Why is PackageId opaque but SourceId not?

[weihanglo]

In rust-lang/cargo#12313, the Cargo team is discussing PackageId and SourceId should be seen as being opaque. We found that for PackageId in cargo_metadata crate it is mentioned as opaque, but for SourceId it's not. I wonder if there was a reason this crate didn't say the inner representations of SourceId is also opaque, or you've spotted some valid use cases. Thank you :)

[oli-obk]

That's just an oversight. We should document it as opaque, too