The d3d12 backend silently falls back to FXC if loading DXC fails

[nical]

I think that this behavior is desirable for most people (prioritizing having something running in as many situations as possible), but it's likely not the behavior that Firefox needs (it should be a hard error), so we may want to add some way to optionally specify that DXC is required and to fail if it isn't available.

[nical]

Relevant part of the code:

wgpu/wgpu-hal/src/dx12/instance.rs

Line 93 in 0b82776

match dxc_container {

[Elabajaba]

I never liked the automatic fallback but it was requested when I implemented support for DXC over returning an error.

IMO automatically falling back to FXC doesn't really make sense for a lot of cases where you explicitly request DXC as you end up with miscompilations if you use shader model 6.0+ features, or if you just have code that FXC doesn't like.

[teoxoy]

Something else we need to do is avoid creating an adapter if it doesn't support SM6. This will fail later when the driver receives DXIL and can't deal with it.

wgpu/wgpu-hal/src/dx12/adapter.rs

Lines 229 to 233 in 48cc33a

	} else {
	break Direct3D12::D3D_SHADER_MODEL_5_1;
	}
	} {
	Direct3D12::D3D_SHADER_MODEL_5_1 => naga::back::hlsl::ShaderModel::V5_1,

[cwfitzgerald]

Such an adapter shouldn't exist, DXIL support was required in launch of dx12 iirc

[teoxoy]

DXIL & SM6 are part of WDDM 2.1, D3D12 was initially released with WDDM 2.