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I wanted `nix-env -iA` back as a scratch install on a declarative host. Turns out you can't persist your way out of a symlink, and the thing I was trying to save wasn't even the thing that mattered.

Moving a directory into the persistence list on an impermanent NixOS host looks trivial until nixos-rebuild dies with 'mount path is not canonical'. Here's why, and the one rule that avoids it.