How can `rustc_plugin` be used to analyze the code of `rustc`?

Hi, thank you for open-sourcing `rustc_plugin`, it has been very helpful for my research. 

However, I want to analyze the source code of `rustc` or the `std` library, and in this case, I cannot use `cargo build` for compilation, so I can't directly use commands like `cargo tool`. Do you have any good suggestions? Thank you so much!