added test cases from strange loop event

tests/showoff/tformatopt.nim

@@ -0,0 +1,57 @@
+discard """
+  output: '''(a: 3
+b: 4
+s: abc
+)'''
+"""
+
+import macros
+
+proc invalidFormatString() =
+  echo "invalidFormatString"
+
+template formatImpl(handleChar: expr) =
+  var i = 0
+  while i < f.len:
+    if f[i] == '$':
+      case f[i+1]
+      of '1'..'9':
+        var j = 0
+        i += 1
+        while f[i] in {'0'..'9'}:
+          j = j * 10 + ord(f[i]) - ord('0')
+          i += 1
+        result.add(a[j-1])
+      else:
+        invalidFormatString()
+    else:
+      result.add(handleChar(f[i]))
+      i += 1
+
+proc `%`*(f: string, a: openArray[string]): string =
+  template identity(x: expr): expr = x
+  result = ""
+  formatImpl(identity)
+
+macro optFormat{`%`(f, a)}(f: string{lit}, a: openArray[string]): expr =
+  result = newNimNode(nnkBracket)
+  let f = f.strVal
+  formatImpl(newLit)
+  result = nestList(!"&", result)
+
+template optAdd1{x = y; add(x, z)}(x, y, z: string) =
+  x = y & z
+
+proc `/&` [T: object](x: T): string =
+  result = "("
+  for name, value in fieldPairs(x):
+    result.add("$1: $2\n" % [name, $value])
+  result.add(")")
+
+type
+  MyObject = object
+    a, b: int
+    s: string
+
+let obj = MyObject(a: 3, b: 4, s: "abc")
+echo(/&obj)