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

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> { ... }