What do you mean by outdated Cargo.lock? You definitely should be leaving a sort of boundary between them. You can use fenix in your devshell to get a toolchai

What do you mean by outdated Cargo.lock? You definitely should be leaving a sort of boundary between them. You can use fenix in your devshell to get a toolchain and then use crane for building. You just use cargo like normal when developing, including dependent crates, but CI can be handled with nix from that point. I've got my flakes organized in a way that they are basically modular, so I can easily reuse files from other ones. If I need a rust toolchain, cargo builds, python toolchail, postgres, or whatever I can just import the file and change it up slightly (mostly names)