返回向量中特征的可变引用

Question

New to rust, I'm trying to build a state stack.

pub trait State {
    fn tick(&mut self);
}

pub struct StateStack {
    stack: Vec<Box<dyn State>>,
}

impl StateStack {
    pub fn push(&mut self, state: Box<dyn State>) {
        self.stack.push(state);
    }

    pub fn current(&mut self) -> &mut dyn State
    {
        &**self.stack.last_mut().expect("No state in stack")
    }
}

When compiling, I get this error:

error[E0308]: mismatched types
  --> src/main.rs:16:9
   |
14 |     pub fn current(&mut self) -> &mut dyn State
   |                                  -------------- expected `&mut dyn State` because of return type
15 |     {
16 |         &**self.stack.last_mut().expect("No state in stack")
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ types differ in mutability
   |
   = note: expected mutable reference `&mut dyn State`
                      found reference `&(dyn State + 'static)`

I don't understand where this State + 'static' come from, is this even the issue? I'm not sure where to begin to analyze this error. I think I get the deref chain right, but I feel I might be missing something here.

As a side question, I'm an OOP developper, and I'm not sure that what I'm doing is going the rustly way. Any other architecture to considere for state stack?

Answer 1

The issue is you downgrade the mutable reference you got with last_mut to an immutable reference when you use &** . You need to instead take a mutable reference and that will solve the issue:

 &mut **self.stack.last_mut().expect("No state in stack")

As for &(dyn State + 'static') , you can mostly just ignore the 'static . Anonymous traits ( dyn Trait ) are not concrete types so they may not all share the same lifetime requirements. dyn State + 'static indicates an anonymous State trait which can survive for a lifetime of 'static . 'static just means that a lifetime in unconstrained and could be placed in a static context. Most types have a lifetime of 'static so it does not get included in regular use. Some types with 'static lifetimes would be primitives like u32 or f64 . Any dyn Trait which does not include a lifetime gets an implicit 'static .