Feature request: NixOS-style assertions support

[shanesveller]

I'd like to offer consumers of my flake modules feedback if they set options to semantically incorrect values, or use mutually-exclusive options simultaneously.

If some guidance is available I'd be happy to try to implement this myself.

[shanesveller]

If the contract were exactly the same as NixOS's version ({ assertion = someBool, message = someStr }), realized at evaluation time, that would satisfy my goals well.

[roberth]

Unlike NixOS, the attributes of a flake are often accessed individually, which means that laziness plays a more important role; somewhat comparable to Nixpkgs. Assertions require more to be evaluated, including things that may not have been evaluated otherwise.
For this reason, and potentially to avoid infinite recursions caused by the extra strictness, this feature should be a bit different from the NixOS assertions. Probably they should come with a flake attribute path that tells flake-parts when to evaluate the assertions.

[shanesveller]

Some of my loose reading of nixpkgs' lib.types implies that I could put some ad-hoc logic in as custom check on the types I declare for my options, if it doesn't require any adjacent information to determine the validity, but I don't know how maintainable or effective that would be. The source has a big cautionary warning above it.