Oh, nice. Thanks for the input. I'm aiming to have the final use look like Const::<f64>::new() resulting in a Const<f64>. I'm wary of having separate f64 and f32 implementations as namespaces that are disconnected from the base type they contain. I think that could lead to issues type checking everything where two separate entities of a given type interact. Eventually it would be nice if things were automatically able to interact using casting from one to the other in the case of differing types. But, to start with, I'd hope that when using the crate to create Item<f64>s, they would then just happily work directly with any OtherItem<f64>.
Here's another discussion relating to this question.