Support --unit-graph

[est31]

rust-lang/cargo#8002

rust-lang/cargo#7977

The new feature resolver has turned metadata inadequate for some purposes, so --unit-graph was introduced.

[jyn514]

Which parts of the output should be stabilized? This structure is very low-level, and thus inherently at high risk of changing in the future and being unstable. I think at a minimum we should document that it is not guaranteed to be backwards compatible. Should we filter some fields on stable?

Hmm, does it make sense for cargo_metadata to expose unstable info?

[oli-obk]

We can put it behind a cargo feature that also enables an unstable rustc feature. That way you can only use it on nightly

[morr0ne]

This would prove very useful but I am not sure what the status on this is on cargo considering that rust-lang/cargo#8002 hasn't been updated since 2020

[morr0ne]

I was trying to implement parsing of --unit-graph but it doesn't seem like this is the most up to date documentation since during testing additional fields where reported. If someone knows where to get the most up to date documentation it would be great.

[morr0ne]

Okay so it looks that https://github.com/rust-lang/cargo/blob/5aad9b302adc2f4cf1f184315f497fb3e3256b78/src/cargo/core/compiler/unit_graph.rs#L39 is getting serialized as is so that could simply copied and pasted and adapted for the crate.