Not really a secret, given that we're an open source project. At the same time, we didn't want to talk about the Azure stuff until we were actually moved over, and the process took a while.

It wasn't a really a secret. They ran into all sorts of issues with Travis, and people from Azure offered to run the Rust CI instead. The initial discussions were public.

Unfortunately, the build times still seem to be close to four hours, which limits the amount of changes that can be merged.

We aggressively use rollups to merge PRs into rust-lang/rust to mitigate the effect of the 4 hour build times. But it would sure be nice to bring it down to less. It would certainly make my life as the maintainer of the bors queue easier. ;)

It's probably a testament to the scope of the build + test that makes it four hours. If it were faster, would the scope tend to increase in order to hit more tiers/more tests?

I'd rather my commit sit in a test queue for several hours than push and cross my fingers like LLVM does it.

With faster builds, would probably reduce the queue latency and land more PRs. We'd probably also allow more toolchains to be tested and whatnot.