Note: I do use scalar-block-layouts if that matters - both enabled in Vulkan1.2, and I pass the command line argument for spirv-builder

We don't care about scalar block layout or std430, we're just using whatever layout rustc gives us to ensure it's the same if you're using the same struct on shader and CPU side (at least since #380). If someone needs higher alignment (for silly wgpu uniform buffers or so), they should just #[repr(align(16))] the struct.

I'm not actually too sure how scalar block layout and spirv-val interact precisely. Even though it's not enabled by default, we've never experienced any issues wrt layout with it being disabled. Even though SpirvBuilder.validator.scalar_bock_layout exists.

Note that you are technically required to #[repr(C)] any struct shared between CPU and GPU, since rustc makes no guarantees about the rust layout. In theory, even two different compile invocations could result in different layouts, though in practice we haven't seen the layout algorithm change, like ever.

Even more minified code:

#[repr(C)]
#[derive(Copy, Clone, Default)]
pub struct ManyFloats {
    a: f32,
    b: f32,
    c: f32,
}

#[spirv(vertex)]
pub fn main(out1: &mut ManyFloats, #[spirv(location = 3)] out2: &mut f32) {
    const {
        assert!(size_of::<ManyFloats>() == 12);
    }
    *out1 = Default::default();
    *out2 = Default::default();
}

This fails as well, as according to spec array elements can't share slots, each array element has to get their own slot.

#[spirv(vertex)]
pub fn main(out1: &mut [f32; 3], out2: &mut f32) {
    *out1 = Default::default();
    *out2 = Default::default();
}

I've reread the Vulkan spec on location assignment and noticed that locations are not assigned for every 16 bytes, but there's a totally custom algorithm on how locations are assigned. Implemented the layout algorithm in SpirvType::location_size(), so our locations are hopefully correct now.

Success! 🥳

I'm not actually too sure how scalar block layout and spirv-val interact precisely. Even though it's not enabled by default, we've never experienced any issues wrt layout with it being disabled. Even though SpirvBuilder.validator.scalar_bock_layout exists.

Fun fact: I actually had it enabled only on the Vulkan12 feature side, and the spirv compiler threw an error and I had to enable it.

I'll let @eddyb review this one as I am not qualified.