I recently tried to publish both category and badge information for some of my crates. I couldn't figure out why this information wasn't being displayed on crates.io, and eventually I figured out that I was using an old version of cargo. If cargo warned on values that it didn't understand when running publish I think this could have been avoided. I'm not certain if very many people put custom things in their Cargo.toml and some crates use that, but I think in general it'd be good to have the package namespace locked down like that. This wouldn't have caught the problem with the new badges namespace, but that might cause too many false positives.

What infrastructure is proposed to resolve this probably overlaps with #2489.