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// Make functions `const` if the `const-fn` feature is active.
// The meta attributes are in place to keep doc comments with the functions.
// The function definition incl. annotations and doc comments must be enclodes
// by the marco invocation.
macro_rules! const_fn {
    ($(#[$attr:meta])* pub const unsafe fn $($f:tt)*) => (

        $(#[$attr])*
        #[cfg(feature = "const-fn")]
        pub const unsafe fn $($f)*

        $(#[$attr])*
        #[cfg(not(feature = "const-fn"))]
        pub unsafe fn $($f)*
    );
    ($(#[$attr:meta])* pub const fn $($f:tt)*) => (

        $(#[$attr])*
        #[cfg(feature = "const-fn")]
        pub const fn $($f)*

        $(#[$attr])*
        #[cfg(not(feature = "const-fn"))]
        pub fn $($f)*
    );
    ($(#[$attr:meta])* const fn $($f:tt)*) => (
        $(#[$attr])*
        #[cfg(feature = "const-fn")]
        const fn $($f)*

        $(#[$attr])*
        #[cfg(not(feature = "const-fn"))]
        fn $($f)*
    );
}