Ideas for a more powerful `FromRequest(Parts)` derive

Feature Request

Motivation

I'd like to derive FromRequestParts from more complex structs (using trait bounds, and where predicates), like:

#[derive(FromRequestParts)]
#[from_request(via(axum::Extension), rejection(MyError))]
pub struct MyType<T1: TX = TTypeX, T2: TY = TTypeY>
where
    TX::SubType: XYType<A = T2>,
{
    pub field1: T1;
    ...
}

Simply using derive(FromRequestParts) would be nice, because the underlying implementation is still pretty straightforward.

The current version of the macro seems to only support newtypes.

Proposal

It would be nice if we could that kind of macro. Either by modifying, or rather by implementing a new macro that supports this kind of thing

I made a proof of concept (#3833), but it breaks the current usecase with newtypes, where it passes on the nested type rather than the newtype.