[naga] Permit only structs as binding array elements.

[jimblandy]

Require T to be a struct in binding_array<T, ...>; do not permit arrays.

In #5428, the validator was changed to accept binding array types that the SPIR-V backend couldn't properly emit. Specifically, the validator was changed to accept binding_array<array<T>>, but the SPIR-V backend wasn't changed to wrap the binding array elements in a SPIR-V struct type, as Vulkan requires. So the type would be accepted by the validator, and then rejected by the backend.

[ErichDonGubler]

CC @teoxoy as reviewer and @kvark as author of the regressor being fixed here.

[ErichDonGubler]

LGTM!

question: Are there plans in the future to relax this restriction, since the OP implies it's possible?

[JMS55]

Wait, so can you no longer do binding_array<array<T>>? That's very unfortunate. I was using that with ray tracing (soon to be merged...) to bind multiple vertex data buffers easily. I can switch to a single uberbuffer of all mesh data, but it's a lot more annoying to manage.

[jimblandy]

@JMS55 wrote:

Wait, so can you no longer do binding_array<array<T>>? That's very unfortunate. I was using that with ray tracing (soon to be merged...) to bind multiple vertex data buffers easily. I can switch to a single uberbuffer of all mesh data, but it's a lot more annoying to manage.

I'm confused. That has never worked:

$ cat in.wgsl
@group(0) @binding(0)
var<storage> a: binding_array<array<u32>>;

fn f(i: i32, j: i32) -> u32 {
   return a[i][j];
}
$ naga in.wgsl out.spv
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.11s
     Running `/home/jimb/rust/wgpu/target/debug/naga in.wgsl out.spv`
$ spirv-val out.spv
error: line 16: Block decoration on target <id> '4[%_runtimearr_uint]' must be a structure type
  OpDecorate %_runtimearr_uint Block

$ 

As the commit message and PR message explains, Naga fails to wrap the array<u32> in a struct, so this doesn't validate.

And this restriction is easy to work around: just put the runtime-sized array in a struct in the WGSL:

$ cat fixed.wgsl
struct S {
  arr: array<u32>,
}

@group(0) @binding(0)
var<storage> a: binding_array<S>;

fn f(i: i32, j: i32) -> u32 {
   return a[i].arr[j];
}
$ naga fixed.wgsl fixed.spv
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.11s
     Running `/home/jimb/rust/wgpu/target/debug/naga fixed.wgsl fixed.spv`
$ spirv-val fixed.spv
$ 

[JMS55]

Oh I see, yeah wrapping it in a struct is what I was doing. Didn't realize that still worked, thanks!