Macro surrounding impl block

help
1

Is it possible to define a macro that can add default implementations of functions for a impl block ?

ie something like

macro_rules! ElementImpl {
    ($tr: ident, $method: ident) => {
      
         // Here I would output the trait implementation but with the added get_name($self) method
    };
}

pub trait Element {

    fn get_name(&self) -> String;
}
    
pub struct Export {
    first_name: String
}

ElementImpl! {
impl Element for Export {

    fn another_implentation(&self) -> String {
       
    }
}
}

So at the end the Element impl for Export would contain the functions get_name and another_implentation.

Thanks

3

I tried that just before asking funny enough. On Rust playground nightly I get

24 | impl Element for Export {
   | ----------------------- first implementation here
...
31 | impl Element for Export {
   | ^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Export`
4

Yeah, sorry, I mixed it up with the regular impl block. The impl for Trait is limited to one :frowning:

5

(If I understood correctly your question)
You can't add a method in a trait impl that is not part of the trait itself: ex

If what you want is having a default impl of some methods of the trait you can do this

6

The problem was

I have quite a few methods of the Element trait that have to be implemented for hundreds of types. Easy things like

fn get_element_name() -> &'static str {
    stringify!("CounterSourceElement")
}

I have a macro that can generate these type of methods in the objects I am implementing Element for ie

element! {
    CounterSourceElement {
    }
}

But I would like some way to reduce having to implement them in the impl trait code where I need them to be ie

impl Element for CounterSourceElement {
    fn get_element_name() -> &'static str {
        stringify!("CounterSourceElement")
    }
  
    .....
}

for every new element type.

Hope that makes sense.

7

In the end I defined the trait for Element to be

pub trait Element: ElementMethods

I then used the macro to create the ElementMethods impl wile leaving the user to define the methods in Element.