Macro phantom_fragment

macro_rules! phantom_fragment {
    ($($_:tt)*) => { ... };
    ($_:block) => { ... };
    ($_:expr) => { ... };
    ($_:ident) => { ... };
    ($_:item) => { ... };
    ($_:lifetime) => { ... };
    ($_:literal) => { ... };
    ($_:meta) => { ... };
    ($_:pat) => { ... };
    ($_:pat_param) => { ... };
    ($_:path) => { ... };
    ($_:stmt) => { ... };
    ($_:ty) => { ... };
    ($_:vis) => { ... };
}

A simple macro that ignores all of its contents and does nothing.

Use

This macro exists to enable expansion of repeat or optional rule groups that may not explicitly make use of that argument. For example, sometimes the conditional inclusion of a token might want to generate something not using that meta-variable – but expansion requires at least one meta-variable from the group to be specified to not be ambiguous. With this macro, you can still “use” the meta-variable without actually actively using it.

Example

macro_rules! generate{
  ($A:ident, $($B:ident)?) => {
    $(
      phantom_fragment!($B);
      impl $A {
        // When $B is present, define this function without using $B.
        fn do_something(&self) { /* ... */ }
      }
    )?
  }
}