While dist: bionic now works for Rust testing the same problem occurs with dist: focal now. Example: https://travis-ci.org/github/klausi/mastodon-twitter-sync/jobs/736883364
The problem is even worse now: instead of building with Ubuntu 20.04 Travis CI simply ignores the configuration and builds with Ubuntu 16.04. That is really confusing because the UI shows “focal”, but in reality the worker uses xenial.
Instead of silently running the build with a completely different Ubuntu version could you throw a build error instead? Like “Ubuntu 20.04 focal is not supported yet for the Rust testing”