Clone an Rc trait object and cast it

Question

This question is related to Rust: Clone and Cast Rc pointer

Let\'s say I have this piece of code which works fine:

Answer 1

You can work around this issue by not using a receiver in TraitAB's casting methods (i.e. by declaring them as associated functions):

trait TraitAB : TraitA + TraitB {
    fn as_a(it: Rc<RefCell<Self>>) -> Rc<RefCell<dyn TraitA>>;
    fn as_b(it: Rc<RefCell<Self>>) -> Rc<RefCell<dyn TraitB>>;
}

The trait can then be implemented as

impl TraitAB for MyType {
    fn as_a(it: Rc<RefCell<MyType>>) -> Rc<RefCell<dyn TraitA>> {it}
    fn as_b(it: Rc<RefCell<MyType>>) -> Rc<RefCell<dyn TraitB>> {it}
}

These functions can then be called using the fully qualified syntax.

a = TraitAB::as_a(ab.clone());
b = TraitAB::as_b(ab.clone());

The TraitAB implementation for all types will be the same. To make this implementation available for all types implementing TraitA and TraitB, you can use a generic impl:

impl<T: TraitA + TraitB + 'static> TraitAB for T {
    fn as_a(it: Rc<RefCell<T>>) -> Rc<RefCell<dyn TraitA>> {it}
    fn as_b(it: Rc<RefCell<T>>) -> Rc<RefCell<dyn TraitB>> {it}
}

Note that T: 'static because the trait objects in the function return types have an implicit 'static lifetime bound.

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