Why is --optional mutually exclusive with --build and --dev?

[jimmycuadra]

--optional      Add as an optional dependency (for use in features). This does not work
                for `dev-dependencies` or `build-dependencies`.

Why is this? Several of my projects have a build dependency that is optional, and I always have to move things around manually since cargo add has this limitation.

[killercup]

Does cargo allow optional dev/build dependencies now? When I implemented this featured, it didn't.

cf. rust-lang/cargo#1596

[jimmycuadra]

Oh, interesting. I think I've only tried to do it with build dependencies, and it definitely works for that. (Here's an example.) Maybe Cargo only disallows optional dependencies for dev dependencies.