use std::{any::TypeId, mem::ManuallyDrop};

/// Safe cast using [`TypeId`].
#[inline]
pub fn cast<T: 'static, R: 'static>(t: T) -> Result<R, T> {
    if TypeId::of::<T>() == TypeId::of::<R>() {
        union U<T, R> {
            t: ManuallyDrop<T>,
            r: ManuallyDrop<R>,
        }

        // SAFETY: we've confirmed the types are the same.
        Ok(unsafe { ManuallyDrop::into_inner(U { t: ManuallyDrop::new(t) }.r) })
    } else {
        Err(t)
    }
}

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn test_cast() {
        assert_eq!(cast::<_, &'static str>("foo"), Ok("foo"));
        assert_eq!(cast::<_, &'static str>(4), Err(4));
    }
}