fix: warn about no pinning strategy for unused features

[kilian-hu]

This PR initially tried to fix #3245 where users noticed that the dependency version pinning strategy is not applied in features that are not used by any environment. This is because right now the solver does not even run for the dependencies in an unused feature because there is no environment in which the solve could happen. One could run a sanity check solve for such an unused feature's dependencies but that would require quite some changes. For now I'd at least improve the warning that is shown when there is an unused feature and inform the user that those dependencies are not being resolved at all.

[lucascolley]

see open thread

[lucascolley]

hey @kilian-hu ! Not sure if this was closed deliberately, but would you be interested in implementing point (1) from #4065 (comment) ?

significant change since

[kilian-hu]

@lucascolley Sorry, no that was not intentional. I just pushed what you suggested about making the warning message more informative.

I noticed that when running e.g. pixi add --feature dev numpy when there is no dev feature in the manifest yet, we will not even get this warning as it can only be emitted while reading the existing manifest AFAIU. So this kind of warning should actually be emitted after the manifest has been written to disk imo so we see it both if the feature was not already there and when it's already there. But I'm not fully aware of all the constraints here, so that's just an idea.

So I'd like to merge this as a first start so users are less confused when this happens. Does the updated warning message look alright to you?

And for the future it remains to maybe implement at least a sanity check solve for unused features and also change that this warning message is shown regardless of whether the feature already existed or not.

[lucascolley]

thanks @kilian-hu, looks good to me! And your plan for potential follow-up also sounds good 👍