parse.expand should use nightly compiler when available like upstream cargo-expand

[segevfiner]

parse.expand currently only works on the nightly toolchain, which means this repos test also only work in the nightly toolchain and anyone that wants to use this feature needs to use nightly. Instead, it should use the nightly toolchain when available like upstream cargo-expand does, which will allow it work on stable, with only the expand process using the nightly toolchain.

P.S. Might be nicer to have a way to specify to expand the current crate without listing its name, e.g. expand = true.

Is there any workaround to not use expand if I have some struct generated by a derive macro (e.g. derive_builder) that I want to export?

[emilio]

We intentionally don't make using nightly-only features easy, that kinda defeats the point of them not being stable. You can achieve what you want with RUSTC_BOOTSTRAP or what not, too, right?