Allow "self" in flake input paths

[alexshpilkin]

Fixes #4931 by making “self” in flake input paths the moral equivalent of “.” in filesystem paths. This takes the lazy approach and interprets any “self” components in the path at parse time.

cc @thufschmitt

[thufschmitt]

I don’t have a very strong opinion on that topic, deferring to @edolstra since he might have one

[roberth]

No particularly strong opinion here either, but if we create a keyword (self), we should disallow it as an input name to avoid potential for ambiguity.

[alexshpilkin]

@roberth The point was (I thought) that self is already effectively a keyword in that even if you declare an input by that name you’ll still get your own outputs instead of it. I agree that you shouldn’t be able to declare one at all, though.

[roberth]

Please add a test case in tests/flakes/flakes.sh. You may want to rebase before you add it, to avoid conflicts in that file.

We'll also want to disallow the flake to define inputs.self = { ... }.

[alexshpilkin]

@roberth OK, that's fair. Except tests/flakes/follow-paths.sh looks more logical to me? And tests/flakes/circular.sh as well just to mirror the docs.

As for disallowing a "self" input, I'm a bit torn because in principle I see no reason why, for example, writing inputs.self.inputs.foo.follows = "bar" shouldn't work as an synonym of inputs.foo.follows = "bar"—it's silly but not nonsense—but trying to make it work seems extremely annoying, e.g. we then need to define how inputs.self.inputs.foo, inputs.foo.inputs.self, and inputs.foo are merged recursively if more than one of them is present. No other attributes of a "self" input seem to make sense, so I think I'll sacrifice this bit of orthogonality and just ban any mention of a self input in the inputs part of a flake.

[Ericson2314]

Triaged in the Nix Team meeting this morning (n.b. I could not make it):

Main takeaway:

• Idea Approved, but requires tests and docs

Discussion:

• @thufschmitt would prefer something like ., but doesn't care too much about the concrete syntax
• self isn't that bad

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-08-04-nix-team-meeting-minutes-77/31487/1

[roberth]

• @thufschmitt would prefer something like ., but doesn't care too much about the concrete syntax

Note that since then we've also accepted flake#.foo as a notation for absolute attribute paths (e.g. to avoid packages.<system>.foo), which is also inspired on unix path notation but with . instead of /.

Things still to do:

• Add test case
• Only operate on the root of the inputs tree. The inputs attribute, such as e.g. returned by getFlake, does not contain any self attributes either.
• Consider . as syntax for self

I'll mark this as draft for the time being. Please undraft when it's ready for review.