Exactly, Rust has been around for a while and it hasn't reached parity in terms of LoC with C and C++ in Gecko's codebase, presumably because it's impossible to

Exactly, Rust has been around for a while and it hasn't reached parity in terms of LoC with C and C++ in Gecko's codebase, presumably because it's impossible to rewrite individual units unless the API is impossibly narrow. This new language will be more like different JVM langs in terms of interop.