Explicit 'count' field in AccelerationStructureBuildGeometryInfoKHR

[sheaf]

I'm wondering why AccelerationStructureBuildGeometryInfoKHR has an explicit geometryCount field, as this is just supposed to be the size of the geometries vector. In other places in this library, this field is usually elided.
I assume that this ended up being a special case, because the original Vulkan function can work with either a pointer to an array of geometries or a pointer to an array of pointers to geometries?

• Only one of pGeometries or ppGeometries can be a valid pointer, the other must be NULL
• If geometryCount is not 0, and pGeometries is not NULL, pGeometries must be a valid pointer to an array of geometryCount valid AccelerationStructureGeometryKHR structures
• If geometryCount is not 0, and ppGeometries is not NULL, ppGeometries must be a valid pointer to an array of geometryCount valid pointers to valid AccelerationStructureGeometryKHR structures

[expipiplus1]

This is a deficiency in the bindings.

The cause is:

Lengths are elided if they can be uniquely determined by the lengths of Vectors being passed. In this case both arrays which are sized by this length are optional:

https://github.com/KhronosGroup/Vulkan-Docs/blob/8f718b4194ed1e0a572d37072e5558dd9ceabcb0/xml/vk.xml#L4772-L4774

This means that (according to just the XML spec) one could pass a length and NULL for both vectors. The XML spec lacks the language to say that only one of these arrays can be NULL.

Usually there's a 1-1 mapping between arrays and Vectors, however in this specific case we ditch ppGeometries and make pGeometries mandatory.

The easiest solution would be to assign it ElidedLength ["pGeometries"] [] (or something like that) here:

vulkan/generate-new/src/Bespoke.hs

Lines 612 to 618 in 8ba913e

	accelerationStructureGeometry :: BespokeScheme
	accelerationStructureGeometry = BespokeScheme $ \case
	"VkAccelerationStructureBuildGeometryInfoKHR" -> \case
	(p :: a) | "ppGeometries" <- name p, Ptr Const (Ptr Const _) <- type' p ->
	Just $ ElidedUnivalued "nullPtr"
	_ -> Nothing
	_ -> const Nothing

. I can take a look at this next week

For now it's safe to set it to zero and it'll be inferred:

vulkan/src/Vulkan/Extensions/VK_KHR_acceleration_structure.hs

Lines 5386 to 5387 in 8ba913e

	geometryCount'' <- lift $ if (geometryCount) == 0
	then pure $ fromIntegral pGeometriesLength

[sheaf]

This obviously isn't causing me any trouble at all so you don't need to worry about it.

[expipiplus1]

Fixed with #251