I'm impressed with how thoughtful the entire Rust development cycle has been. They've managed not just to strike a balance between stability and language redesign, but to approach to API and syntax standardization in a way that made tracking the changes relatively painless. Some of this is done in the language itself, with opt-in statements and stability annotations in the API, but the tooling around compatibility in the ecosystem is quite useful as well. Now they're going to doing CI on their own nightlies against packages in the ecosystem:
> To help ensure that we don’t accidentally introduce breakage as we add new features, we’ve also been working on an exciting new CI infrastructure to allow us to monitor which packages are building with the Nightly builds and detect regressions across the entire Rust ecosystem, not just our own test base.
CI against package ecosystems is a really great idea. We do it for Julia too [1], and it can identify some really subtle issues that would otherwise take longer to become apparent.
I'd imagine that such a bank of tests would be helpful in tuning performance too. Though perhaps the reality of each tweak only contributing a fraction of a percent would be considered depressing compared with the standard practice of saying that a microbenchmark is now 2x or 4x faster.
> To help ensure that we don’t accidentally introduce breakage as we add new features, we’ve also been working on an exciting new CI infrastructure to allow us to monitor which packages are building with the Nightly builds and detect regressions across the entire Rust ecosystem, not just our own test base.