rustc should error instead of panic when the generator trait is not found

[bjorn3]

This somehow doesn't occur when putting the abc function in a non #![no_core] crate, yet I can't find any difference between the libcore version of FnOnce and this one.

#![feature(no_core, lang_items, unboxed_closures)]
#![no_core]

#[lang = "sized"]
pub trait Sized {}

#[lang = "fn_once"]
#[rustc_paren_sugar]
pub trait FnOnce<Args> {
    type Output;

    extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
}

pub fn abc() -> impl FnOnce(f32) {
    |_| {}
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', src/libcore/option.rs:378:21
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.35/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.35/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:47
   3: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:36
   4: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:200
   5: std::panicking::default_hook
             at src/libstd/panicking.rs:214
   6: rustc::util::common::panic_hook
   7: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:481
   8: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:384
   9: rust_begin_unwind
             at src/libstd/panicking.rs:311
  10: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
  11: core::panicking::panic
             at src/libcore/panicking.rs:49
  12: rustc_typeck::check::closure::<impl rustc_typeck::check::FnCtxt>::deduce_sig_from_projection
  13: rustc_typeck::check::closure::<impl rustc_typeck::check::FnCtxt>::check_expr_closure
  14: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  15: rustc_typeck::check::FnCtxt::check_block_with_expected
  16: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  17: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_return_expr
  18: rustc_typeck::check::check_fn
  19: rustc::ty::context::GlobalCtxt::enter_local
  20: rustc_typeck::check::typeck_tables_of
  21: rustc::ty::query::__query_compute::typeck_tables_of
  22: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::typeck_tables_of>::compute
  23: rustc::dep_graph::graph::DepGraph::with_task_impl
  24: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  25: rustc_typeck::collect::checked_type_of
  26: rustc_typeck::collect::type_of
  27: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::type_of>::compute
  28: rustc::dep_graph::graph::DepGraph::with_task_impl
  29: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  30: rustc::ty::util::<impl rustc::ty::context::TyCtxt>::try_expand_impl_trait_type::OpaqueTypeExpander::expand_opaque_ty
  31: rustc::ty::util::<impl rustc::ty::context::TyCtxt>::try_expand_impl_trait_type
  32: rustc_typeck::check::check_item_type
  33: rustc::hir::map::Map::visit_item_likes_in_module
  34: rustc_typeck::check::check_mod_item_types
  35: rustc::ty::query::__query_compute::check_mod_item_types
  36: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::check_mod_item_types>::compute
  37: rustc::dep_graph::graph::DepGraph::with_task_impl
  38: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  39: rustc_typeck::check_crate::{{closure}}
  40: rustc::util::common::time
  41: rustc_typeck::check_crate
  42: rustc_interface::passes::analysis
  43: rustc::ty::query::__query_compute::analysis
  44: rustc::dep_graph::graph::DepGraph::with_task_impl
  45: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  46: rustc::ty::context::tls::enter_global
  47: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  48: rustc_interface::passes::create_global_ctxt::{{closure}}
  49: rustc_interface::passes::BoxedGlobalCtxt::enter
  50: rustc_interface::interface::run_compiler_in_existing_thread_pool
  51: std::thread::local::LocalKey<T>::with
  52: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
query stack during panic:
#0 [typeck_tables_of] processing `abc`
#1 [type_of] processing `abc::{{opaque}}#0`
#2 [check_mod_item_types] checking item types in top-level module
#3 [analysis] running analysis passes on this crate
end of query stack

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 1.39.0-nightly (9eae1fc0e 2019-08-23) 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

error: Could not compile `playground`.

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

[bjorn3]

Seems like it panic when the Generator trait is not found:

rust/src/librustc_typeck/check/closure.rs

Line 269 in e2b4165

let gen_trait = tcx.lang_items().gen_trait().unwrap();

Maybe it can use tcx.require_lang_item instead.