Cargo metadata subcommand should work with workspaces

[matklad]

Currently it gives

error: manifest path `/home/matklad/projects/vulkano/Cargo.toml` is a virtual manifest, but this command requires running against an actual package in this workspace

Ideally it should return the info in the same format as metadata command for the non-virtual manifest. The only obstacle seems to be the root node in the resolve.

And perhaps for any package that is a part of the workspace cargo metadata should indicate it.

[matklad]

Ok, so to solve this we need to get rid of the root field in the Resolve structure. It is not used in a significant way during resolve procedure and basically just duplicates Workspace::current.

Also, we currently serialize arbitrary root into the lockfile. It should be possible to get rid of this in a backwards compatible way.

[matklad]

Note: check_cycles in the resolver may be buggy because it currently only checks for cycles in sub graph reachable from the root.

[alexcrichton]

Note that unfortunately we can't get rid of root in the lockfile (and transitively Resolve), but it's fine to just ignore it and not really treat it as the "root" any more. Seems reasonable to me that this'd be expected to work!

[matklad]

Note that unfortunately we can't get rid of root in the lockfile (and transitively Resolve)

But why? What are the reasons besides backwards compatibility?

[alexcrichton]

No reason beyond backwards compatibility. The thinking is that older Cargo instances should be able to work with newer lock files wherever possible.

[matklad]

But we don't use root from the Resolve when we are writing a lock file: https://github.com/rust-lang/cargo/blob/master/src/cargo/core/resolver/encode.rs#L279-L281.

So it should be possible to remove it?

[matklad]

That is, remove from the Resolve struct, not from the lockfile of course (though we can accept lockfiles without root to be able to delete it later).

[alexcrichton]

Oh! We should totally do that if we can!

[matklad]

And is there any way to get the list of workspace members? Perhaps this should be a part of cargo metadata json?

We don't need this in IntelliJ Rust yet, but it seems to be generally useful.

[alexcrichton]

Oh yeah certainly, I believe you can iterate over packages with ws.members()