add associated constructor to Dimension trait #1330

I am working on the ndarray-interp crate, which is generic over the number of dimensions. This causes difficulties when the I need to construct a Dimension that is bigger than 6 (IxDyn).

Specifically the method interp_array needs to generically construct its return array of Dimension <Dq as DimAdd<D::Smaller>>::Output where Dq: Dimension + DimAdd<D::Smaller>.
This works fine for static dimensions as I can use the Default trait to create the correct Dimension. But when <Dq as DimAdd<D::Smaller>>::Output happens to be IxDyn this does not work because the information about the number of dimensions is lost.

This is my current, very ugly solution:

let shape = match <Dq as DimAdd<D::Smaller>>::Output::NDIM {
    Some(_) => {
        let mut dim = <Dq as DimAdd<D::Smaller>>::Output::default();
        dim.as_array_view_mut()
            .into_iter()
            // ... set the correct axis lenghts
            ;
        dim
    }
    None => {
        let lenghts: Vec<usize> = ; // ... collect the correct axis lenghts
        let dyn_dim = IxDyn(&lenghts);
        (&dyn_dim as &dyn Any)
            .downcast_ref::<<Dq as DimAdd<D::Smaller>>::Output>()
            .unwrap_or_else(|| unimplemented!())
            .clone()
    }
};

This requires ndarray to guarantee that Dimension::NDIM is only ever None when the underlying type is IxDyn. If there is ever a impl Dimension for NewType this code will probably brake.

I might have missed something, but there does not seem to be a good solution for this problem currently.

I suggest extending the Dimenion trait, possibly by requiring Dimension: TryFrom<&[Ix]> or adding a associated constructor:

trait Dimension {
    /// try to create a new Dimension from the provided slice
    /// fails when `Self::NDIM.is_some_and(|ndim| ndim != ix.len())`
    fn try_new(ix: &[Ix]) -> Option<Self>;
}