Generic structs with additional fields for certain type constraints

Right now generic types can implement traits if its type parameters match certain constraints. But sometimes implementing these additional traits requires additional struct fields. Since the struct field list is currently fixed, that means that the programmer has to decide between adding additional fields that are unused in most instances of the struct, or not implementing traits that it would be nice to have.

For example, I think it would be nice if std::io::Take<T> implemented std::io::Seek if T: std::io::Seek. But in order to do this, std::io::Take would need an additional field to keep track of the original limit (in order to support SeekFrom::Start). It seems very wasteful to put this extra field on all implementations of Take when most of them would never touch the field.

I'm not sure what the syntax of this should look like.

Start with the issue's generic-struct and conditional-trait example involving std::io::Take and std::io::Seek. Research how Rust represents fields and trait implementations under type constraints, then define a viable syntax and its expected behavior; completion would require a concrete, agreed-upon design rather than a localized code change.