thread 'rustc' panicked at 'Failed to get parent for DefId(0:5 ~ playground[5c45]::{{impl}}[1])', src/librustc/traits/specialize/specialization_graph.rs:390:52

[steffahn]

Didn’t find a way (yet?) to do this for traits that are not FnOnce, also didn’t find a way to trigger the panic without having a conflicting trait implementation.

Where it currently says &F, you can also use F, &mut F, && F, ect, ..., or Box<F>. Seems like, possibly, everything that already implements FnOnce triggers the panic.

This also panicks on stable(1.40.0), also with features commented out (though it’ll complain about unstable/missing features before the panic).

#![feature(unboxed_closures)]
#![feature(type_alias_impl_trait)]
#![feature(fn_traits)]

trait MyTrait {}
impl MyTrait for () {}

impl<F> FnOnce<()> for &F {
    type Output = impl MyTrait ;
    fn call_once(self, _:() ) -> Self::Output { }
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0119]: conflicting implementations of trait `std::ops::FnOnce<()>` for type `&_`:
 --> src/lib.rs:8:1
  |
8 | impl<F> FnOnce<()> for &F {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: conflicting implementation in crate `core`:
          - impl<A, F> std::ops::FnOnce<A> for &F
            where F: std::ops::Fn<A>, F: ?Sized;

thread 'rustc' panicked at 'Failed to get parent for DefId(0:5 ~ playground[5c45]::{{impl}}[1])', src/librustc/traits/specialize/specialization_graph.rs:390:52
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:77
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1057
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1426
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:195
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:215
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:476
  12: rust_begin_unwind
             at src/libstd/panicking.rs:376
  13: std::panicking::begin_panic_fmt
             at src/libstd/panicking.rs:327
  14: rustc::traits::specialize::specialization_graph::Graph::parent::{{closure}}
  15: rustc::traits::specialize::specialization_graph::Graph::parent
  16: rustc::traits::specialize::specialization_graph::Ancestors::leaf_def
  17: rustc::traits::project::assoc_ty_def
  18: rustc::infer::InferCtxt::commit_if_ok
  19: rustc::traits::project::opt_normalize_projection_type
  20: rustc::traits::project::normalize_projection_type
  21: <rustc::traits::project::AssocTypeNormalizer as rustc::ty::fold::TypeFolder>::fold_ty
  22: <smallvec::SmallVec<A> as core::iter::traits::collect::FromIterator<<A as smallvec::Array>::Item>>::from_iter
  23: rustc::ty::fold::TypeFoldable::fold_with
  24: rustc::traits::project::normalize
  25: rustc_typeck::check::Inherited::normalize_associated_types_in
  26: rustc::ty::context::tls::with_context::{{closure}}
  27: rustc_typeck::check::typeck_tables_of
  28: rustc::ty::query::__query_compute::typeck_tables_of
  29: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::typeck_tables_of>::compute
  30: rustc::dep_graph::graph::DepGraph::with_task_impl
  31: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  32: rustc_typeck::collect::find_opaque_ty_constraints::ConstraintLocator::check
  33: rustc::hir::intravisit::Visitor::visit_nested_impl_item
  34: rustc::hir::intravisit::walk_item
  35: rustc_typeck::collect::find_opaque_ty_constraints
  36: rustc_typeck::collect::type_of
  37: rustc::ty::query::__query_compute::type_of
  38: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::type_of>::compute
  39: rustc::dep_graph::graph::DepGraph::with_task_impl
  40: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  41: <rustc_typeck::collect::CollectItemTypesVisitor as rustc::hir::intravisit::Visitor>::visit_impl_item
  42: rustc::hir::map::Map::visit_item_likes_in_module
  43: rustc_typeck::collect::collect_mod_item_types
  44: rustc::ty::query::__query_compute::collect_mod_item_types
  45: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::collect_mod_item_types>::compute
  46: rustc::dep_graph::graph::DepGraph::with_task_impl
  47: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  48: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::ensure_query
  49: rustc_typeck::check_crate::{{closure}}::{{closure}}
  50: rustc::util::common::time
  51: rustc_typeck::check_crate
  52: rustc_interface::passes::analysis
  53: rustc::ty::query::__query_compute::analysis
  54: rustc::dep_graph::graph::DepGraph::with_task_impl
  55: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  56: rustc::ty::context::tls::enter_global
  57: rustc_interface::interface::run_compiler_in_existing_thread_pool
  58: scoped_tls::ScopedKey<T>::set
  59: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: internal compiler error: unexpected panic

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

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.42.0-nightly (da3629b05 2019-12-29) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type lib

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

query stack during panic:
#0 [typeck_tables_of] processing `<impl std::ops::FnOnce<()> for &F>::call_once`
#1 [type_of] processing `<impl std::ops::FnOnce<()> for &F>::Output`
#2 [collect_mod_item_types] collecting item types in top-level module
#3 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

For more information about this error, try `rustc --explain E0119`.
error: could not compile `playground`.

To learn more, run the command again with --verbose.