Add cargo-metadata crate.

[ahmedcharles]

This allows parsing the result of cargo metadata without having to
implement the json serialization in each caller, independently.

This is intended to be used by: rust-lang/rust#36032

This is the bare minimum required to implement the functionality required by rustc's bootstrap to determine the transitive dependency closure. Obviously, it would be nice if this were more fully featured. I initially tried copying the implementations from cargo with the intent of having cargo depend on this crate through a path dependency, just like it does with crates-io. However, this ended up being non-trivial, so I decided to limit the scope and start from scratch. Any thoughts on whether that would be a worthwhile approach (likely longer term)?

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @brson (or someone else) soon.

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[ahmedcharles]

r? @alexcrichton

[alexcrichton]

Thanks for the PR! I wouldn't mind including this, but how about we hook up the tests to literally the output of cargo metadata itself? That is, we could make this a dev-dependency of Cargo itself, and then modify tests/metadata.rs to exercise this crate?

[alexcrichton]

ping @ahmedcharles, any update on avoiding vendoring the json and using the output of a literal execution of cargo metadata instead?

[ahmedcharles]

Sorry, I've gotten busy and won't be able to work on this soon.

[alexcrichton]

Ah ok, no worries! I'm going to close this for now to help clear out the queue, but feel free to resubmit!