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)*
);
}