How to implement quote::ToTokens for an enum?

Question

I am implementing a procedural macro and I need to provide my own structures to the outside. I need to implement quote::ToTokens trait for my structures, but I don't understand how. I have tried this code but did not succeed. This code is compilable, but it is not valid:

#[derive(Debug, Serialize, Clone)]
pub enum Enum {
    A,
    B,
}
impl quote::ToTokens for Enum {
    fn to_tokens(&self, tokens: &mut quote::Tokens) {
        tokens.append("Enum::B");
    }
}

The quote::ToTokens example does not explain this.

Saddletree

Answer 1

I think you are looking for something like:

impl quote::ToTokens for Enum {
    fn to_tokens(&self, tokens: &mut quote::Tokens) {
        tokens.append("Enum");
        tokens.append("::");
        match *self {
           Enum::A => tokens.append("A"),
           Enum::B => tokens.append("B")
        }
    }
}

If you are concerned with how the enum has been imported, then you should re-import it to make sure it is not conflicting with anything else:

impl quote::ToTokens for Enum {
    fn to_tokens(&self, tokens: &mut quote::Tokens) {
        tokens.append("{");
        tokens.append("use");
        tokens.append_separated(&["module_name", "Enum"], "::");
        tokens.append(";");
        tokens.append("Enum");
        tokens.append("::");
        match *self {
           Enum::A => tokens.append("A"),
           Enum::B => tokens.append("B")
        }
        tokens.append("}");
    }
}

Which will produce:

 { use module_name::Enum; Enum::A }

Extinguisher

What if we have "use Enum as E;" in our code or we did not even provided use for it so it must be prepended with "cratename::" ? Personally I think there must be a method of getting fully qualified name for the requested item. P.S. My question implies that your code in your answer is incorrect too :) – Saddletree

Probably you should always use the fully qualified name, including crate, just to be safe. It's quite ugly though. I'll update the answer. – Extinguisher

You can use it once inside the function, then you would only need to FQN it in the impl and in the use itself. – Madlin

This answer is out of date. tokens.append(&str) no longer works. – Quirinus

@FordO. what is the alternative now? just ran into this issue – Donata

I'm also running into this. I think we're now supposed to use quote::TokenStreamExt ("TokenStream extension trait with methods for appending tokens") but I haven't made it work, I get: "the trait cannot be made into an object because method append has generic type parameters" – Floriaflorian