I basically exclusively make PRs to the Rust repo to contribute to std, and I don't do so frequently. In march I wrote this post about my experience: Experience report contributing to rust-lang/rust Having made another small patch this week, my experience largely remains the same.
I wish it was much easier to contribute to std than it is. I do not contribute frequently enough that it is ever worthwhile learn the incantations the build system needs to build only std. As a result, I just don't build locally before making a PR (if my change is complicated, I prototype it in a separate crate and make sure that builds). Then I wackamoll build failures resulting from silly things like having too many newlines in my diff because the project blocks CI on a very stringent tidy check. This wastes time and our donated CI resources, but there's no process that doesn't waste substantially more of my time, which I am donating as well.