The following code works fine without the `effects` feature, but triggers a warning (soon to [become a hard error](https://github.com/rust-lang/rust/pull/120805)) when the feature is enabled: ```rust #![feature(effects)] #[derive(Eq, PartialEq)] pub struct TypeThatIsPartialEq(u8); pub const GREEN: TypeThatIsPartialEq = TypeThatIsPartialEq(4); pub const fn is_green(x: TypeThatIsPartialEq) -> bool { match x { GREEN => true, _ => false } } ``` [Playground](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=415cd88942231f06b9f2e79fb70da64b)