'assertion failed: fields.len() > 0' ICE with struct variants

[tomjakubowski]

Simple reproduction:

#![feature(struct_variant)]

enum Foo {
    Bar {}
}

fn main() {}

error: internal compiler error: unexpected panic
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: http://doc.rust-lang.org/complement-bugreport.html
note: run with `RUST_BACKTRACE=1` for a backtrace
task 'rustc' panicked at 'assertion failed: fields.len() > 0', /build/rust-git/src/rust/src/librustc/middle/ty.rs:4273

stack backtrace:
   1:     0x7fc1d4523f90 - rt::backtrace::imp::write::h599f1affd1b0bc63Bgt
   2:     0x7fc1d4527010 - <unknown>
   3:     0x7fc1d4cdb5f0 - unwind::begin_unwind_inner::h9a24fa62b68e1061E9c
   4:     0x7fc1d50886b0 - <unknown>
   5:     0x7fc1d54bfcd0 - middle::ty::VariantInfo::from_ast_variant::hfdc74922dfb784f6HCK
   6:     0x7fc1d57fef20 - middle::typeck::variance::ConstraintContext<'a, 'tcx>.Visitor<'v>::visit_item::h21525de41ade7650gun
   7:     0x7fc1d57f9d80 - middle::typeck::variance::infer_variance::h403b90f07402dc1cM6m
   8:     0x7fc1d50a36c0 - <unknown>
   9:     0x7fc1d5804680 - middle::typeck::check_crate::h4c6a3e5eabf06e46lrp
  10:     0x7fc1d5866610 - driver::driver::phase_3_run_analysis_passes::ha2a07c2b2bcd993dM9B
  11:     0x7fc1d5861b00 - driver::driver::compile_input::h62b35130395852dcxQB
  12:     0x7fc1d58e9390 - <unknown>
  13:     0x7fc1d58e9280 - <unknown>
  14:     0x7fc1d50bcbf0 - <unknown>
  15:     0x7fc1d50bc9e0 - <unknown>
  16:     0x7fc1d6114a10 - <unknown>
  17:     0x7fc1d4d28820 - <unknown>
  18:     0x7fc1d4d28810 - rust_try
  19:     0x7fc1d4cd8f70 - unwind::try::ha5be98cf90518e14mYc
  20:     0x7fc1d4cd8e00 - task::Task::run::h2157d702c714dd0du4b
  21:     0x7fc1d6114750 - <unknown>
  22:     0x7fc1d4cda610 - <unknown>
  23:     0x7fc1cfedc250 - start_thread
  24:     0x7fc1d49b53b9 - clone
  25:                0x0 - <unknown>

Version is rustc 0.13.0-dev (7e43f419c 2014-11-15 13:22:24 +0000)

[tomjakubowski]

I'll submit a fix shortly. I guess it's an open question whether that should be a nullary variant or an error, related to the question about whether struct Foo {} is acceptable.