message: Dropping slices works again
diff: @@ -116,7 +116,17 @@ impl<'a, 'tcx> EvalContext<'a, 'tcx> {
  arg = Value::ByValPair(PrimVal::Ptr(src_ptr), PrimVal::Bytes(n as u128));
  ::eval_context::MirRef::clone(&self.seq_drop_glue)
  },
  - ty::TySlice(ref elem) => unimplemented!(),
  + ty::TySlice(elem) => {
  + instance.substs = self.tcx.mk_substs([
  + Kind::from(elem),
  + ].iter().cloned());
  + if let Lvalue::Ptr { ptr, extra: LvalueExtra::Length(len) } = lval {
  + arg = Value::ByValPair(PrimVal::Ptr(ptr), PrimVal::Bytes(len as u128));
  + ::eval_context::MirRef::clone(&self.seq_drop_glue)
  + } else {
  + panic!("slice without length: {:?}", lval);
  + }
  + },
  _ => {
  let src_ptr = self.force_allocation(lval)?.to_ptr();
  arg = Value::ByVal(PrimVal::Ptr(src_ptr));
  