ICE when referencing non-existent variant/item in C-like enum

[quantheory]

Modified from an example that @ahwatts provided in issue #22933:

struct Apple;

enum Delicious {
    ApplePie = Apple::PIE,
}

fn main() {}

Output with a recent nightly:

test_argl.rs:5:16: 5:26 error: no associated item named `PIE` found for type `Apple` in the current scope
test_argl.rs:5     ApplePie = Apple::PIE,
                              ^~~~~~~~~~
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
thread 'rustc' panicked at 'path not fully resolved: PathResolution { base_def: DefTy(DefId { krate: 0, node: 4 }, false), last_private: LastMod(AllPublic), depth: 1 }', ../src/librustc/middle/def.rs:81

stack backtrace:
   1:      0x317dd4983d3 - sys::backtrace::write::h42760ef5ab82bcd2irs
   2:      0x317dd4a0521 - panicking::on_panic::h12f68dc29fcc5657Tax
   3:      0x317dd45f28a - rt::unwind::begin_unwind_inner::h68e746642e433893DQw
   4:      0x317dd45fec7 - rt::unwind::begin_unwind_fmt::h22d77de4e1204a17JPw
   5:      0x317db414e0b - middle::const_eval::eval_const_expr_with_substs::h6757944525066075748
   6:      0x317db3d94c1 - middle::ty::enum_variants::hf5833ad9292c8ac5HAb
   7:      0x317dcc71c9d - check::check_enum_variants::h52e7fb747609e307ALs
   8:      0x317dcc63995 - check::check_item_type::ha7187a1df3f0e4ddEco
   9:      0x317dcc6aa44 - check::check_item_types::h8615aaad832dd957tRn
  10:      0x317dcd2bbb0 - check_crate::h15b94c5d979eb877VgD
  11:      0x317dd9ffcb9 - driver::phase_3_run_analysis_passes::h535099e77e0d0b0aHGa
  12:      0x317dd9e155a - driver::compile_input::hf94b88e60b92b1d3Sba
  13:      0x317dda9d2e7 - run_compiler::hd38788069be51651F7b
  14:      0x317dda9a8d6 - boxed::F.FnBox<A>::call_box::h10971823990562187917
  15:      0x317dda9a0d9 - rt::unwind::try::try_fn::h13260011850501376318
  16:      0x317dd51cea8 - rust_try_inner
  17:      0x317dd51ce95 - rust_try
  18:      0x317dd48b147 - rt::unwind::try::inner_try::hb6dce8ad1d668192wMw
  19:      0x317dda9a2f9 - boxed::F.FnBox<A>::call_box::h7963608429107561708
  20:      0x317dd49f151 - sys::thread::Thread::new::thread_start::h82e421ca205cff91sWv
  21:      0x317d702369b - <unknown>
  22:      0x317dd0f932c - clone
  23:                0x0 - <unknown>

The original example gets an error through a different code path (check_casts -> middle::CastTy::from_ty -> ty::type_is_c_like_enum), but the result is still that ty::enum_variants is called, which then attempts to evaluate the constant expression, causing an error.

For unresolved paths I think we should have the const_eval functions return an error rather than panicking. This is a bit of a mess (e.g. lots of unhelpful uses of Option rather than Error for error handling), and should be fixed anyway.

[mitaa]

Fixed by #28686

Now errors with

<anon>:4:16: 4:26 error: constant evaluation error: unresolved path in constant expression [E0080]
<anon>:4     ApplePie = Apple::PIE,
                        ^~~~~~~~~~

[alexcrichton]

Yay!