Macro maybe_const_fn

macro_rules! maybe_const_fn {
    (
      #[cfg($cfg_predicate:meta)]
      $(#[$attr:meta])*
      $vis:vis $(unsafe $($unsafe:lifetime)?)? fn $name:ident $($rest:tt)*
  ) => { ... };
}

A macro for conditionally const-ifying a function. #[allow(unused)] because currently it is only used with the must_cast feature.