Panic when iterator passed to alloc_extend itself tries to allocate in the same arena

[lierdakil]

TL;DR: see subject.

A slightly convoluted example:

#[test]
fn test() {
    use typed_arena::Arena;
    enum List<'a> {
        Nil,
        Elt(&'a Self),
        Seq(&'a [Self]),
    }

    let arena: Arena<List> = Arena::new();
    let _ =
        List::Seq(arena.alloc_extend([List::Nil].into_iter().map(|x| List::Elt(arena.alloc(x)))));
}

Expectation: the code works.

Reality:

thread 'test' panicked at 'already borrowed: BorrowMutError', .../typed-arena-2.0.2/src/lib.rs:194:38

For additional context, I wanted to represent a rose-tree-like data structure and save on allocations, constructing it from another data structure, but in effect I have to collect all my iterators into Vecs before alloc_extend and that kinda defeats the purpose.

Now, I understand why it panics. The question is, can we make it not panic? Seems like it should be possible in principle (although tricky)

[lierdakil]

A simple(-ish) approach may be to change ChunkList::current to be an Option, take it out at the start of alloc_extend. If the iterator tries to allocate, make a new Vec for current. At the end of alloc_extend either put the taken out current back (if current is None) or push it to rest. Depending on the allocation pattern, this may be rather wasteful, but it arguably beats an undocumented panic.