Content-addressable derivations support

[domenkozar]

This issue tracks CA derivations support and requests for it.

[TerrorJack]

I tried adding contentAddressedByDefault = true; to nixpkgs config, building and pushing all build-time/run-time dependencies of pkgs.hello to a cachix bucket, but in another nix environment, it fails to find the artifacts in the cachix bucket and instead starts rebootstrapping stdenv. So indeed this seems to be unimplemented at the time being.

[domenkozar]

I'd appreciate those that want to use CA support to say a few words why and how you're using them.

I'm currently not motivated yet to implement this, so it would help a lot!

[ncfavier]

I'm using a CA derivation to avoid unnecessary rebuilds of the NixOS man page cache, which are slow:

https://github.com/ncfavier/config/blob/e78cb1b7191bfaa043bcd6d960ccf36052b863f5/modules/environment/default.nix#L5-L13

[domenkozar]

cc @robbins @bbigras as you thumbed up

[domenkozar]

There's absolutely zero documentation for this, so it's going to be a while to get it all right.