rustdoc should also be passed RUSTFLAGS

[nox]

I know about RUSTDOCFLAGS, but I would like to understand the rationale against passing RUSTFLAGS to rustdoc when running cargo test.

My use case is the following: I'm using unstable Rust code in a 1.0 crate, and I want to cfg-gate it like Rayon does with RUSTFLAGS="--cfg rayon_unstable". It works great, but my issue is that some code that is now using unstable Rust is tested through doc tests and given rustdoc isn't passed RUSTFLAGS, putting # #![cfg_attr(foo_unstable, feature(some_unstable_feature))] in the doc test does nothing and it fails.

[alexcrichton]

This unfortunately cannot be done because rustdoc does not support all of rustc's flags, and I think supporting that would be an upstream rust-lang/rust issue.

[nox]

See rust-lang/rust#46189.

[Kixunil]

Could there be a warning when RUSTFLAGS is set for doc? I just spent a lot of time trying to figure out why it doesn't work for me. Would be nice to at least save others.