Very nice! One of the issues of using Nix has been the ad-hoc methods for pinning versions, such as rust-overlay. (Although rust-overlay is great for toolchain customization which might make it worth keeping around.)

Even with some ad-hoc pieces I'm happier using Nix than using completely unrelated tools for each ecosystem, like Rustup and Nvm. If we could have a more unified Nix solution that would be even better.

It could certainly make my old Python versions test matrix easier to maintain.

Oh, there is also a cli that can be used to resolve which package versions exist in which commit, and that works entirely offline.

I recently had to pin some stuff and it was an annoying experience. Now I have to check this out...