Should warn on optional dep not used in any feature

[gdesmott]

Cargo could raise a warning when user defines an optional dependency which is not part of any feature.

I've been confused for a while trying to figure out why building my single feature project with --all-features was working while building it with --features=my-feature was not. Turned out I forgot to include one optional dep with the feature. Such warning would have prevent me from doing such stupid mistake. :)

[jplatte]

I think it was an intentional design choice to have optional crates exposed as features of their own. But maybe once the two namespaces are separated, that could be changed such that an optional dependency can only be activated through an explicitly declared feature.

In the current situation however, if you want a feature foo that (only) activates the dependency foo, there's no good way to express that other than making the dependency optional without creating an explicit feature. (you can rename the dependency and create a feature with the original dependency name that activates the dependency, but that's pretty ugly)

[epage]

We are removing implicit features in Edition 2024 and warning the user that the dependency is unused.

Closing in favor of the tracking issue: #12826