ICE: `deeply_normalize should not be called with pending obligations`

[matthiaskrgr]

auto-reduced (treereduce-rust):

pub trait TraitA {
    type AssocB = T;
}
pub trait TraitB {
    type AssocB;
}

pub trait MethodTrait {
    fn method(self) -> impl for<'a> FnMut(&'a ()) -> Self::Assoc<'a>;
}

impl<T: TraitB> MethodTrait for T
where
    <T::AssocB as TraitA>::AssocB: TraitA,
{
    // }
    fn method(self) -> impl for<'a> FnMut(&'a ()) -> Self::Assoc<'a> {}
}

original code

original:

pub struct Wrapper<T>(T);
struct Struct;

pub trait TraitA {
    type AssocB = T;
}
pub trait TraitB {
    type AssocB;
}

pub fn helper(v: impl MethodTrait) {
    // monomorphization instantiates something it then normalizes to:
    //
    // Closure(
    //   DefId(0:27 ~ unnamed_1[00e7]::{impl#0}::method::{closure#0}),
    //   [
    // must be a method (through Self), the example below doesn't work (as a standalone function)
    //     i16,
    //     Binder {
    //       value: extern "RustCall" fn((&'^0 (),)) -> Alias(Projection, AliasTy { args: [StructX, '^0], def_id: DefId(0:10 ~ unnamed_1[00e7]::TraitA::AssocA), .. }),
    //       bound_vars: [Region(BrAnon)]
    //     },
    //     ()
    //   ]
    // ),
    //
    // This should be completely normalized but isn't.
    // so, normalizing again gives (StructX is inserted) for
    // Alias(Projection, AliasTy { args: [StructX, '^0], def_id: DefId(0:10 ~ unnamed_1[00e7]::TraitA::AssocA), .. })
    //
    // Closure(
    //   DefId(0:27 ~ unnamed_1[00e7]::{impl#0}::method::{closure#0}),
    pub struct Wrapper<T>(T);
    //     Wrapper1<StructX>,
    //     i16,
    //     Binder {
    //       value: extern "RustCall" fn((&'^0 (),)) -> StructX, bound_vars: [Region(BrAnon)]
    //     },
    //     ()
    //   ]
    // ).
    let _local_that_causes_ice = v.method();
}

pub fn main() {
    helper(WrapperT::AssocB as TraitA);
}

pub trait MethodTrait {
    type Assoc<'a>;

    fn method(self) -> impl for<'a> FnMut(&'a ()) -> Self::Assoc<'a>;
}

impl<T: TraitB> MethodTrait for T
where
    <T::AssocB as TraitA>::AssocB: TraitA,
{
    type Assoc<'a> = <T::AssocB as TraitA>::AssocA<'t>;

    // must be a method (through Self), the example below doesn't work (as a standalone function)
    // fn helper2<M: MethodTrait>(_v: M) -> impl for<'a> FnMut(&'a ()) -> M::Assoc<'a> {
    //    move |_| loop {}
    // }
    fn method(self) -> impl for<'a> FnMut(&'a ()) -> Self::Assoc<'a> {
        move |_| loop {}
    }
}

impl<T, B> TraitB for Wrapper<B>
where
    B: TraitB<AssocB = T>,
{
    type AssocB = T;
}

impl TraitB for Struct {
    type AssocB = Struct;
}

impl TraitA for Struct {
    type AssocA<'t> = Self;
}

Version information

rustc 1.85.0-nightly (96e51d948 2024-12-04)
binary: rustc
commit-hash: 96e51d9482405e400dec53750f3b263d45784ada
commit-date: 2024-12-04
host: x86_64-unknown-linux-gnu
release: 1.85.0-nightly
LLVM version: 19.1.5

Possibly related line of code:

rust/compiler/rustc_trait_selection/src/traits/normalize.rs

Lines 63 to 75 in 96e51d9

	{
	if self.infcx.next_trait_solver() {
	crate::solve::deeply_normalize(self, value)
	} else {
	if fulfill_cx.has_pending_obligations() {
	let pending_obligations = fulfill_cx.pending_obligations();
	span_bug!(
	pending_obligations[0].cause.span,
	"deeply_normalize should not be called with pending obligations: \
	{pending_obligations:#?}"
	);
	}
	let value = self

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc

Program output

error[E0412]: cannot find type `T` in this scope
 --> /tmp/icemaker_global_tempdir.oqycY4Xka193/rustc_testrunner_tmpdir_reporting.FxaXXxPy3JEq/mvce.rs:2:19
  |
2 |     type AssocB = T;
  |                   ^ not found in this scope

error[E0658]: associated type defaults are unstable
 --> /tmp/icemaker_global_tempdir.oqycY4Xka193/rustc_testrunner_tmpdir_reporting.FxaXXxPy3JEq/mvce.rs:2:5
  |
2 |     type AssocB = T;
  |     ^^^^^^^^^^^^^^^^
  |
  = note: see issue #29661 <https://github.com/rust-lang/rust/issues/29661> for more information
  = help: add `#![feature(associated_type_defaults)]` to the crate attributes to enable
  = note: this compiler was built on 2024-12-04; consider upgrading it if it is out of date

error[E0601]: `main` function not found in crate `mvce`
  --> /tmp/icemaker_global_tempdir.oqycY4Xka193/rustc_testrunner_tmpdir_reporting.FxaXXxPy3JEq/mvce.rs:18:2
   |
18 | }
   |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.oqycY4Xka193/rustc_testrunner_tmpdir_reporting.FxaXXxPy3JEq/mvce.rs`

error[E0277]: the trait bound `<T as TraitB>::AssocB: TraitA` is not satisfied
  --> /tmp/icemaker_global_tempdir.oqycY4Xka193/rustc_testrunner_tmpdir_reporting.FxaXXxPy3JEq/mvce.rs:17:5
   |
17 |     fn method(self) -> impl for<'a> FnMut(&'a ()) -> Self::Assoc<'a> {}
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `TraitA` is not implemented for `<T as TraitB>::AssocB`
   |
help: consider further restricting the associated type
   |
17 |     fn method(self) -> impl for<'a> FnMut(&'a ()) -> Self::Assoc<'a> where <T as TraitB>::AssocB: TraitA {}
   |                                                                      +++++++++++++++++++++++++++++++++++

error[E0220]: associated type `Assoc` not found for `Self`
 --> /tmp/icemaker_global_tempdir.oqycY4Xka193/rustc_testrunner_tmpdir_reporting.FxaXXxPy3JEq/mvce.rs:9:60
  |
9 |     fn method(self) -> impl for<'a> FnMut(&'a ()) -> Self::Assoc<'a>;
  |                                                            ^^^^^ associated type `Assoc` not found

error[E0220]: associated type `Assoc` not found for `Self`
  --> /tmp/icemaker_global_tempdir.oqycY4Xka193/rustc_testrunner_tmpdir_reporting.FxaXXxPy3JEq/mvce.rs:17:60
   |
17 |     fn method(self) -> impl for<'a> FnMut(&'a ()) -> Self::Assoc<'a> {}
   |                                                            ^^^^^ associated type `Assoc` not found

error[E0277]: the trait bound `<T as TraitB>::AssocB: TraitA` is not satisfied
  --> /tmp/icemaker_global_tempdir.oqycY4Xka193/rustc_testrunner_tmpdir_reporting.FxaXXxPy3JEq/mvce.rs:17:24
   |
17 |     fn method(self) -> impl for<'a> FnMut(&'a ()) -> Self::Assoc<'a> {}
   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `TraitA` is not implemented for `<T as TraitB>::AssocB`
   |
help: consider further restricting the associated type
   |
17 |     fn method(self) -> impl for<'a> FnMut(&'a ()) -> Self::Assoc<'a> where <T as TraitB>::AssocB: TraitA {}
   |                                                                      +++++++++++++++++++++++++++++++++++

error[E0277]: the trait bound `<T as TraitB>::AssocB: TraitA` is not satisfied
  --> /tmp/icemaker_global_tempdir.oqycY4Xka193/rustc_testrunner_tmpdir_reporting.FxaXXxPy3JEq/mvce.rs:12:1
   |
12 | / impl<T: TraitB> MethodTrait for T
13 | | where
14 | |     <T::AssocB as TraitA>::AssocB: TraitA,
   | |__________________________________________^ the trait `TraitA` is not implemented for `<T as TraitB>::AssocB`
   |
help: consider further restricting the associated type
   |
14 |     <T::AssocB as TraitA>::AssocB: TraitA, <T as TraitB>::AssocB: TraitA
   |                                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

error[E0277]: the trait bound `<T as TraitB>::AssocB: TraitA` is not satisfied
  --> /tmp/icemaker_global_tempdir.oqycY4Xka193/rustc_testrunner_tmpdir_reporting.FxaXXxPy3JEq/mvce.rs:12:1
   |
12 | / impl<T: TraitB> MethodTrait for T
13 | | where
14 | |     <T::AssocB as TraitA>::AssocB: TraitA,
15 | | {
16 | |     // }
17 | |     fn method(self) -> impl for<'a> FnMut(&'a ()) -> Self::Assoc<'a> {}
18 | | }
   | |_^ the trait `TraitA` is not implemented for `<T as TraitB>::AssocB`
   |
help: consider further restricting the associated type
   |
14 |     <T::AssocB as TraitA>::AssocB: TraitA, <T as TraitB>::AssocB: TraitA
   |                                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

error[E0277]: the trait bound `<T as TraitB>::AssocB: TraitA` is not satisfied
  --> /tmp/icemaker_global_tempdir.oqycY4Xka193/rustc_testrunner_tmpdir_reporting.FxaXXxPy3JEq/mvce.rs:17:24
   |
17 |     fn method(self) -> impl for<'a> FnMut(&'a ()) -> Self::Assoc<'a> {}
   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `TraitA` is not implemented for `<T as TraitB>::AssocB`
   |
help: consider further restricting the associated type
   |
14 |     <T::AssocB as TraitA>::AssocB: TraitA, <T as TraitB>::AssocB: TraitA
   |                                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

error: internal compiler error: compiler/rustc_trait_selection/src/traits/normalize.rs:69:17: deeply_normalize should not be called with pending obligations: [
                                    Obligation(predicate=Binder { value: TraitPredicate(<_ as TraitA>, polarity:Positive), bound_vars: [] }, depth=1),
                                ]
 --> /tmp/icemaker_global_tempdir.oqycY4Xka193/rustc_testrunner_tmpdir_reporting.FxaXXxPy3JEq/mvce.rs:9:24
  |
9 |     fn method(self) -> impl for<'a> FnMut(&'a ()) -> Self::Assoc<'a>;
  |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

thread 'rustc' panicked at compiler/rustc_trait_selection/src/traits/normalize.rs:69:17:
Box<dyn Any>
stack backtrace:
   0:     0x7f404913b59a - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h7d5d2ef8548ea862
   1:     0x7f4049813e26 - core::fmt::write::hc7f1fcf2cd4d0b24
   2:     0x7f404a7f4751 - std::io::Write::write_fmt::h5dae3cf637b1b14c
   3:     0x7f404913b3f2 - std::sys::backtrace::BacktraceLock::print::h603d5439a4d48e27
   4:     0x7f404913d91a - std::panicking::default_hook::{{closure}}::h317c28a5ebbd850f
   5:     0x7f404913d763 - std::panicking::default_hook::h0f4f8ead395a2965
   6:     0x7f40482bbb88 - std[b886acd6fb60b0a8]::panicking::update_hook::<alloc[e4e71b9a99f0d22f]::boxed::Box<rustc_driver_impl[c07627fafa57377e]::install_ice_hook::{closure#0}>>::{closure#0}
   7:     0x7f404913e0d8 - std::panicking::rust_panic_with_hook::he1329bac1e03080f
   8:     0x7f40482f10a1 - std[b886acd6fb60b0a8]::panicking::begin_panic::<rustc_errors[c4cb0ec99f59abcb]::ExplicitBug>::{closure#0}
   9:     0x7f40482e6246 - std[b886acd6fb60b0a8]::sys::backtrace::__rust_end_short_backtrace::<std[b886acd6fb60b0a8]::panicking::begin_panic<rustc_errors[c4cb0ec99f59abcb]::ExplicitBug>::{closure#0}, !>
  10:     0x7f40482e2d29 - std[b886acd6fb60b0a8]::panicking::begin_panic::<rustc_errors[c4cb0ec99f59abcb]::ExplicitBug>
  11:     0x7f40482fb041 - <rustc_errors[c4cb0ec99f59abcb]::diagnostic::BugAbort as rustc_errors[c4cb0ec99f59abcb]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  12:     0x7f404884f8bc - <rustc_errors[c4cb0ec99f59abcb]::DiagCtxtHandle>::span_bug::<rustc_span[5535563d387a33e2]::span_encoding::Span, alloc[e4e71b9a99f0d22f]::string::String>
  13:     0x7f40488e2227 - rustc_middle[67be29f945e3ecf0]::util::bug::opt_span_bug_fmt::<rustc_span[5535563d387a33e2]::span_encoding::Span>::{closure#0}
  14:     0x7f40488ca74a - rustc_middle[67be29f945e3ecf0]::ty::context::tls::with_opt::<rustc_middle[67be29f945e3ecf0]::util::bug::opt_span_bug_fmt<rustc_span[5535563d387a33e2]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  15:     0x7f40488ca5db - rustc_middle[67be29f945e3ecf0]::ty::context::tls::with_context_opt::<rustc_middle[67be29f945e3ecf0]::ty::context::tls::with_opt<rustc_middle[67be29f945e3ecf0]::util::bug::opt_span_bug_fmt<rustc_span[5535563d387a33e2]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  16:     0x7f404738acf7 - rustc_middle[67be29f945e3ecf0]::util::bug::span_bug_fmt::<rustc_span[5535563d387a33e2]::span_encoding::Span>
  17:     0x7f4049b83090 - <rustc_trait_selection[8b7799daf48e843f]::traits::engine::ObligationCtxt<rustc_trait_selection[8b7799daf48e843f]::traits::FulfillmentError>>::assumed_wf_types_and_report_errors
  18:     0x7f404a004b26 - rustc_hir_analysis[e6847c1464713c9a]::check::compare_impl_item::check_type_bounds
  19:     0x7f404a22849d - rustc_hir_analysis[e6847c1464713c9a]::check::compare_impl_item::compare_impl_item
  20:     0x7f404a225dd1 - rustc_query_impl[d2f5d943c2280edc]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[d2f5d943c2280edc]::query_impl::compare_impl_item::dynamic_query::{closure#2}::{closure#0}, rustc_middle[67be29f945e3ecf0]::query::erase::Erased<[u8; 1usize]>>
  21:     0x7f404a1cf730 - rustc_query_system[254f9b8bf6b2b5ec]::query::plumbing::try_execute_query::<rustc_query_impl[d2f5d943c2280edc]::DynamicConfig<rustc_data_structures[d98f95e3c765a7d0]::vec_cache::VecCache<rustc_span[5535563d387a33e2]::def_id::LocalDefId, rustc_middle[67be29f945e3ecf0]::query::erase::Erased<[u8; 1usize]>, rustc_query_system[254f9b8bf6b2b5ec]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[d2f5d943c2280edc]::plumbing::QueryCtxt, false>
  22:     0x7f404a1cf201 - rustc_query_impl[d2f5d943c2280edc]::query_impl::compare_impl_item::get_query_non_incr::__rust_end_short_backtrace
  23:     0x7f4045dbbf66 - rustc_hir_analysis[e6847c1464713c9a]::check::check::check_item_type
  24:     0x7f40471754b4 - rustc_hir_analysis[e6847c1464713c9a]::check::wfcheck::check_well_formed
  25:     0x7f404a1cf487 - rustc_query_impl[d2f5d943c2280edc]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[d2f5d943c2280edc]::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle[67be29f945e3ecf0]::query::erase::Erased<[u8; 1usize]>>
  26:     0x7f404a1cf748 - rustc_query_system[254f9b8bf6b2b5ec]::query::plumbing::try_execute_query::<rustc_query_impl[d2f5d943c2280edc]::DynamicConfig<rustc_data_structures[d98f95e3c765a7d0]::vec_cache::VecCache<rustc_span[5535563d387a33e2]::def_id::LocalDefId, rustc_middle[67be29f945e3ecf0]::query::erase::Erased<[u8; 1usize]>, rustc_query_system[254f9b8bf6b2b5ec]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[d2f5d943c2280edc]::plumbing::QueryCtxt, false>
  27:     0x7f404a1cf462 - rustc_query_impl[d2f5d943c2280edc]::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
  28:     0x7f404a1d01ec - rustc_hir_analysis[e6847c1464713c9a]::check::wfcheck::check_mod_type_wf
  29:     0x7f404a1d000b - rustc_query_impl[d2f5d943c2280edc]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[d2f5d943c2280edc]::query_impl::check_mod_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle[67be29f945e3ecf0]::query::erase::Erased<[u8; 1usize]>>
  30:     0x7f404a62b784 - rustc_query_system[254f9b8bf6b2b5ec]::query::plumbing::try_execute_query::<rustc_query_impl[d2f5d943c2280edc]::DynamicConfig<rustc_query_system[254f9b8bf6b2b5ec]::query::caches::DefaultCache<rustc_span[5535563d387a33e2]::def_id::LocalModDefId, rustc_middle[67be29f945e3ecf0]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[d2f5d943c2280edc]::plumbing::QueryCtxt, false>
  31:     0x7f404a62b518 - rustc_query_impl[d2f5d943c2280edc]::query_impl::check_mod_type_wf::get_query_non_incr::__rust_end_short_backtrace
  32:     0x7f4049ac9adc - rustc_hir_analysis[e6847c1464713c9a]::check_crate
  33:     0x7f404a13a57c - rustc_interface[14bb8bd49e3c2011]::passes::run_required_analyses
  34:     0x7f404a3cfd5e - rustc_interface[14bb8bd49e3c2011]::passes::analysis
  35:     0x7f404a3cfd2f - rustc_query_impl[d2f5d943c2280edc]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[d2f5d943c2280edc]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[67be29f945e3ecf0]::query::erase::Erased<[u8; 1usize]>>
  36:     0x7f404a7bc37a - rustc_query_system[254f9b8bf6b2b5ec]::query::plumbing::try_execute_query::<rustc_query_impl[d2f5d943c2280edc]::DynamicConfig<rustc_query_system[254f9b8bf6b2b5ec]::query::caches::SingleCache<rustc_middle[67be29f945e3ecf0]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[d2f5d943c2280edc]::plumbing::QueryCtxt, false>
  37:     0x7f404a7bc04e - rustc_query_impl[d2f5d943c2280edc]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  38:     0x7f404a88c239 - rustc_interface[14bb8bd49e3c2011]::interface::run_compiler::<core[dd48f16c736a3ea3]::result::Result<(), rustc_span[5535563d387a33e2]::ErrorGuaranteed>, rustc_driver_impl[c07627fafa57377e]::run_compiler::{closure#0}>::{closure#1}
  39:     0x7f404a71a3c7 - std[b886acd6fb60b0a8]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[14bb8bd49e3c2011]::util::run_in_thread_with_globals<rustc_interface[14bb8bd49e3c2011]::util::run_in_thread_pool_with_globals<rustc_interface[14bb8bd49e3c2011]::interface::run_compiler<core[dd48f16c736a3ea3]::result::Result<(), rustc_span[5535563d387a33e2]::ErrorGuaranteed>, rustc_driver_impl[c07627fafa57377e]::run_compiler::{closure#0}>::{closure#1}, core[dd48f16c736a3ea3]::result::Result<(), rustc_span[5535563d387a33e2]::ErrorGuaranteed>>::{closure#0}, core[dd48f16c736a3ea3]::result::Result<(), rustc_span[5535563d387a33e2]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[dd48f16c736a3ea3]::result::Result<(), rustc_span[5535563d387a33e2]::ErrorGuaranteed>>
  40:     0x7f404a71a062 - <<std[b886acd6fb60b0a8]::thread::Builder>::spawn_unchecked_<rustc_interface[14bb8bd49e3c2011]::util::run_in_thread_with_globals<rustc_interface[14bb8bd49e3c2011]::util::run_in_thread_pool_with_globals<rustc_interface[14bb8bd49e3c2011]::interface::run_compiler<core[dd48f16c736a3ea3]::result::Result<(), rustc_span[5535563d387a33e2]::ErrorGuaranteed>, rustc_driver_impl[c07627fafa57377e]::run_compiler::{closure#0}>::{closure#1}, core[dd48f16c736a3ea3]::result::Result<(), rustc_span[5535563d387a33e2]::ErrorGuaranteed>>::{closure#0}, core[dd48f16c736a3ea3]::result::Result<(), rustc_span[5535563d387a33e2]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[dd48f16c736a3ea3]::result::Result<(), rustc_span[5535563d387a33e2]::ErrorGuaranteed>>::{closure#1} as core[dd48f16c736a3ea3]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  41:     0x7f404a7197ab - std::sys::pal::unix::thread::Thread::new::thread_start::h7cc6adb00f817196
  42:     0x7f4044a6a39d - <unknown>
  43:     0x7f4044aef49c - <unknown>
  44:                0x0 - <unknown>

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 1.85.0-nightly (96e51d948 2024-12-04) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [compare_impl_item] checking assoc item `<impl at /tmp/icemaker_global_tempdir.oqycY4Xka193/rustc_testrunner_tmpdir_reporting.FxaXXxPy3JEq/mvce.rs:12:1: 14:43>::{synthetic#0}` is compatible with trait definition
#1 [check_well_formed] checking that `<impl at /tmp/icemaker_global_tempdir.oqycY4Xka193/rustc_testrunner_tmpdir_reporting.FxaXXxPy3JEq/mvce.rs:12:1: 14:43>` is well-formed
end of query stack
error: aborting due to 11 previous errors

Some errors have detailed explanations: E0220, E0277, E0412, E0601, E0658.
For more information about an error, try `rustc --explain E0220`.

[matthiaskrgr]

mvce

trait a {
    type b;
}
trait c {
    fn d->impl
}
impl<e> c e 
where<e as a>::b: a { 
    fn d
}

[compiler-errors]

@matthiaskrgr: That's not an MCVE. That's "i'm trying to make a test with the minimal amount of tokens", but I'm certain I've noted before that such tests are actually significantly worse than minimizing it conceptually (which is a task that actually requires some puzzle solving to do, rather than just getting a machine to do it). a test that has minimal tokens introduces a bunch of unrelated parser errors and is a bad candidate for a ui test, frankly.

[matthiaskrgr]

MUTS? (minimum unviable token set)? 🙈

[compiler-errors]

A less esoteric MCVE:

trait Foo {
    type Assoc;
}

trait Bar {
    fn method() -> impl Sized;
}
impl<T> Bar for T where <T as Foo>::Assoc: Sized
 { 
    fn method() {}
}

[cuviper]

Crater #139827 found one of these too:

• https://crater-reports.s3.amazonaws.com/beta-1.87-2/beta-2025-04-13/gh/privi-metaverse.chainbridge-substrate/log.txt

The same code also failed with errors on 1.86.0, but no ICE:

• https://crater-reports.s3.amazonaws.com/beta-1.87-2/1.86.0/gh/privi-metaverse.chainbridge-substrate/log.txt