flag to dump proc-macro's generated tokenstream to file for debugging

[programmerjake]

I think it would be useful for rustc to have flags to dump a proc-macro's generated tokenstream to a file when rustc generates an error or if you choose to always dump the macro's output.

the existing Zunpretty=expanded flag is not suitable because it fails if rustc can't parse the generated tokenstream, it expands all macros in the generated tokenstream instead of showing exactly what the proc-macro generated, and it usually generates a huge amount of output which makes finding the output of any particular proc macro invocation very annoying.

This came up here where the workaround I'm currently using is inspired by iirc https://github.com/drahnr/expander where the proc-macro writes an output file itself and generates include!("path/to/generated/file.rs")

[tgross35]

Could unpretty=expanded change to print the token stream even if it is broken? Maybe it could gain some better filtering options so you could only emit a specific file, macro name, or module.

[programmerjake]

Could unpretty=expanded change to print the token stream even if it is broken? Maybe it could gain some better filtering options so you could only emit a specific file, macro name, or module.

If it also had an option to produce the output of a particular macro invocation without further expanding it, I think that would solve the majority of the problem. One important thing that wouldn't solve though is rustc error messages pointing to where in the generated file errors occur.