Rapid feedback

Ok.. This is a flight of fancy I had a few months ago, but its still in my head, so I thought I would let it free on the web.

Imagine an IDE where as you type in new code you not only get told if the code compiles, but also if the unit tests pass with this new code.

IDEs now hold a parsed version of your code so they can highlight compiler errors as you type. Its not that big a step to then interpret the tests over the in memory parse-tree to see how the code will execute. The IDE could keep track of which lines of code are executed by which test easily enough. Then it would be a case of only re-interpreting the tests that are potentially effected by each edit.

This would be the ultimate in rapid feedback :)