Add tests for various ways to write 'do' and 'for' without args

src/test/run-pass/do-for-empty-args.rs

@@ -0,0 +1,16 @@
+// no-reformat
+// Testing various forms of `do` and `for` with empty arg lists
+
+fn f(f: fn() -> bool) {
+}
+
+fn main() {
+    do f() || { true }
+    do f() { true }
+    do f || { true }
+    do f { true }
+    for f() || { }
+    for f() { }
+    for f || { }
+    for f { }
+}