Yeah, thats fair, but if I understood right- this is a custom built tool to be compatible with Ninja.

That work building “yet another build tool” could have gone in to programmatically generating bazel BUILD files. So, there was an active choice here somewhere; we just don’t know all the information as to why effort was diverted away from Bazel and toward building a new tool.

I trust them to make good decisions, so I would like to understand more. :)

Seems like Siso supports Starlark, so maybe its a step in Bazels direction after all.

Same can be extended for other tools that are generally used in builds like make. I never heard someone say that they need to build make from source so they can build X, unless of course you're using something like Linux From Scratch.

Not really relevant here, but this is actually exactly how it's done in embedded systems like Yocto, everything from gcc, make, etc. is built from source (I believe the host compiler is used in a 3-stage bootstrapping process for gcc).

And in these cases you really see the impact of internal dependencies (building rust/llvm takes around 30-40% of the entire build). The upside is that you can patch and debug absolutely any part of the system.

You compile the compiler from source, then you use the compiler compiled from source to compile the compiler from source again, and then the compiler that you compiled from source using the compiler compiled from source should be essentially identical to the compiler compiled from source by itself (unless anything like Reflections on Trusting Trust is in play, but then a lot of bets are off).