Meaningless `pin!` in wgpu_hal::dx12

[jimblandy]

In #8495 (comment) @Wumpf asked @inner-daemons to pin an ArrayVec because we're going to take a pointer to it, resulting in this code:

        let mut view_instancing =
            core::pin::pin!(ArrayVec::<Direct3D12::D3D12_VIEW_INSTANCE_LOCATION, 32>::new());

However, this doesn't help and isn't necessary.

"Doesn't help": like most types, ArrayVec<T> implements Unpin if T does, meaning that Pin<ArrayVec<T>> is just an ordinary newtype, with no unusual effect. You could simply use Pin::get_mut to get a mutable reference to the ArrayVec and then use std::mem::replace to swap out a different one and drop the original.

"Isn't necessary": You can actually do what you intend here without using pin, using ordinary Rust:

1. Before building the D3D12_VIEW_INSTANCING_DESC, declare a local variable that borrows a shared reference to view_instancing.

2. Use this reference directly to initialize pViewInstanceLocations, since &T converts to *const T automatically.

3. Say drop(<reference var>); after you're done using the pointer. This ensures that the shared reference lives this long.

As long as view_instancing is borrowed for sharing, it can't be moved.

[inner-daemons]

Thanks for writing this up. I rarely have access to windows these days, so I'd prefer if someone could address this. The fix should be simple, just a few lines.

[Wumpf]

Thanks for the clarification, will fix since I caused this in the first place!

[jimblandy]

I'm pretty sure there's nothing wrong with the code even if you just say:

let mut view_instancing = ArrayVec<...>::new();
... build view_instancing ...
pViewInstanceLocations: &view_instancing.as_ptr(),

If you wanted to ensure that view_instancing isn't modified after it's built, you could say:

let view_instancing = view_instancing; // remove `mut`

But you can still move out of variables even when they're not mut. The nice thing about the shared reference is that you'll get an error if you do anything to view_instancing at all, even just to move out of it.

I'm honestly not sure how many pairs of suspenders I want to wear simultaneously here.