Builtins.sformat is nice

Idea mentioned in
https://trello.com/c/uTPysoYP/15-find-useful-calls-producing-niceness

features/ops_sformat.feature

@@ -0,0 +1,24 @@
+Feature: sformat
+
+  Builtins.sformat results in a nice value that even if the input is ugly!
+  Well, it does return nil if the format argument is nil, but
+  that can be checked, and mostly we will see string literal formats.
+
+  Scenario: translates `Ops.add` of a literal and a regular sformat
+    Given the original code is
+      """
+      Ops.add("nice", Builtins.sformat("%1", nil))
+      """
+    When the cop Yast/Ops autocorrects it
+    Then the code is converted to
+      """
+      "nice" + Builtins.sformat("%1", nil)
+      """
+
+  Scenario: doesn't translate `Ops.add` of a literal and a ugly sformat
+    Given the original code is
+      """
+      Ops.add("nice", Builtins.sformat(ugly, nil))
+      """
+    When the cop Yast/Ops autocorrects it
+    Then the code is unchanged

lib/rubocop/cop/yast/ops.rb

@@ -93,12 +93,4 @@ def autocorrectable?(node)
     _ops, _method, a, b = *node
     nice(a) && nice(b)
   end
-
-  def call?(node, namespace, message)
-    n_receiver, n_message = *node
-    n_receiver && n_receiver.type == :const &&
-      n_receiver.children[0].nil? &&
-      n_receiver.children[1] == namespace &&
-      n_message == message
-  end
 end

lib/rubocop/yast/niceness.rb

@@ -18,7 +18,7 @@ module Niceness
 
   def nice(node)
     nice_literal(node) || nice_variable(node) || nice_send(node) ||
-      nice_begin(node)
+      nice_sformat(node) || nice_begin(node)
   end
 
   def nice_literal(node)
@@ -54,7 +54,25 @@ def nice_send(node)
     args.size == arity && args.all? { |a| nice(a) }
   end
 
+  # Builtins.sformat is special in that it can produce nil
+  # but only if the format string is nil which is fortunately
+  # easy to rule out. Other args may be ugly but we don't care.
+  def nice_sformat(node)
+    return false unless node.type == :send
+    return false unless call?(node, :Builtins, :sformat)
+    _builtins, _sformat, format_string, *_other_args = *node
+    nice(format_string)
+  end
+
   def nice_begin(node)
     node.type == :begin && nice(node.children.last)
   end
+
+  def call?(node, namespace, message)
+    n_receiver, n_message = *node
+    n_receiver && n_receiver.type == :const &&
+      n_receiver.children[0].nil? &&
+      n_receiver.children[1] == namespace &&
+      n_message == message
+  end
 end