I posted today a pretty cool PoC where I was able to provide real-time 'security vulnerability feedback' to a developer as he is writing code in VisualStudio.
You can see the video at Real-time Vulnerability Creation Feedback inside VisualStudio (with Greens and Reds) where every time the user makes a change to the code there is an auto-compilation (using Roslyn's C# compiler) and a SAST scan (using Cat.NET)
Although this PoC is quite agressive (I do a compilation and scan on every keystoke which is a bit OTT), here is another video that shows a bigger compilation+scan on save: Real-Time C# Solution Compilation and Security Scanning (using Roslyn and Cat.NET)
What is key about this PoC is that is represent the real-time loop (almost a REPL) that we need for (some types) of security issues.
Next we need to think about the best way to present this information to developers. For example I was thinking that we might want to have a number of colours shown depending on the type of issue, how serious it could be, its exploitability, etc...
Another cool idea is to change the cursor's color or its shape (think bigger or smaller), depending on the number of issues currently outstanding :)