The really awesome thing with rust is how easy it is to make library code which can be called from any language including C and C++ (in most situations) - just as easily as writing a library in C or C++. In some situations even easier than C++.

Thus I can easily see rust replacing C/C++ as the default language for writing 'high performance libraries' in, for use in Ruby/Python/etc. It's usually more similar to what higher level programmers are used to too. You get to use functional code for a start.

But also exactly as in Firefox - libraries / modules can be written in Rust, and then brought in to replace existing C++ code. And I'm sure the ergonomics of this process will improve too.

If a C++ project lead / manager can say, in all honesty: "The x component needs to be re-written for whatever reason - either major refactoring for a desired feature, or performance, or to split out to use in another product. We can either do it in C++, or Rust, and Rust will garentee no new segfaults, we can trivially parellelise a lot of it for improved performance, etc. etc. with no major disadvantages..." then Rust will inevitably grow.