Cross-registry concerns

[CAD97]

The primary goal of parent::child packages is a strong indication that the child package is "part of" the parent package/project. This is enforced by the registry only permitting an owner of the parent package to publish a parent::child package.

So what happens with the cross-registry case? E.g. with

[dependencies]
parent = { registry = "a", version = "1" }
"parent::child" = { registry = "b", version = "1" }

there's entirely no guarantee that [a]::parent and [b]::parent are related. (It's desirable that they don't need to be.)

Should this be some sort of error/warning, or is this just a "sharp tools can cut you" kind of case?

Relates some to #18, which constructs a similar scenario in a single registry with dependency renaming.

[Manishearth]

It's an interesting idea; I can see arguments for users of a local registry using this to perform surgery on their crates, which is fine to me. As with #18 I think the right call is to let people do what they want in their Cargo.toml, and handle it transparently.