Provide a way to allow (or check) to fail a particular combination of features

[taiki-e]

usecases:

• https://github.com/rust-lang/regex/blob/d6bc7a4c3b58e1d618024aaededa722df32fa6e8/src/lib.rs#L613
• https://github.com/rust-lang/futures-rs/blob/3601bb76708ea4bcc14c3491237abc33b685ba0b/futures/src/lib.rs#L100
• https://doc.rust-lang.org/cargo/reference/features.html#mutually-exclusive-features

[CAD97]

Another use case, roughly:

cfg_if! {
    if #[cfg(feature = "unstable")] {
        // do something unstable
    } else if #[cfg(feature = "polyfill")] {
        // do something polyfilled
    } else {
        compile_error!("requires the `unstable` or `polyfill` feature");
    }
}

This is useful when the polyfill feature is a dependency. (Example: std::lazy and once_cell.)

Though this is achievable with

cargo hack check --feature-powerset --features unstable
cargo hack check --feature-powerset --features polyfill

[tamird]

Either this or #182 would be quite helpful.

[taiki-e]

As already said in #182, I would accept a PR to implement #182.

[taiki-e]

Closing in favor of #182.