Support `sources.json` being JSON5 to allow comments

[hansl]

[yaitskov]

+1 to have comments

[zimbatm]

This is not possible without changing Nix itself. If you want JSON5 to be added then it will require to extend builtins.fromJSON with that support, or introduce a new builtins.fromJSON5 builtin.

[refnil]

I agree with zimbatm that we should wait for a nix builtins before supporting JSON5. The alternative is to use the "import from derivation", which may not be appropriate in all niv use cases.

I tried implementing the feature by using by catching the fromJSON error in hope of falling back to a implementation of fromJSON using import from derivation. It didn't looks it was possible to catch this fromJSON error. This solution would also need changing nix itself.

[zimbatm]

Actually, a different possibility would be to serialize the file as Nix. It supports JSON-like data structures, and comments.