[hobofan]

It's a block though, isn't it?

Color::Green is an enum variant while { Color::Green } is a (constant) block (expression) that evaluates to an instance which is used as a generic type parameter.

The difference might be more easily understandable if we look at an enum variant that holds a value, where the syntactic differences between variant and instance constructor are more clearly visible.

Color::RGB(u64, u64, u64) vs { Color::RGB(10, 20, 30) }

[DougBTX]

Hm, the RFC says the braces are needed if it is not an "identity expression" with the examples:

    const X: usize = 7;
    
    let x: RectangularArray<i32, 2, 4>;
    let y: RectangularArray<i32, X, {2 * 2}>;

So I'm still not sure if Color::Green is an identity expression, they say:

> Identity expression: An expression which cannot be evaluated further except by substituting it with names in scope. This includes all literals as well all idents

So... maybe? Depends on whether Color::Green is an ident or not. At the very least I'd expect this to work, but it doesn't yet:

    use Color::*;
    
    impl State<Green> { ... }

[fluffything]

A name in scope is a path to a name, and Color::Green is also a path, so if impl<Green> works impl<Color::Green> would work as well.