Panic when no adapters are available

[parasyte]

Similar to #105
Originally reported in parasyte/pixels#36

We try to catch any errors from wgpu::Adapter::request to provide a "helpful" error message, but our error handler never gets called: https://github.com/parasyte/pixels/blob/98983d201db0639f51c264ade5cf876f8df65d1e/src/lib.rs#L412-L413

The backtrace points to the panic occurring here in wgpu-native: https://github.com/gfx-rs/wgpu/blob/951641dcc50b355eda894c9a5cd6da057d48765b/wgpu-native/src/instance.rs#L473-L475

The wgpu::Adapter::request method itself is suspicious. The docs claim it returns None when an adapter cannot be found, but it is clearly hardcoded to return Some<T>:

wgpu-rs/src/lib.rs

Lines 538 to 547 in cbdd74f

	/// Retrieves an [`Adapter`] which matches the given options.
	///
	/// Some options are "soft", so treated as non-mandatory. Others are "hard".
	///
	/// If no adapters are found that suffice all the "hard" options, `None` is returned.
	pub fn request(options: &wgn::RequestAdapterOptions) -> Option<Self> {
	Some(Adapter {
	id: wgn::wgpu_request_adapter(Some(options)),
	})
	}

Given these two issues, we're unable to capture the error (transform the None into Result<_, E>) because this method never returns None. I would work on this, but unfortunately I don't know the best way to approach the extern "C" interface. Should it just return Option<AdapterId>?

[fogti]

Actually, the unwrap lies in rust code here:

https://github.com/gfx-rs/wgpu/blob/c33b1e81fddf480f24e543cd3689b8991029d893/wgpu-native/src/instance.rs#L473-L475

It would be a good idea to call request_adapter directly.

[grovesNL]

It would be a good idea to call request_adapter directly.

In general we should avoid skipping the C API for functional changes like this, since we also want to allow other languages using the C API to fail gracefully too.

I would work on this, but unfortunately I don't know the best way to approach the extern "C" interface. Should it just return Option<AdapterId>?

We can't return Option<AdapterId> in the C API because we don't have Option available in C, but we need to be able to use the return value there.

We haven't implemented full WebGPU error handling yet, so for now the C API could use the special Id::ERROR for None and check for this in wgpu-rs. This is similar to the following check which we already have for swapchain errors:

wgpu-rs/src/lib.rs

Line 1358 in cbdd74f

if output.view_id == wgn::Id::ERROR {

[parasyte]

Duplicate of gfx-rs/wgpu#368