Your project’s nix-env.
lorri is a nix-shell replacement for project development. lorri is based around fast direnv integration for robust CLI and editor integration.
When changes are made that would affect a project's developement shell, lorri builds the new shell in the background, and applies the result on the next shell prompt. The result is that development tools are kept in sync with the current Nix shell configuration (even e.g. as you switch branches) without blocking your use of the terminal.