生命周期必须在 static 生命周期内有效

Question

trait X {}

trait Y {}

struct A {}

impl X for A {}

struct B<'r> {
    x: &'r mut Box<dyn X + 'r>,
    id: i32,
}

impl <'r> Y for B<'r> {}


struct Out {
    x: Box<dyn X>,
}

impl Out {
    pub fn new() -> Self {
        return Out {
            x: Box::new(A{})
        }
    }

    pub fn get_data(&mut self) -> Box<dyn Y> {
        return Box::new(B{
            id: 1,
            x: &mut self.x
        })
    }
}

Run it here on playground.

I am getting this note from the compiler:

note: but, the lifetime must be valid for the static lifetime...
   = note: ...so that the expression is assignable:
           expected &mut std::boxed::Box<dyn X>
              found &mut std::boxed::Box<(dyn X + 'static)>

I understand where static lifetime is coming from, but doesn't the same lifetime is passed to struct B during its creation which accepts any generic lifetime.

[Edit After below answer]

I also tried making struct Out generic but then was unable to use it after initialization.

Answer 1

I fixed it ( playground ). Here's the relevant code:

// note the lifetime!
struct Out<'a> {
    x: Box<dyn X + 'a>,
}

// note the lifetime!
impl<'a> Out<'a> {
    pub fn new() -> Self {
        return Out {
            x: Box::new(A{})
        }
    }

    pub fn get_data(&'a mut self) -> Box<dyn Y + 'a> {
        return Box::new(B {
            id: 1,
            x: &mut self.x,
        })
    }
}

Why is this necessary?

Trait objects always have a lifetime. If no lifetime is specified or inferred, it defaults to 'static . Therefore you have to make Out generic over its lifetime and use it in the implementation.