struct A<'a> {
    other: &'a mut Box<dyn B>,
    pother: Box<dyn X>,
}

trait B { }

struct C;
impl B for C { }

trait X { }

impl<'a> X for A<'a> { }

impl<'a> A<'a> {
    pub fn new(o: &'a mut Box<dyn B>, p: Box<dyn X>) -> Self {
        Self { other: o, pother: p }
    }
}