Look for a nightly compiler

[dtolnay]

$ rustup run stable cargo expand >/dev/null

  warning: the option `Z` is unstable and should only be used on the nightly
    compiler, but it is currently accepted for backwards compatibility; this
    will soon change, see issue #31847 for more details
  warning: the option `pretty` is unstable and should only be used on the nightly
    compiler, but it is currently accepted for backwards compatibility; this
    will soon change, see issue #31847 for more details

Let's run rustc --version and rustup run nightly rustc --version and choose the one that provides a nightly compiler.

If neither is nightly, keep the current behavior with warnings.

[ehuss]

Note: as of Rust 1.19 you must use nightly in order for cargo expand to work.

I'm curious if you have any thoughts about the future of cargo expand. In rust-lang/rust#31847 it is mentioned that --pretty is broken and may be removed or replaced.

[dtolnay]

Fixed in db455ab.