Clone trait error in recursive

[ivanka2012]

This isn't necessarily an issue per se, but I thought the following problem I encountered would be worth documenting since this took me a while to finally figure out:

If you use recursive and call a function in it that returns a parser, then make sure that the parser that function returns also implements Clone.

This doesn't work:

fn another_parser<'a>() -> impl Parser<'a, &'a str, ()> {
    // ...
}

fn test(){
    // the trait bound `impl chumsky::Parser<'_, &str, ()>: Clone` is not satisfied
    // the trait `Clone` is not implemented for `impl chumsky::Parser<'_, &str, ()>`
    recursive(|r| {
        let parser = another_parser();

        parser
    });
}

This works:

// Note the Clone
fn another_parser<'a>() -> impl Parser<'a, &'a str, ()> + Clone {
    // ...
}

fn test(){
    recursive(|r| {
        let parser = another_parser();

        parser
    });
}

This also applies to cases where the parser passes useful data i.e. impl Parser<'a, &'a str, SomeType> instead of impl Parser<'a, &'a str, ()>.

[zesterer]

I've been tempted for a while to add a + Clone bound to Parser. It's not required, but this seems to be a common enough thing that people run into that it might be worth it. Alternatively, the compiler could provide a more useful diagnostic here.