If I were you, I wouldn't do that.
Really.
I strongly advise against it.
However, if you must;
There is editable code. This is how you get to it:
In eclipse, look for the "build" folder. Navigate to "tmp", then to "recompSrc".
Inside there, there is a huge repository of editable files. The Minecraft folders are located in recompSrc's "net", then "minecraft" folders. I don't know whether these are the actually running files when you start Minecraft in eclipse, or if it is the folders in forgeSrc in "Referenced Libraries" that contains uneditable code. Either way, the code in recompSrc is fully editable.