detail::GenericFeaturesPNextNode's constructor is error-prone due to inconsistent struct alignment

[n0F4x]

I had an issue where creating a Vulkan feature struct ended up with a rounded alignment (32 bytes), but the same structure created in another context was not aligned (28 bytes).
This made the resulting detail::GenericFeaturesPNextNodes different.

I was trying to use PhysicalDevice::are_extension_features_present with the 28-byte version as a parameter, and the function returned an unexpected result as the stored feature had been aligned to 32 bytes.
Based on the differing "last" 4 bytes I got the result that the requested feature is not supported, whereas this is not true.

Compiled with:
Clang version 17.0.6
Target x86_64-w64-windows-gnu

[n0F4x]

Here are some more tests.

Calling PhysicalDevice::are_extension_features_present normally:

Calling PhysicalDevice::are_extension_features_present with a static variable:

[n0F4x]

The problem is not that the struct doesn't get aligned correctly, but that the value of the extra alignment bits are inconsistent.

[charles-lunarg]

I didn't forget about this - I just had other priorities since this issue was created.
The real solution is to generate all the struct comparisons. I cannot see a way to do generic struct comparisons that ultimately doesn't leave the code liable to padding concerns. sizeof cannot be trusted to yield useful values.

I started working on a branch which does the codegen, but it became a bigger task than I could finish in a day and had to put it to the side for the time being.

The WIP branch with my progress is here:
https://github.com/charles-lunarg/vk-bootstrap/tree/generate_feature_struct_comparisons

(I swear I had typed out a reply but maybe I never hit enter, and it was lost on a machine reset?)