Now that --pretty=expanded is gone, how do you debug the output of a procedural macro?

[tomaka]

I'd like to debug the code generated by a #[derive(MyCustomDerive)].

Just like this question says, you were supposed to use --pretty=expanded:

Is there a way to see generated code for `derive` attributes?

The available doc does not give much details about the code generated through the derive attribute. In fact the spec only talks about deriving two traits (PartialEq and Clone). My understanding is that derive is handled in an ad hoc fashion inside the compiler. So yes, a proper specification would be of great help. My question is more pragmatic. In the mean time do you know of any any way to get the generated code? For example through a flag of rustc.

However --pretty=expanded is now gone. Someone suggested trace_macros!(true);, but that doesn't show the expansions of custom derives.
Is there some way then, or is it a terrible overlook?

[dtolnay]

Nightly still has it.

cargo +nightly rustc -- -Z unstable-options --pretty=expanded

[tomaka]

After your reply I investigated some more, and turns out you're right.

What confused me is that rustc now entirely compiles the crate (which takes a few minutes in my case), and then only prints the macro expansion. This is something it didn't do in the past.

Thanks