Is it really sensible to call this `sm6_signature_element`? AFAIU it is not specific for SM6, it is used for all shader models. Maybe `internal_signature_element` would be a more neutral name?
True. We wouldn't need the "internal_" prefix either though, because the public version is called "vkd3d_shader_signature_element".
Relatedly, why isn't the new field exposed to the public API? Just because we don't want to go through the hassle of updating the public API, or is it genuinely uninteresting for our clients?
Mostly some variant of the former; that doesn't prevent us from exposing this at some point in the future, of course. I think we'd want to wait with exposing this in the public API until broader DXIL support is settled a bit, in case there are additional API changes we'd like to make.
Is there a reason why this couldn't be a `.shader_test`, beside missing HLSL compiler support?
I don't think so, no; see also https://gitlab.winehq.org/wine/vkd3d/-/merge_requests/181#note_31337