Is there a way to pre- & un-leak a value?

Question

I'm currently looking into doing more stuff with arrays, but I think the performance of those operations could be even better if we were allowed to somehow transmute into a Leaked<T> the array up front, only to un-leak it when the function ends. This would let us use leak amplification without a) introducing unsafety and b) setting up a catch_panic(_). Is this somehow possible in Rust?

For example, creating a generic array from an iterator (this obviously does not work):

#[inline]
fn map_inner<I, S, F, T, N>(list: I, f: F) -> GenericArray<T, N>
where I: IntoIterator<Item=S>, F: Fn(&S) -> T, N: ArrayLength<T> {
     unsafe {
        // pre-leak the whole array, it's uninitialized anyway
        let mut res : GenericArray<Leaked<T>, N> = std::mem::uninitialized();
        let i = list.into_iter();
        for r in res.iter_mut() {
            // this could panic anytime
            std::ptr::write(r, Leaked::new(f(i.next().unwrap())))
        }
        // transmuting un-leaks the array
        std::mem::transmute::<GenericArray<Leaked<T>, N>,
                              GenericArray<T, N>>(res)
    }
}

I should note that if we either had compile-time access to the size of T or a type that can hide its innards from borrowck (like Leaked<T> in the example), this is perfectly feasible.

Answer 1

It is possible using nodrop, but it could leak.

fn map_inner<I, S, F, T, N>(list: I, f: F) -> GenericArray<T, N>
where I: IntoIterator<Item=S>, F: Fn(&S) -> T, N: ArrayLength<T> {
     unsafe {
        // pre-leak the whole array, it's uninitialized anyway
        let mut res : NoDrop<GenericArray<T, N>> = NoDrop::new(std::mem::uninitialized());
        let i = list.into_iter();
        for r in res.iter_mut() {
            // this could panic anytime
            std::ptr::write(r, f(i.next().unwrap()))
        }
        res.into_inner()
    }
}

Let's suppose that after the first item (a) is consumed from i and written to r, a panic happens. The remaining items from i would be drop, but the item a would not. Although leaking memory is not considered unsafe, it is not desirable.

I think that the approach described in the question link is the way to go. It is similar to the Vec and ArrayVec implementations. I'm using a similar approach in array library that I'm writing.