[sleepy_keita]

Maybe - but in this case, Ruby is written in C, it uses C extensions when performance matters, but tooling for the Ruby language itself is all in Ruby. Rust isn't replacing the use of C in the core of Ruby (yet) - it's stepping in to the area where Ruby would have been traditionally used.

[frollogaston]

Similar thing is in motion with the JS toolchains. Rewriting in Rust is easier than rewriting in C, but why didn't they previously rewrite in something like C++ or Go? I'm guessing because people were simply not interested.