Macro unreachable_unchecked

macro_rules! unreachable_unchecked {
    () => { ... };
    ($msg:expr) => { ... };
    ($msg:expr,) => { ... };
    ($fmt:expr, $($arg:tt)*) => { ... };
}

Indicates unreachable code that we are confident is truly unreachable.

This is essentially a compromise between core::unreachable!() and core::hint::unreachable_unchecked(). In debug mode builds and in tests, this expands to unreachable!(), causing a panic. However, in release mode non-test builds, this expands to unreachable_unchecked. Thus, this is a somewhat safer form of unreachable_unchecked that will allow cases where unreachable_unchecked would be invalid to be detected early.

Nonetheless, this must still be used with caution! If code is not adequately tested, it is entirely possible for the unreachable_unchecked to be reached in a scenario that was not reachable in tests.