[mhlakhani]

yeah. I am talking about repos with hundreds of thousands (probably millions) of tests. Here's just one example of the scale: https://engineering.fb.com/2018/11/21/developer-tools/predic...

if you change a low level library that's the equivalent of the C++ standard library and you want to test the changes, you effectively have to rebuild the world. And you don't want to.

[danpalmer]

Exactly, or when "one" build rule connecting a service to another is actually a hundred deeply nested build rules doing a lot more work, and every one of those code paths would need to correctly convey whether a dependency is required at the test level or not.