A new Nix store implementation — local-overlay — that lets you create a writable layer over another underlying store.
Nix has various different store implementations. For example the daemon store, which provides store access to unprivileged users on multiuser systems.
Some caveats apply, but the new local-overlay store lets you combine an existing lower store with an upper layer, similar to the Linux Overlay Filesystem. Anything in the lower store will be accessible via the overlay store, while modifying the overlay store will only change the upper layer and leave the lower store untouched.