This helps create shell binaries, which build their corresponding
packages, only when run.

By default, it uses package names identical to command
names. Binaries, contained by packages with different name, can be
listed in a catalog.

Motivation for this change

I often want to update my system, but I also want to have access to many packages on my system path. This creates a tension: installing many packages means lots of download volume and / or build time (especially if you're on master).

Considering the pervasive laziness of nix, wouldn't it be nice, if there was a sort of lazy nix-shell, that would build packages only at the last second, before calling into binaries from them?

Things done

This is a proof of concept, mainly for personal use and to gauge interest.
It might already be a workable prototype, please have a look at the code comments, to get a sense of the direction, I can see this going.

I'm open to suggestions for the interface, especially on collision-detection/prioritization/runtime-dispatch and renames.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

not sure who to cc this to, in particular. I need feedback about having the catalog built on hydra and other integrations. I think, right now, this should already be of interest to everybody doing nixos development, but a nixos module for it could also benefit infrequent users of heavy packages, replacing nix-shell -p.