Introducing Bazel to a Rust Project

Before I left my previous company, a large and frequent complaint was how long our CI (Continus Integration) took to complete. The fastest I remember it taking was 40 minutes. (It also had an unfortunate amount of flakey tests, which is a separate issue…) We were able to separate our CI into front-end and back-end pipelines, but we had many integration tests that spanned both areas.

Bazel came up as an idea to potentially speed up our CI times by caching and reusing artifacts. I had heard about Bazel in browsing the Rust sub-reddit as well. So, I wanted to see how easy it would be to integrate Bazel into an existing project and whether it was worth including in my personal projects. The biggest issue that I ran into was that the initial directory structure was very important---the tooling assumed that directories were laid out in a certain way. I decided to write up my experiences, with some of the hardships and uncertainties included. Too many blogs and guides skip over the difficult portions---I have kept some of them in but left out some initial churn:

https://prestontw.github.io/bazel-rust-guided-experiment/

The source is available at:

https://github.com/prestontw/bazel-rust-guided-experiment

P.S.: Bazel might run afoul of Matklad’s O(1) build file rule. This is necessary since Bazel has to be aware of tests and their dependencies to know whether their results are up to date, but it’s just a little more manual effort or initial automated tooling setup than I’m used to.