Python: implement ToString on mappings

Author: yoff

python/ql/src/experimental/dataflow/internal/DataFlowPrivate.qll

@@ -372,13 +372,15 @@ private module ArgumentPassing {
     abstract int getParamN(int argN);
 
     /** Gets a textual representation of this element. */
-    string toString() { result = "ArgParamMapping" }
+    abstract string toString();
   }
 
   /** A mapping that passes argument `n` to parameter `n`. */
   class NoShift extends ArgParamMapping, TNoShift {
     NoShift() { this = TNoShift() }
 
+    override string toString() { result = "NoShift [n -> n]" }
+
     bindingset[argN]
     override int getParamN(int argN) { result = argN }
   }
@@ -387,6 +389,8 @@ private module ArgumentPassing {
   class ShiftOneUp extends ArgParamMapping, TShiftOneUp {
     ShiftOneUp() { this = TShiftOneUp() }
 
+    override string toString() { result = "ShiftOneUp [n -> n+1]" }
+
     bindingset[argN]
     override int getParamN(int argN) { result = argN + 1 }
   }