Feature suggestion: Provide visual cues for readonly/const/non-mutating methods

One of the huge benefits of immutability is being able to reason that an operation is 'safe' because it cannot produce non-local side-effects or that a read/write access is idempotent.  It seems to me that Reharper could provide a very helpful hint here by, for example, rendering const/readonly variables in bold/different color or by doing further analysis to show that a method is non-mutating. To provide a concrete example, for the (bad!) class below it would be highly desirable if references to "Multiply" and "HalfMultiply" had different visual cues so that I didn't have to devote brainpower to worrying that the Multiply method might change the internal state of the multiplier I happen to be using at the time.

I do appreciate that solving this fully is a "hard problem" (TM) but even an incomplete solution that gives assurance over the things you _can_ prove are non-mutating would be beneficial imho.

public class Multiplier
{   
   private readonly int  _multiplier;
   public Multiplier(int m) 
   {
      _multiplier=m;
   }
   //'good' method - no side-effects :-)    
   public int Multiply(int input) 
   { 
      return input * _multiplier;
   } 
   //'bad' method - changes internal state of object non-obviously  :-(   
   public int HalfMultiply(int input) 
   {
     _multiplier=_multiplier/2;
     return input *_multiplier;
   }
}