You assume that you will write code and check in without testing. Code should pass all unit tests locally before check in. Some tools, like TFS, can be configured with a gated check in. If unit tests don't pass, check in is prohibited.

>Craig. I often believe the software world can learn from other industries. So using an engine as a mainline branch. A developer wishes to refine the spark plug. That takes months of iterative testing and discarding unworkable designs. Why should any of these unworkable ones reach the mainline branch?