Forbid classes with no fields

Classes with no fields don't really make sense, so forbid them
(just as records with no fields aren't allowed). Closes #2509

src/rustc/middle/typeck/check.rs

@@ -387,6 +387,12 @@ fn check_item(ccx: @crate_ctxt, it: @ast::item) {
           };
           // typecheck the members
           for members.each {|m| check_class_member(class_ccx, class_t, m); }
+          // Check that there's at least one field
+          let (fields,_) = split_class_items(members);
+          if fields.len() < 1u {
+              ccx.tcx.sess.span_err(it.span, "A class must have at least one \
+                field");
+          }
           // Check that the class is instantiable
           check_instantiable(ccx.tcx, it.span, it.id);
       }

src/test/compile-fail/issue-2509-a.rs

@@ -0,0 +1,9 @@
+class c { //! ERROR A class must have at least one field
+    new() { }
+}
+
+fn main() {
+    let a = c();
+    let x = [a];
+    let _y = x[0];
+}