message: Fix single field by val tuples
diff: @@ -224,6 +224,9 @@ impl<'a, 'tcx> EvalContext<'a, 'tcx> {
  )?;
  let mut arg_locals = self.frame().mir.args_iter();
  + trace!("ABI: {:?}", sig.abi);
  + trace!("arg_locals: {:?}", self.frame().mir.args_iter().collect::<Vec<_>>());
  + trace!("arg_operands: {:?}", arg_operands);
  match sig.abi {
  Abi::Rust => {
  for (arg_local, (arg_val, arg_ty)) in arg_locals.zip(args) {
  @@ -245,16 +248,27 @@ impl<'a, 'tcx> EvalContext<'a, 'tcx> {
  let (arg_val, arg_ty) = args.remove(0);
  let layout = self.type_layout(arg_ty)?;
  if let (&ty::TyTuple(fields, _), &Layout::Univariant { ref variant, .. }) = (&arg_ty.sty, layout) {
  + trace!("fields: {:?}", fields);
  if self.frame().mir.args_iter().count() == fields.len() + 1 {
  let offsets = variant.offsets.iter().map(|s| s.bytes());
  - if let Value::ByRef(ptr) = arg_val {
  + match arg_val {
  + Value::ByRef(ptr) => {
  for ((offset, ty), arg_local) in offsets.zip(fields).zip(arg_locals) {
  let arg = Value::ByRef(ptr.offset(offset));
  let dest = self.eval_lvalue(&mir::Lvalue::Local(arg_local))?;
  + trace!("writing arg {:?} to {:?} (type: {})", arg, dest, ty);
  self.write_value(arg, dest, ty)?;
  }
  + },
  + Value::ByVal(PrimVal::Undef) => {},
  + other => {
  + assert_eq!(fields.len(), 1);
  + let dest = self.eval_lvalue(&mir::Lvalue::Local(arg_locals.next().unwrap()))?;
  + self.write_value(other, dest, fields[0])?;
  + }
  }
  } else {
  + trace!("manual impl of rust-call ABI");
  // called a manual impl of a rust-call function
  let dest = self.eval_lvalue(&mir::Lvalue::Local(arg_locals.next().unwrap()))?;
  self.write_value(arg_val, dest, arg_ty)?;
  