Help test Cargo's `-Zrustdoc-depinfo`

[weihanglo]

The unstable feature -Zrustdoc-info in Cargo is now calling for testing.
As it is more precise in rebuild detection when any file path referenced is outside the package root, I think it could potentially make document-features more robust and performance.

It would be much appreciated if you can help test and provide feedback!

Here are test instructions: rust-lang/cargo#15370 (comment)

Thank you in advance!