Migrating src/lib.rs from 2018 edition to 2021
error: internal compiler error: compiler/rustc_traits/src/normalize_erasing_regions.rs:54:32: could not fully normalize `<<I as std::iter::Iterator>::Item as std::iter::Iterator>::Item`

thread 'rustc' panicked at 'Box<dyn Any>', compiler/rustc_errors/src/lib.rs:1147:9
stack backtrace:
   0: std::panicking::begin_panic
   1: std::panic::panic_any
   2: rustc_errors::HandlerInner::bug
   3: rustc_errors::Handler::bug
   4: rustc_middle::ty::context::tls::with_opt
   5: rustc_middle::util::bug::opt_span_bug_fmt
   6: rustc_middle::util::bug::bug_fmt
   7: rustc_infer::infer::InferCtxtBuilder::enter
   8: core::ops::function::FnOnce::call_once
   9: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  10: rustc_data_structures::stack::ensure_sufficient_stack
  11: rustc_query_system::query::plumbing::try_execute_query
  12: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::normalize_generic_arg_after_erasing_regions
  13: <rustc_middle::ty::normalize_erasing_regions::NormalizeAfterErasingRegionsFolder as rustc_middle::ty::fold::TypeFolder>::fold_ty
  14: rustc_middle::ty::normalize_erasing_regions::<impl rustc_middle::ty::context::TyCtxt>::normalize_erasing_regions
  15: <rustc_ty_utils::needs_drop::NeedsDropTypes<F> as core::iter::traits::iterator::Iterator>::next
  16: <core::iter::adapters::ResultShunt<I,E> as core::iter::traits::iterator::Iterator>::next
  17: <alloc::vec::Vec<T> as alloc::vec::spec_from_iter::SpecFromIter<T,I>>::from_iter
  18: core::iter::adapters::process_results
  19: rustc_ty_utils::needs_drop::adt_significant_drop_tys
  20: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  21: rustc_data_structures::stack::ensure_sufficient_stack
  22: rustc_query_system::query::plumbing::try_execute_query
  23: rustc_query_system::query::plumbing::get_query
  24: rustc_ty_utils::needs_drop::has_significant_drop_raw
  25: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  26: rustc_data_structures::stack::ensure_sufficient_stack
  27: rustc_query_system::query::plumbing::try_execute_query
  28: rustc_query_system::query::plumbing::get_query
  29: rustc_middle::ty::util::<impl rustc_middle::ty::TyS>::has_significant_drop
  30: rustc_typeck::check::upvar::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::analyze_closure
  31: <rustc_typeck::check::upvar::InferBorrowKindVisitor as rustc_hir::intravisit::Visitor>::visit_expr
  32: rustc_hir::intravisit::walk_expr
  33: rustc_hir::intravisit::walk_stmt
  34: rustc_hir::intravisit::walk_block
  35: rustc_hir::intravisit::walk_body
  36: rustc_infer::infer::InferCtxtBuilder::enter
  37: rustc_typeck::check::typeck
  38: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  39: rustc_data_structures::stack::ensure_sufficient_stack
  40: rustc_query_system::query::plumbing::try_execute_query
  41: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::typeck
  42: rustc_typeck::check::typeck
  43: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  44: rustc_data_structures::stack::ensure_sufficient_stack
  45: rustc_query_system::query::plumbing::try_execute_query
  46: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::typeck
  47: rustc_middle::hir::map::Map::par_body_owners
  48: rustc_typeck::check::typeck_item_bodies
  49: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  50: rustc_data_structures::stack::ensure_sufficient_stack
  51: rustc_query_system::query::plumbing::try_execute_query
  52: rustc_query_system::query::plumbing::get_query
  53: rustc_session::utils::<impl rustc_session::session::Session>::time
  54: rustc_typeck::check_crate
  55: rustc_interface::passes::analysis
  56: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  57: rustc_data_structures::stack::ensure_sufficient_stack
  58: rustc_query_system::query::plumbing::try_execute_query
  59: rustc_query_system::query::plumbing::get_query
  60: rustc_interface::passes::QueryContext::enter
  61: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  62: rustc_span::with_source_map
  63: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

note: 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 1.58.0-nightly (efd048394 2021-10-20) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental

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

query stack during panic:
#0 [normalize_generic_arg_after_erasing_regions] normalizing `<<I as core::iter::traits::iterator::Iterator>::Item as core::iter::traits::iterator::Iterator>::Item`
#1 [adt_significant_drop_tys] computing when `Foo` has a significant destructor
#2 [has_significant_drop_raw] computing whether `Foo<core::iter::sources::empty::Empty<()>>` has a significant drop
#3 [typeck] type-checking `foo`
#4 [typeck] type-checking `foo::{closure#0}`
#5 [typeck_item_bodies] type-checking all item bodies
#6 [analysis] running analysis passes on this crate
end of query stack