Rust has gained so much traction at this point with other major companies (Cloudflare, Microsoft, Dropbox, etc), because of the unique value it offers, that I assume it would have to be picked up if it got dropped by Mozilla.

Rust has multiple implementations, see mrustc.

mrustc doesn't support all of Rust, its only purpose is to bootstrap the normal Rust compiler; and it only works on x86 and x86_64 Linux and isn't very much optimized (eg. needs about 10GB of RAM for the final linking).

And it's lagging behind by a year or two (which is about 15 intermediate releases of the Rust compiler you need to compile to get the current one); because it only has a single developer and keeping up is a huge amount of work.

How exactly do you know which implementation is correct without a document to point to?

Dlang has three implementations, one front-end, and a specification you can learn in a day. https://dlang.org/spec/statement.html