Right but your call to Default::default() gives the game away that we do have inference. Default::default() is generic, if we didn't have inference we'd need to tell it which of the enormous number of implementations of that trait it should call.

Would you take syntax like clear_value: _ { r: 0.25, g: 0.5, g: 0.75, a: 1.0 } ?? Then we're saying that we know we need to pick a type here but the type can be inferred where our underscore was, just like when we

let words: Vec<_> = "A sentence broken by spaces".split_whitespace().collect();