(Reproducible) Found unstable fingerprints for evaluate_obligation(fe6269799238ead4-76dd6f107d00ff42): Ok(EvaluatedToAmbig)

[moxian]

Code

https://github.com/moxian/rust-mini-things/tree/unstable-eval-ambig

git clone https://github.com/moxian/rust-mini-things.git --branch unstable-eval-ambig
cd rust-mini-things/unstable-eval-ambig
git checkout 5efe9ae
cargo build # succeeds
git checkout 9e4910c
cargo build # ICE

Meta

rustc --version --verbose:

rustc 1.86.0 (05f9846f8 2025-03-31)
binary: rustc
commit-hash: 05f9846f893b09a1be1fc8560e33fc3c815cfecb
commit-date: 2025-03-31
host: x86_64-pc-windows-msvc
release: 1.86.0
LLVM version: 19.1.7

But also present on nightly (rustc 1.88.0-nightly (2e6882ac5 2025-05-05))
If compiling on stable, requires RUSTC_BOOTSTRAP=1 due to code shenanigans

Error output

error: internal compiler error: encountered incremental compilation error with evaluate_obligation(d26c3d7d51bae428-a81b738ddcca7476)
  |
  = help: This is a known issue with the compiler. Run `cargo clean -p reqwest` or `cargo clean` to allow your project to compile
  = note: Please follow the instructions below to create a bug report with the provided information
  = note: See <https://github.com/rust-lang/rust/issues/84970> for more information


[backtrace here]

error[E0046]: not all trait items implemented, missing: `Error`, `Future`
  --> vendor\reqwest\src\dns\resolve.rs:48:1
   |
48 | impl Service<HyperName> for DynResolver {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `Error`, `Future` in implementation
   |
   = help: implement the missing item: `type Error = /* Type */;`
   = help: implement the missing item: `type Future = /* Type */;`

For more information about this error, try `rustc --explain E0046`.
warning: `reqwest` (lib) generated 17 warnings
error: could not compile `reqwest` (lib) due to 2 previous errors; 17 warnings emitted

backtrace

thread 'rustc' panicked at /rustc/05f9846f893b09a1be1fc8560e33fc3c815cfecb\compiler\rustc_query_system\src\query\plumbing.rs:731:9:
Found unstable fingerprints for evaluate_obligation(fe6269799238ead4-76dd6f107d00ff42): Ok(EvaluatedToAmbig)
stack backtrace:
   0: std::panicking::begin_panic_handler
             at /rustc/05f9846f893b09a1be1fc8560e33fc3c815cfecb/library\std\src\panicking.rs:695
   1: core::panicking::panic_fmt
             at /rustc/05f9846f893b09a1be1fc8560e33fc3c815cfecb/library\core\src\panicking.rs:75
   2: rustc_query_system::query::plumbing::incremental_verify_ich_failed::<rustc_middle::ty::context::TyCtxt>
   3: RINvNtNtCs1bonBNpMvFl_18rustc_query_system5query8plumbing17try_execute_queryINtCs5uFTMav0Vyp_16rustc_query_impl13DynamicConfigINtNtCsdmqHcUI6doO_21rustc_data_structures9vec_cache8VecCacheNtNtCslRMtD7Kp4gh_10rustc_span6def_id8CrateNumINtNtNtCs2vqUEYxsi4E_12
   4: RINvNtNtCs1bonBNpMvFl_18rustc_query_system5query8plumbing17try_execute_queryINtCs5uFTMav0Vyp_16rustc_query_impl13DynamicConfigINtNtB4_6caches12DefaultCacheINtNtCs1KkNgFnBCUn_13rustc_type_ir9canonical19CanonicalQueryInputNtNtNtCs2vqUEYxsi4E_12rustc_middle2t
   5: rustc_query_impl::plumbing::query_key_hash_verify_all
   6: <rustc_trait_selection::traits::fulfill::FulfillProcessor as rustc_data_structures::obligation_forest::ObligationProcessor>::process_obligation
   7: <rustc_data_structures::obligation_forest::ObligationForest<rustc_trait_selection::traits::fulfill::PendingPredicateObligation>>::process_obligations::<rustc_trait_selection::traits::fulfill::FulfillProcessor>
   8: <rustc_trait_selection::traits::engine::ObligationCtxt<rustc_trait_selection::traits::FulfillmentError>>::assumed_wf_types_and_report_errors
   9: <alloc::raw_vec::RawVec<rustc_hir_typeck::method::probe::Candidate>>::grow_one
  10: rustc_hir_typeck::method::probe::method_autoderef_steps
  11: rustc_hir_typeck::method::probe::method_autoderef_steps
  12: rustc_hir_typeck::typeck
  13: <<rustc_hir_typeck::fn_ctxt::FnCtxt>::deduce_closure_signature_from_predicates::MentionsTy as rustc_type_ir::visit::TypeVisitor<rustc_middle::ty::context::TyCtxt>>::visit_ty
  14: <alloc::raw_vec::RawVec<rustc_hir_typeck::method::probe::Candidate>>::grow_one
  15: <<rustc_hir_typeck::fn_ctxt::FnCtxt>::deduce_closure_signature_from_predicates::MentionsTy as rustc_type_ir::visit::TypeVisitor<rustc_middle::ty::context::TyCtxt>>::visit_ty
  16: rustc_hir_typeck::typeck
  17: <<rustc_hir_typeck::fn_ctxt::FnCtxt>::deduce_closure_signature_from_predicates::MentionsTy as rustc_type_ir::visit::TypeVisitor<rustc_middle::ty::context::TyCtxt>>::visit_ty
  18: <alloc::raw_vec::RawVec<rustc_hir_typeck::method::probe::Candidate>>::grow_one
  19: <<rustc_hir_typeck::fn_ctxt::FnCtxt>::deduce_closure_signature_from_predicates::MentionsTy as rustc_type_ir::visit::TypeVisitor<rustc_middle::ty::context::TyCtxt>>::visit_ty
  20: rustc_hir_typeck::typeck
  21: <<rustc_hir_typeck::fn_ctxt::FnCtxt>::deduce_closure_signature_from_predicates::MentionsTy as rustc_type_ir::visit::TypeVisitor<rustc_middle::ty::context::TyCtxt>>::visit_ty
  22: <<rustc_hir_typeck::fn_ctxt::FnCtxt>::deduce_closure_signature_from_predicates::MentionsTy as rustc_type_ir::visit::TypeVisitor<rustc_middle::ty::context::TyCtxt>>::visit_ty
  23: <<rustc_hir_typeck::fn_ctxt::FnCtxt>::deduce_closure_signature_from_predicates::MentionsTy as rustc_type_ir::visit::TypeVisitor<rustc_middle::ty::context::TyCtxt>>::visit_ty
  24: <<rustc_hir_typeck::fn_ctxt::FnCtxt>::deduce_closure_signature_from_predicates::MentionsTy as rustc_type_ir::visit::TypeVisitor<rustc_middle::ty::context::TyCtxt>>::visit_ty
  25: rustc_hir_typeck::typeck
  26: <<rustc_hir_typeck::fn_ctxt::FnCtxt>::deduce_closure_signature_from_predicates::MentionsTy as rustc_type_ir::visit::TypeVisitor<rustc_middle::ty::context::TyCtxt>>::visit_ty
  27: <<rustc_hir_typeck::fn_ctxt::FnCtxt>::deduce_closure_signature_from_predicates::MentionsTy as rustc_type_ir::visit::TypeVisitor<rustc_middle::ty::context::TyCtxt>>::visit_ty
  28: <<rustc_hir_typeck::fn_ctxt::FnCtxt>::deduce_closure_signature_from_predicates::MentionsTy as rustc_type_ir::visit::TypeVisitor<rustc_middle::ty::context::TyCtxt>>::visit_ty
  29: <<rustc_hir_typeck::fn_ctxt::FnCtxt>::deduce_closure_signature_from_predicates::MentionsTy as rustc_type_ir::visit::TypeVisitor<rustc_middle::ty::context::TyCtxt>>::visit_ty
  30: <<rustc_hir_typeck::fn_ctxt::FnCtxt>::deduce_closure_signature_from_predicates::MentionsTy as rustc_type_ir::visit::TypeVisitor<rustc_middle::ty::context::TyCtxt>>::visit_ty
  31: <<rustc_hir_typeck::fn_ctxt::FnCtxt>::deduce_closure_signature_from_predicates::MentionsTy as rustc_type_ir::visit::TypeVisitor<rustc_middle::ty::context::TyCtxt>>::visit_ty
  32: rustc_hir_typeck::typeck
  33: rustc_hir_typeck::typeck
  34: <<rustc_hir_typeck::fn_ctxt::FnCtxt>::deduce_closure_signature_from_predicates::MentionsTy as rustc_type_ir::visit::TypeVisitor<rustc_middle::ty::context::TyCtxt>>::visit_ty
  35: rustc_hir_typeck::typeck
  36: <rustc_hir_typeck::upvar::InferBorrowKind as rustc_hir_typeck::expr_use_visitor::Delegate>::borrow
  37: rustc_hir_typeck::typeck
  38: rustc_query_impl::plumbing::query_key_hash_verify_all
  39: RINvNtNtCs1bonBNpMvFl_18rustc_query_system5query8plumbing17try_execute_queryINtCs5uFTMav0Vyp_16rustc_query_impl13DynamicConfigINtNtCsdmqHcUI6doO_21rustc_data_structures9vec_cache8VecCacheNtNtCslRMtD7Kp4gh_10rustc_span6def_id10LocalDefIdINtNtNtCs2vqUEYxsi4E
  40: <alloc::sync::Arc<rustc_session::cstore::CrateSource>>::drop_slow
  41: <rustc_query_system::query::plumbing::JobOwner<()> as core::ops::drop::Drop>::drop
  42: RINvMs6_NtCs8UpmKsNDoY5_9hashbrown3rawINtB6_8RawTableTTNtNtNtCs2vqUEYxsi4E_12rustc_middle2ty8instance8InstanceNtNtNtBX_3mir4mono14CollectionModeETINtNtNtBX_5query5erase6ErasedAhj20_ENtNtNtCs1bonBNpMvFl_18rustc_query_system9dep_graph5graph12DepNodeIndexEEE1
  43: RINvMs6_NtCs8UpmKsNDoY5_9hashbrown3rawINtB6_8RawTableTTNtNtNtCs2vqUEYxsi4E_12rustc_middle2ty8instance8InstanceNtNtNtBX_3mir4mono14CollectionModeETINtNtNtBX_5query5erase6ErasedAhj20_ENtNtNtCs1bonBNpMvFl_18rustc_query_system9dep_graph5graph12DepNodeIndexEEE1
  44: RINvMs6_NtCs8UpmKsNDoY5_9hashbrown3rawINtB6_8RawTableTTNtNtNtCs2vqUEYxsi4E_12rustc_middle2ty8instance8InstanceNtNtNtBX_3mir4mono14CollectionModeETINtNtNtBX_5query5erase6ErasedAhj20_ENtNtNtCs1bonBNpMvFl_18rustc_query_system9dep_graph5graph12DepNodeIndexEEE1
  45: RINvMs6_NtCs8UpmKsNDoY5_9hashbrown3rawINtB6_8RawTableTTNtNtNtCs2vqUEYxsi4E_12rustc_middle2ty8instance8InstanceNtNtNtBX_3mir4mono14CollectionModeETINtNtNtBX_5query5erase6ErasedAhj20_ENtNtNtCs1bonBNpMvFl_18rustc_query_system9dep_graph5graph12DepNodeIndexEEE1
  46: RINvMs6_NtCs8UpmKsNDoY5_9hashbrown3rawINtB6_8RawTableTTNtNtNtCs2vqUEYxsi4E_12rustc_middle2ty8instance8InstanceNtNtNtBX_3mir4mono14CollectionModeETINtNtNtBX_5query5erase6ErasedAhj20_ENtNtNtCs1bonBNpMvFl_18rustc_query_system9dep_graph5graph12DepNodeIndexEEE1
  47: RINvNtNtCs1bonBNpMvFl_18rustc_query_system5query8plumbing15ensure_must_runINtCs5uFTMav0Vyp_16rustc_query_impl13DynamicConfigINtNtB4_6caches12DefaultCacheNtNtCslRMtD7Kp4gh_10rustc_span6def_id13LocalModDefIdINtNtNtCs2vqUEYxsi4E_12rustc_middle5query5erase6Era
  48: rustc_query_impl::query_system
  49: rustc_hir_analysis::check_crate
  50: <rustc_interface::passes::LintStoreExpandImpl as rustc_expand::base::LintStoreExpand>::pre_expansion_lint
  51: rustc_interface::passes::analysis
  52: <alloc::sync::Arc<rustc_session::cstore::CrateSource>>::drop_slow
  53: RINvNtNtCs1bonBNpMvFl_18rustc_query_system5query8plumbing17try_execute_queryINtCs5uFTMav0Vyp_16rustc_query_impl13DynamicConfigINtNtB4_6caches11SingleCacheINtNtNtCs2vqUEYxsi4E_12rustc_middle5query5erase6ErasedAhj0_EEKb0_KB3s_KB3s_ENtNtB1f_8plumbing9QueryCtx
  54: rustc_query_impl::query_system
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: the compiler unexpectedly panicked. this is a bug.

note: using internal features is not supported and expected to cause internal compiler errors when used incorrectly

note: please attach the file at `H:\work\my\trash\rust-mini\req\rustc-ice-2025-05-08T11_30_07-20116.txt` to your bug report

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 `dns::resolve::DynResolver: hyper_util::client::legacy::connect::dns::sealed::Resolve`
#1 [typeck] type-checking `connect::<impl at vendor\reqwest\src\connect.rs:199:1: 199:22>::connect_with_maybe_proxy`
#2 [analysis] running analysis passes on this crate
end of query stack
there was a panic while trying to force a dep node
try_mark_green dep node stack:
#0 type_of_opaque(reqwest[4048]::connect::{impl#2}::connect_with_maybe_proxy::{opaque#0})
#1 type_of(reqwest[4048]::connect::{impl#2}::connect_with_maybe_proxy::{opaque#0})
#2 check_well_formed(reqwest[4048]::connect::{impl#2}::connect_with_maybe_proxy::{opaque#0})
#3 check_mod_type_wf(reqwest[4048]::connect)
end of try_mark_green dep node stack

Other issues with the same EvaluatedToAmbig: #127432 #127051 , although we probably can't know whether this one is an exact duplicate...

@rustbot label: +A-incr-comp

[cyrgani]

working on a reduction

[cyrgani]

reduction with renaming: cargo clean && cargo rustc && cargo rustc -- --cfg=a

struct Foo;

trait First {
    type Assoc;
}

trait Second {
    type Assoc;
}

trait Final {
    fn call();
}

impl First for Foo {
    #[cfg(not(a))]
    type Assoc = ();
}

impl<T> Second for T
where
    T: First,
    T::Assoc: Into<Box<()>>,
{
    type Assoc = ();
}

impl<T> Final for T
where
    T: Second,
    T::Assoc: Clone,
{
    fn call() {}
}

fn main() {
    Foo::call();
}

@rustbot label S-has-mcve