It is, and that is cool. That said, what it's shipping is essentially a separate library written in Rust that exposes a C API for parsing mp4s - it replaced one libraries usage with another, with a presumably sable API consisting of nothing but C struct's and C functions. Perhaps a key to point out is that it doesn't actually interface with Firefox very much at all - it isn't capable of accessing any Firefox state for instance. I also didn't list Firefox as you'll note - it's not a C project, it's a mishmash of languages already (Mostly JS and C++ from my understanding). I'm really talking about projects that are currently C-only adding in Rust code to the mix - they are structured much differently then a project like Firefox and the interfaces are generally much more complex as far as C goes.
I will add though, servo is making pretty impressive efforts. It is again a complete rewrite though, not an integration of Rust into Gecko which is the type of thing I'm talking about. A rewrite of a library in Rust against a stable API is generally possible, depending on the API - though the amount of work may make it prohibitive to get something usable.
> There are various examples of Linux kernel modules written in Rust.
I would like to see an example of a legitimate Linux Kernel module written in Rust that is in some form of use. I have never seen one besides a toy implementation and I can all but guarantee you it doesn't exist, for the reason that it would be way to much of a hassle to attempt to get it working with the mixture of macros, inline assembly, inline functions, gcc attributes, etc.
> There are rewrites of the coreutils in Rust
Coreutils is admittedly not a very good example, if only because coreutils isn't actually that big/complex of a project (Though supporting all the GNU flags and arguments is a pretty big task). They're also mostly just separate exe's anyway, so you could replace one or two with Rust code without much of a difference - I'd gladly remove Coreutils from the list if you would like.