[wffurr]

Is that really a “golden rule”?

I have worked on lots of cross language codebases. While it’s extremely useful to have experts in language or part, one can meaningfully contribute to parts written in other languages without being an expert. Certainly programmers on the level of kernel developers should readily be able to learn the basics of Rust.

There’s lots of use cases for shared business logic or rendering code with platform specific wrapper code, e.g. a C++ or Rust core with Swift, Kotlin, and TypeScript wrappers. Lots of high level languages have a low level API for fast implementations, like CPython, Ruby FFI, etc. The other way around lots of native code engines have scripting APIs for Lua, Python, etc.

[w0m]

I don't know if its golden rule or common-sence when applicable.

If our testing framework is in Python; writing a wrapper to code tests for your feature in Perl because you're more comfortable with it is the Wrong way to do it imo.

But if writing a FluentD plugin in Ruby solves a significant problem in the same infra; the additional language could be worth it.

Everything is about tradeoffs.