If you ever used AtCoder, you should know about its many features. One of the features is running your program on all of the testcases and then displaying how much WA's, how many AC's, how many RE's, etc. I think that this feature should be implemented in Codeforces. You can have the option to turn it on or turn it off.
Why Would I Want It
I recently saw Errichto use this feature of AtCoder to check if his thought process is right before trying to optimize the code. I think that he was trying to do Problem E from AtCoder Grand Contest 48. I also believe that many people like Errichto would also want to see if their thought process is right before continuing on to optimize the code.
Please share your thoughts in the comments below.
This would probably drastically slow down the queue. AtCoder usually only has at most 40 testcases, while CF has a lot of problems with at least 70 testcases (there are also problems with 400 testcases). In addition, CF reruns your code multiple times if you TLE on a testcase. Now, imagine the grading queue and also the amount of time it'd take to judge your submission (right now, even an ILE takes like 5 minutes to judge).
Good point.