Make #[spirv(block)] decoration force ABI type into Struct

[fu5ha]

In a situation where one wants to use a non-struct primitive type as a push constant or uniform, you need to put it into a struct and apply the block decoration to it. However, if we try to just put it in a wrapper struct the ABI will not actually change. For example this

#[spirv(block)]
#[derive(Clone, Copy)]
#[allow(unused_attributes)]
pub struct Constants {
    direction: Vec2,
    _make_struct: f32,
}

without the _make_struct field, this will still be an AbiVector and so it's illegal to apply the block decoration to it. This also occurs for two f32s in which case it will be a ScalarPair, like here:

#[spirv(block)]
#[derive(Copy, Clone)]
#[allow(unused_attributes)]
pub struct TonemapConstants {
    pub exposure: f32,
    pub aspect: f32,
    _make_struct: f32,
}

It would be nice to not have to add this extra member to the struct.

[khyperia]

Just checking, does adding #[repr(C)] fix the issue?

[fu5ha]

Evidently not, doesn't compile

[fu5ha]

The other case also doesn't work

[khyperia]

Right, the first issue is #373, but I'm really confused by the second issue, that's really bizarre and like #[repr(C)] didn't actually apply (or I don't understand #[repr(C)])

[eddyb]

We can fix this with a query override, and I've been meaning to try it, but would it be better to just not need #[spirv(block)], and wrap every uniform automatically in an OpTypeStruct with a Block decoration?

I can't find anywhere in SPIR-V or Vulkan specs why extra layers of single-field structs would change anything - there's some stuff about Block-decorated OpTypeStruct having Offset decorations on their members, but I don't really see that resulting in different values than a single struct-typed field with an Offset of 0.

[hrydgard]

I think auto-wrapping any structs used as various kinds of buffers would be great.

Yeah, Vulkan shouldn't treat a 4 byte struct any different from a 128 byte one or whatever.