Can I write a declarative macro that does nothing with its input?

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1

I'd like to write a declarative macro that I can use to annotate my source code for use by a non-Rust parser. Ideally, for purposes of Rust code, this macro would consume its input (typically a string constant) and generate no corresponding syntax at all in the compiled output.

The error I'm getting at the moment is this:

error: macro expansion ignores token `{` and any following
 --> src/tests/sdd.rs:6:43
  |
6 | macro_rules! track_file( ($($tt:tt)*) => {{}} );
  |                                           ^
  |
 ::: src/tests/asciidoc_lang/root/index.rs:3:1
  |
3 | track_file!("docs/modules/ROOT/pages/index.adoc");
  | ------------------------------------------------- caused by the macro expansion here
  |
  = note: the usage of `track_file!` is likely invalid in item context

I don't typically work with declarative macros, so please be gentle. If there's a better way to accomplish this, I'd love to hear about it. The non-Rust parser is malleable (I'm writing it), but I'd prefer not to parse raw comments.

Edit: For context, here's a PR (not yet working, obvs) where I'd like to use this pattern: chore: Add macros to allow spec coverage tool to track spec implementation by scouten · Pull Request #167 · scouten/asciidoc-parser · GitHub. The intent here is to write an external parser that recognises specific tags (i.e. the no-op decl macros).

2

The issue is that you're making your macro expand to {}, you can just remove it to make the macro expand to nothing at all:

macro_rules! track_file( ($($tt:tt)*) => {} );
2 Likes
3

Boom. Thank you for the super-fast response, @SkiFire13. That seems to work.