The ComPtr API is unsound.

[Brezak]

Description
The ComPtr API let's you de reference a null pointer in safe code.

Repro steps

1. Safely construct a null ComPtr with the ComPtr::null function.
   A.2 Clone your new pointer.
   A.3 Look on in horror as as_unknown de references a null pointer.
   B.2 Be a bit careless and let rust autoderef your pointer under your nose.
   B.3 Watch as your pointer Deref impl executes a rapid unplanned program exit.

Possible solutions

The simplest solution would be two fold. The clone impl should only call AddRef when the pointer isn't null. The deref impl should be replaced by a deref function that's either safe and returns a Option<&T> or is unsafe and returns a &T.

I'm also attempting a solution where ComPtr is turned into a basic wrapper over a *mut ptr and a new type is introduced that always contains a valid (non null and pointing to a valid allocation) pointer to a Interface. The problem is it touches a lot of code and I'm not very familiar with this repo.

Platform
Windows

[teoxoy]

It looks like we do have debug asserts for this but I agree our implementation is not great. Rather than rewriting it, I think we should just move over to windows-rs or a subset of it (see #3207).

[ErichDonGubler]

Speaking for myself and @cwfitzgerald (with whom I've spoken about this directly), I think we have no attachment to the d3d12 crate; we'd also rather migrate directly to the windows ecosystem, rather than fix d3d12.

[onkoe]

it's probably not scarier than trusting in libloading, but the bindings for d3d12 are automatically generated and kinda verbose.

keep msdn handy if you tackle this issue! https://github.com/microsoft/windows-rs/tree/master/crates/libs/windows/src/Windows/Win32/Graphics/Direct3D12

[MarijnS95]

@onkoe per #3207 (comment) the migration to the windows crate ecosystem is already being worked on 🙂

[onkoe]

sounds great! just wanted to warn anyone who expected a drop-in replacement

that said, your branch looks very nice already. can see why you'd start immediately! 😄