Але, оп!

pub struct Struct;

pub fn test<T>()
where
T: From<Struct>,
{
inner::<T>();
}

pub fn inner<T>() -> T
where
T: From<Struct>,
{
T::from(Struct)
}

fn main() {
test::<Struct>();
test::<Box<Struct>>();
}

Мейджик!