b1aa0f68bd]::panicking::panic_with_hook
   7:     0x7d98567692f8 - std[e28293b1aa0f68bd]::panicking::panic_handler::{closure#0}
   8:     0x7d9856760029 - std[e28293b1aa0f68bd]::sys::backtrace::__rust_end_short_backtrace::<std[e28293b1aa0f68bd]::panicking::panic_handler::{closure#0}, !>
   9:     0x7d985676ad5d - __rustc[b7974e8690430dd9]::rust_begin_unwind
  10:     0x7d98534de82c - core[c1f1a4ba060b9bfa]::panicking::panic_fmt
  11:     0x7d9855d987d8 - rustc_middle[9956b31dc9b62104]::verify_ich::incremental_verify_ich_failed
  12:     0x7d9857367d2b - rustc_middle[9956b31dc9b62104]::verify_ich::incremental_verify_ich::<rustc_middle[9956b31dc9b62104]::query::erase::ErasedData<[u8; 2usize]>>
  13:     0x7d98576e4929 - rustc_query_impl[b21371cac9e22a84]::execution::try_execute_query::<rustc_middle[9956b31dc9b62104]::query::caches::DefaultCache<rustc_type_ir[9c1281d1b8893e1d]::canonical::CanonicalQueryInput<rustc_middle[9956b31dc9b62104]::ty::context::TyCtxt, rustc_middle[9956b31dc9b62104]::ty::ParamEnvAnd<rustc_middle[9956b31dc9b62104]::ty::predicate::Predicate>>, rustc_middle[9956b31dc9b62104]::query::erase::ErasedData<[u8; 2usize]>>, true>
  14:     0x7d98576e2982 - rustc_query_impl[b21371cac9e22a84]::query_impl::evaluate_obligation::execute_query_incr::__rust_end_short_backtrace
  15:     0x7d98575f6d87 - <rustc_trait_selection[79d0130c2732d367]::traits::fulfill::FulfillProcessor as rustc_data_structures[2c167692117eb14d]::obligation_forest::ObligationProcessor>::process_obligation
  16:     0x7d9856e06c01 - <rustc_data_structures[2c167692117eb14d]::obligation_forest::ObligationForest<rustc_trait_selection[79d0130c2732d367]::traits::fulfill::PendingPredicateObligation>>::process_obligations::<rustc_trait_selection[79d0130c2732d367]::traits::fulfill::FulfillProcessor>
  17:     0x7d98572f30f9 - <rustc_trait_selection[79d0130c2732d367]::traits::engine::ObligationCtxt>::make_canonicalized_query_response::<()>
  18:     0x7d98572ed0a0 - rustc_traits[bd42b8ac0103835d]::type_op::type_op_prove_predicate
  19:     0x7d98576f7d1e - rustc_query_impl[b21371cac9e22a84]::execution::try_execute_query::<rustc_middle[9956b31dc9b62104]::query::caches::DefaultCache<rustc_type_ir[9c1281d1b8893e1d]::canonical::CanonicalQueryInput<rustc_middle[9956b31dc9b62104]::ty::context::TyCtxt, rustc_middle[9956b31dc9b62104]::ty::ParamEnvAnd<rustc_middle[9956b31dc9b62104]::traits::query::type_op::ProvePredicate>>, rustc_middle[9956b31dc9b62104]::query::erase::ErasedData<[u8; 8usize]>>, true>
  20:     0x7d98576f6a18 - rustc_query_impl[b21371cac9e22a84]::query_impl::type_op_prove_predicate::execute_query_incr::__rust_end_short_backtrace
  21:     0x7d9857476908 - <rustc_borrowck[70f000df51ab9cb4]::type_check::TypeChecker>::normalize_and_prove_instantiated_predicates
  22:     0x7d985401ab26 - <rustc_borrowck[70f000df51ab9cb4]::type_check::TypeChecker as rustc_middle[9956b31dc9b62104]::mir::visit::Visitor>::visit_operand
  23:     0x7d9857d46393 - <rustc_borrowck[70f000df51ab9cb4]::type_check::TypeChecker as rustc_middle[9956b31dc9b62104]::mir::visit::Visitor>::visit_body
  24:     0x7d9857d2ff29 - rustc_borrowck[70f000df51ab9cb4]::type_check::type_check
  25:     0x7d9857ea7def - <rustc_borrowck[70f000df51ab9cb4]::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
  26:     0x7d9857ea3736 - rustc_borrowck[70f000df51ab9cb4]::mir_borrowck
  27:     0x7d9857ea3567 - rustc_query_impl[b21371cac9e22a84]::query_impl::mir_borrowck::invoke_provider_fn::__rust_begin_short_backtrace
  28:     0x7d9856f3cc7d - rustc_query_impl[b21371cac9e22a84]::execution::try_execute_query::<rustc_data_structures[2c167692117eb14d]::vec_cache::VecCache<rustc_span[e3c9a1967c4921fd]::def_id::LocalDefId, rustc_middle[9956b31dc9b62104]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[9956b31dc9b62104]::dep_graph::graph::DepNodeIndex>, true>
  29:     0x7d9856fa83d1 - rustc_query_impl[b21371cac9e22a84]::query_impl::mir_borrowck::execute_query_incr::__rust_end_short_backtrace
  30:     0x7d9857ba98f0 - rustc_mir_transform[2be6bdc629d1da0e]::mir_drops_elaborated_and_const_checked
  31:     0x7d9857ba8e99 - rustc_query_impl[b21371cac9e22a84]::query_impl::mir_drops_elaborated_and_const_checked::invoke_provider_fn::__rust_begin_short_backtrace
  32:     0x7d9856f3c1fc - rustc_query_impl[b21371cac9e22a84]::execution::try_execute_query::<rustc_data_structures[2c167692117eb14d]::vec_cache::VecCache<rustc_span[e3c9a1967c4921fd]::def_id::LocalDefId, rustc_middle[9956b31dc9b62104]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[9956b31dc9b62104]::dep_graph::graph::DepNodeIndex>, true>
  33:     0x7d9856fa8021 - rustc_query_impl[b21371cac9e22a84]::query_impl::mir_drops_elaborated_and_const_checked::execute_query_incr::__rust_end_short_backtrace
  34:     0x7d9856f37888 - <rustc_middle[9956b31dc9b62104]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface[c8170c4b7b680ee8]::passes::run_required_analyses::{closure#1}::{closure#0}>::{closure#0}
  35:     0x7d9856f36c6f - rustc_interface[c8170c4b7b680ee8]::passes::analysis
  36:     0x7d98583ec447 - rustc_query_impl[b21371cac9e22a84]::execution::try_execute_query::<rustc_middle[9956b31dc9b62104]::query::caches::SingleCache<rustc_middle[9956b31dc9b62104]::query::erase::ErasedData<[u8; 0usize]>>, true>
  37:     0x7d98583ebdb4 - rustc_query_impl[b21371cac9e22a84]::query_impl::analysis::execute_query_incr::__rust_end_short_backtrace
  38:     0x7d985805632d - rustc_interface[c8170c4b7b680ee8]::interface::run_compiler::<(), rustc_driver_impl[e4ca9dd5dd6cf2a8]::run_compiler::{closure#0}>::{closure#1}
  39:     0x7d985802663e - std[e28293b1aa0f68bd]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[c8170c4b7b680ee8]::util::run_in_thread_with_globals<rustc_interface[c8170c4b7b680ee8]::util::run_in_thread_pool_with_globals<rustc_interface[c8170c4b7b680ee8]::interface::run_compiler<(), rustc_driver_impl[e4ca9dd5dd6cf2a8]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  40:     0x7d9858026ca0 - <std[e28293b1aa0f68bd]::thread::lifecycle::spawn_unchecked<rustc_interface[c8170c4b7b680ee8]::util::run_in_thread_with_globals<rustc_interface[c8170c4b7b680ee8]::util::run_in_thread_pool_with_globals<rustc_interface[c8170c4b7b680ee8]::interface::run_compiler<(), rustc_driver_impl[e4ca9dd5dd6cf2a8]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[c1f1a4ba060b9bfa]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  41:     0x7d9858027b2c - <std[e28293b1aa0f68bd]::sys::thread::unix::Thread>::new::thread_start
  42:     0x7d985189caa4 - start_thread
                               at ./nptl/pthread_create.c:447:8
  43:     0x7d9851929c6c - clone3
                               at ./misc/../sysdeps/unix/sysv/linux/x86_64/clone3.S:78:0
  44:                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: rustc 1.95.0 (59807616e 2026-04-14) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib -C embed-bitcode=no -C debuginfo=2 -C incremental=[REDACTED]

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

query stack during panic:
#0 [evaluate_obligation] evaluating trait selection obligation `{closure@libclient/src/impostor/regionimpostortask.rs:245:20: 245:27}: core::marker::Send`
#1 [type_op_prove_predicate] evaluating `type_op_prove_predicate` `ProvePredicate { predicate: Binder { value: TraitPredicate(<{closure@libclient/src/impostor/regionimpostortask.rs:245:20: 245:27} as core::marker::Send>, polarity:Positive), bound_vars: [] } }`
... and 3 other queries... use `env RUST_BACKTRACE=1` to see the full query stack

query stack during panic:
#0 [evaluate_obligation] evaluating trait selection obligation `{closure@libclient/src/impostor/regionimpostortask.rs:245:20: 245:27}: core::marker::Send`
#1 [type_op_prove_predicate] evaluating `type_op_prove_predicate` `ProvePredicate { predicate: Binder { value: TraitPredicate(<{closure@libclient/src/impostor/regionimpostortask.rs:245:20: 245:27} as core::marker::Send>, polarity:Positive), bound_vars: [] } }`
#2 [mir_borrowck] borrow-checking `impostor::regionimpostortask::<impl at libclient/src/impostor/regionimpostortask.rs:220:1: 220:31>::start`
#3 [mir_drops_elaborated_and_const_checked] elaborating drops for `impostor::regionimpostortask::<impl at libclient/src/impostor/regionimpostortask.rs:220:1: 220:31>::start::{closure#0}`
#4 [analysis] running analysis passes on crate `libclient`
end of query stack

Start with libclient/src/impostor/regionimpostortask.rs around lines 220-245 and the evaluate_obligation query stack; reproduce the incremental compilation failure using the reported rustc version and compiler flags. Reduce the oversized example to a minimal reproduction, then verify that the unstable fingerprint no longer occurs after the relevant compiler change.