> Okay, suppose someone backporting a Rust update runs a big batch of tests and finds, say, two dozen packages with regressions.
> Now what?
> Spend two weeks investigating all the test failures? Backporting updates to these packages as well, all while users are patiently waiting for their Firefox to have its zero‐day fixed? Are the tests even correct? Were they failing before and nobody noticed?
Yes, people do exactly that. It's part of running a "rolling" distro, see e.g. the Debian Testing transition tracker at https://release.debian.org/transitions/ - These transitions are running essentially all the time; they're only "put on hold" as a first step in the process of making a new stable release. And even then, newer versions of packages such as rust can still enter stable as part of an "unrelated" security update.