One of the first uses for Rust was rewriting parts of Firefox yet "Cargo was not designed for integration into existing build systems"? That's hard to believe.
Firefox uses cargo and has to go through a lot of hoops for the build system to work. And the result is flaky and far from ideal for fast builds. Build plans support would help but has not progressed in the past few years. Worse, it was removed. https://github.com/rust-lang/cargo/issues/5579#issuecomment-...
Google just don't think it's worth integrating their stuff nicely with pre-existing things :-/
It seems that Google has more build systems that messengers: GN, Bazel, Soong, there were few more but I can not remember them.
I think that ninja is fine, but I was wrong with the terms. All those are build automation tools[1] and ninja is a build system. Also, that article refers to GYP[2] yet another build automation tool from Google.
At least Rust has far less building systems than C and C++ building system jungle: C, GNU Make, BSD Make, Windows Make, GNU autotools, and much more. Google may have a valid reason for not using Cargo, since they may need to integrate Rust with existing C and C++ code. It would be nice if Cargo had a way to build and link C and C++ dependencies in the same source tree as there are many C libraries which the functionality is still missing in Rust.