This is a matter that concerns the Rust team a lot. From the Rust side we want to make it possible for Debian and Fedora (as a proxy for all distros) to be able to bootstrap off of their own rustc's and keep up with the Rust release process.
We have a good idea 1 about how to accomplish that part, though I haven't followed through on the implementation 2. In particular, as noted in this thread, you can't bootstrap off of either the current version of Rust or the previous. This is actually a problem that could be fixed in about one release cycle. There's little excuse not to do it yesterday, though some on the team have lingering worry about having to wait 6 weeks to complete unstable language changes.
We have less understanding of what it will take to get the major distros to a) actually deploy rust in a stable release, b) keep rust updated every 6 weeks.
Do we have a plan being executed to make sure that when Gecko components written in Rust ship on the release channel in Mozilla-produced builds the also ship in the Firefox packages of various Linux distributions?
On the Rust side we have a plan to unblock the distros ability to package Rust 1. We don't have a plan to actually get Rust into distros beyond counting on the respective package maintainers to make it happen once everything is clear on our end. I have not been actively working on it this year.
When is rustc expected to reach that level of bootstrapping predictability?
There's no ETA. It's not being worked on.
I think it's unacceptable to limit our ability to leverage Rust in Gecko by forgoing the ability to co-evolve Rust and Gecko at a rapid pace.
Even after hurdles of getting stable rustc into distros is solved I think this is one of the most difficult issues. If Firefox is not on stable Rust then distros still won't have compilers that can build it. If we get into a situation where distros must package the Rust nightly compiler to build the one major piece of Rust software they need, then that severely compromises Rust's release strategy.
I think we must not allow ourselves to wait for a Debian or Ubuntu LTS cycle before Rust improvements can be used in Gecko.
I agree with Brian Smith's sentiment at https://internals.rust-lang.org/t/perfecting-rust-packaging/2623/67
I agree that Rust being stuck on the distro LTS cycle is impracticle for real software.
If Rust produces 'universal' debs and rpms as suggested by @briansmith that would be enough for distro users to build Firefox, but it's not sufficient for the distro's themselves to keep their Firefox's up to date. I think Rust should do this but it isn't clear that it solves a critical problem.
Firefox's release model also has this tension with distros, and even LTS distros do update Firefox, right? Is there any chance Rust can recieve updates like Firefox?