Investigate caching on travis to improve build times

[thomaseizinger]

At the moment, we cache several things on travis:

• $HOME/.rustup (Rust installation)
• target/ (Compiled binaries from last build)
• $HOME/.cargo (Downloaded dependencies)

We should run some investigation if all of that actually improves the build time on travis. At the moment, our cache is about 1.5 GB in size, which takes quite a time to up and download. (2.5 minutes)

Things to try out:

• Clean build artifacts of workspace crates only (cargo clean -p for all workspace members)

  Doesn't work because cargo clean -p doesn't clean all files generated for a particular crate.

• Not caching Rust installation

• Using Travis cache only for the Rust installation + downloaded dependencies AND cache the target folder on Google Cloud Storage using rsync (https://cloud.google.com/storage/docs/gsutil/commands/rsync)

[thomaseizinger]

@bonomat Could you please create a "service-user" or however these things are called on GCS? I would like to experiment with caching "manually" by syncing the target folder to Google Cloud Storage.

I basically need the same thing as for the log-storage.

[bonomat]

I'm not sure if GCS should be used in this case. Each build would cost us a few cents then

[thomaseizinger]

Could you nevertheless create an account? I would like to play around with it on Friday to see the impact!