nixpkgs-multiverse: fast mode

The nixpkgs-multiverse package offers a "fast mode" that allows users to directly access the store path for every indexed version of every package, eliminating the need to download and evaluate the entire 378 MB Nixpkgs tree. This capability, explained by Sun Tzu's famous quote, "the fastest evaluation is the one that never happens," has the potential to revolutionize the way we think about Nixpkgs and the entire ecosystem.

By providing a concrete store path, users can skip the costly evaluation process and retrieve the desired package from the cache, resulting in faster builds and reduced disk usage. The fast mode works seamlessly with the complete Nix API, except for releases, and does not require the use of --impure or an experimental environment.

The multiverse index, a map from (attribute, version) to the revision that shipped it, enables the creation of a unique address for every historical version of a package. A clever implementation by tomberek uses builtins.storePath and builtins.appendContext to attach the string as a store path without requiring impure evaluation.

The resulting "fake" derivation maintains a bit-for-bit identical graph to what Nixpkgs would have produced, while still remaining pure and cacheable. The performance benefits are significant, with a census showing that 271,187 indexed versions of every package are still accessible from the cache.nixos.org binary cache as of August 14, 2026.

This extensive archive, containing 14.8 TB of unpacked software from 2013 onward, can be accessed quickly with a single fast command. However, it's worth noting that the system relies on the cache.nixos.org to continue serving the archive, which has never performed garbage collection, ensuring the growth of the binary cache and its associated costs.