Support duplicate interpolations inside a repetition

[dtolnay]

quote! { #a #a } works but quote! { #(#a #a)* } does not.

[dtolnay]

I don't think this is possible with quote! being a macro_rules! macro because macro_rules does not expose any way to compare idents for equality.

Rewriting quote! as a procedural macro (using proc-macro-hack) would make it possible to fix this but I would prefer to wait until functionlike!() procedural macros are supported for real.

[purpleposeidon]

This is the work-around I have been using for this issue:

let a2 = &a
quote! { #(#a #a2)* }

[purpleposeidon]

Nope, that doesn't actually compile!

#[macro_use]
extern crate quote;

fn main() {
    let foo: Vec<&str> = Vec::new();
    let foo2 = &foo;
    quote! {
        #(#foo #foo2)*
    };
}

error[E0505]: cannot move out of `foo` because it is borrowed
 --> src/main.rs:7:5
  |
6 |       let foo2 = &foo;
  |                   --- borrow of `foo` occurs here
7 |       quote! {
  |  _____^ starting here...
8 | |         #(#foo #foo2)*
9 | |     };
  | |______^ ...ending here: move out of `foo` occurs here
  |
  = note: this error originates in a macro outside of the current crate

--pretty expanded makes it obvious why:

fn main() {
    let foo: Vec<&str> = Vec::new();
    let foo2 = &foo;
    {
        let mut _s = $crate::Tokens::new();
        for (foo, foo2) in foo.into_iter().zip(foo2) {
            $crate::ToTokens::to_tokens(&foo, &mut _s);
            $crate::ToTokens::to_tokens(&foo2, &mut _s);
        };
        _s
    };
}

[dtolnay]

let ref foo: Vec<&str> = Vec::new();
let foo2 = foo;
quote! {
    #(#foo #foo2)*
};

[JeanMertz]

For reference, I ran into this today.

I am porting a fairly complex macro_rules! macro to a proc-macro implementation, using quote! as the first step to keep as much of the existing code the same. As soon as I got to some of the repetition parts, it failed with this error.

Since funclike proc-macro's are (almost?) a thing now, perhaps it's time to start thinking about that rewrite for quote! as well (I'm saying this, knowing full well that such a thing takes time, and time is something we could all use a bit more).

Until that moment, is the best workaround to do the looping outside of quote! and use extend to stitch multiple quotes together?

Not sure why I didn't realise the post above was the current workaround, but that works as expected 👍