[Question] Why does parse.expand use an unstable flag to rustc?

[bavalpey]

Currently, [parse.expand] uses the -Z unstable pretty=expand option to rustc. This requires using the nightly rustc compiler. There is nothing in the documentation that mentions why this is required, and there isn't much discussion as to why. If macro expansion is needed, is there a reason that cbindgen can't just do cargo expand which does not require any nightly rustc options?

I think that macro expansion is a relatively common requirement, and gating it behind having to use a nightly toolchain is a big cost.

[bavalpey]

I've been looking into this more and I think that cbindgen should set RUSTC_BOOTSTRAP in the command line where it invokes rustc. This is what cargo-expand does.
That way, we can avoid having needless error messages when using cbindgen without a nightly compiler.