Improved Nix Repl Startup Speed

Replit aims to transition all Repls to a Nix-based foundation to make more than 80,000 Linux packages available. However, the existing Nix integration approach had an issue where evaluating replit.nix and exporting environment variables added several seconds to tens of seconds of extra loading time.

To solve this, Replit introduced caching environment variables technology. It stores the environment variables exported by nix-shell in the Repl's file system, allowing the Nix evaluation step to be skipped on subsequent startups for immediate execution.

With this optimization, Nix-based Repls now start just as fast as the existing Polygott-based Repls.

• Caching mechanism:
  • Environment variables generated by nix-shell are permanently stored in the file system for reuse.
  • If a user modifies replit.nix to add a new package, the environment is automatically rebuilt when the cached paths are no longer valid, maintaining accuracy.