Varying Generic Parameters with features

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You might be interested in this trick that I cobbled together recently. It's a hack, but it does allow a proc macro to determine the true/false state of a #[cfg(...)] predicate in a downstream crate, and it doesn't break any of Rust's rules for macros. In your particular case, if you know all the features ahead of time, it's actually easier. You could do something like the following:

  #[cfg(feature = "first")]
  macro_rules! check_first{
    (($($bools:expr),*), $($args:tt)*) => {
      check_second!(($($bools,)* true), $($args)*);
    }
  }
  #[cfg(not(feature = "first"))]
  macro_rules! check_first{
    (($($bools:expr),*), $($args:tt)*) => {
      check_second!(($($bools,)* false), $($args)*);
    }
  }

  #[cfg(feature = "second")]
  macro_rules! check_second{
    (($($bools:expr),*), $($args:tt)*) => {
      my_proc_macro!(($($bools,)* true), $($args)*);
    }
  }
  #[cfg(not(feature = "second"))]
  macro_rules! check_second{
    (($($bools:expr),*), $($args:tt)*) => {
      my_proc_macro!(($($bools,)* false), $($args)*);
    }
  }

And then using

check_first!((), args...);

will expand to

my_proc_macro!((true, false), args...);

where the bool literals are the state of each cfg predicate, in order. The proc macro will then see and be able to parse those literals to know each cfg predicate state.

One other alternative is that if you're only ever going to be generating these generics in your own crate, you could just forward the features directly to the proc macro in your Cargo.toml when you pull the proc macro crate in as a dependency. That way it will have the same features enabled/disabled locally.

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