`invariant!` macro to declare an invariant

invariant_macro.rs

// If `unsafe` and `nightly` are enabled, enable unstable `core_intrinsics` feature
// with #![feature(core_intrinsics)] (Nightly only).

macro_rules! invariant {
    ($expr: expr) => {
        cfg_if::cfg_if! {
            if #[cfg(all(feature = "unsafe", feature = "nightly"))] {
                core::intrinsics::assume($expr);
            }
            else if #[cfg(feature = "unsafe")] {
                if !($expr) {
                    core::hint::unreachable_unchecked();
                }
            }
            else {
                debug_assert!($expr);
            }
        }
    };
}