How do I call concat_idents inside a recursive macro expansion?

Question

I'm trying to create a macro to expand a set of names and define fields for a struct:

macro_rules! expand {
  ($($name:ident),*) => {
     pub struct Test {
       $(
          concat_idents!(var_, $name) : Vec<$name>
        ),*
     }
  }
}
//fails
expand!(a,b,c);

This fails as the compiler does not recognize concat_idents! as a macro. How am I supposed to work around this?

score 5 · Accepted Answer · answered May 08 '15 at 00:43

5

It is not at present possible to do this in any way. Substitutions ($foo) are fine there, but not macro calls.

answered May 08 '15 at 00:43

Chris Morgan

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This response is outdated, its perfectly legal to do macro calls inside macro differentiation. I'm using version1.24.0-nightly. – Luke Dupin Dec 25 '17 at 18:56
@LukeDupin: no; the answer is still correct, nothing has changed: the parser does not accept macros in field name position. To remove `macro_rules!` from the equation, try `struct Test { concat_idents!(var_, a): u8 }`: It fails to parse. – Chris Morgan Dec 29 '17 at 01:51

How do I call concat_idents inside a recursive macro expansion?

1 Answers1