RUSTFLAGS is gone

[dtolnay]

It appears Cargo is no longer propagating RUSTFLAGS into the build script execution as of nightly-2021-07-22.

// build.rs

use std::env;
use std::io::{self, Write};
use std::process;

fn main() {
    let _ = writeln!(io::stderr(), "{:?}", env::var_os("RUSTFLAGS"));
    process::exit(1);
}

$ RUSTFLAGS='-Z allow-features=' cargo +nightly-2021-07-21 check
  --- stderr
  Some("-Z allow-features=")

$ RUSTFLAGS='-Z allow-features=' cargo +nightly-2021-07-22 check
  --- stderr
  None

That currently breaks this CI build:

https://github.com/alexcrichton/proc-macro2/blob/e71bc70db21eb6f217d8ed9f8ece0d53ba7cfac5/.github/workflows/ci.yml#L46

due to this logic not being able to observe that RUSTFLAGS value:

https://github.com/alexcrichton/proc-macro2/blob/e71bc70db21eb6f217d8ed9f8ece0d53ba7cfac5/build.rs#L135