Enable RUST_BOOTSTRAP=1 environment

[kvark]

This would allow having parse.expand.crates in cbindgen.toml without requiring the nightly compiler. Related to #38 . Would make Gecko's life easier.
cc @eqrion @staktrace

[staktrace]

Should be RUSTC_BOOTSTRAP=1 (not RUST_BOOTSTRAP=1). It's basically a hack that allows using nightly -Z flags on stable.

[emilio]

Why do you want to expand macros in Gecko? We wouldn't be able to do this easily anyhow, I'd think, since cbindgen runs as part of the export phase, and thus cannot depend on other compiled artifacts.

Though maybe since it invokes rustc by itself it'd be fine.

That being said, I think it'd be better to just enable it in Gecko if we were to do this, you can do it easily changing RunCbindgen.py to run with the new environment variable. So I think this is wontfix, in this repo at least.

[eliaslevy]

I would be useful the documentation mentioned that parse.expand requires the use of the nightly compiler.

BTW, what is the reason for this? It seems cargo expand works with stable.

[emilio]

cbindgen doesn't use cargo expand, it uses rustc --pretty=expanded.

[eliaslevy]

I am aware. I was wondering why cbindgen requires rustc and the use of a nightly only flag while cargo expand seemingly performs the same function under stable.