workgroup_size attribute does not support variable expression

[haoyunfeix]

Failed shader:

 var a : i32 = 256u;
 @compute @workgroup_size(a, 1u, 1u)
        fn main(@builtin(local_invocation_id) LocalId : vec3<u32>,
                  @builtin(global_invocation_id) GlobalId : vec3<u32>,
                  @builtin(num_workgroups) NumWorkgroups : vec3<u32>) {
          var localId = LocalId;
          var globalId = GlobalId;
          var numWorkgroups = NumWorkgroups;
          }

Output:

naga .\6842.wgsl
error: expected 32-bit signed integer literal, found 'a'
  ┌─ .\6842.wgsl:2:27
  │
2 │  @compute @workgroup_size(a, 1u, 1u)
  │                           ^ expected 32-bit signed integer literal

Could not parse WGSL

This shader should be acceptable due to spec https://gpuweb.github.io/gpuweb/wgsl/#attributes
Each parameter must be a const-expression or an override-expression.

Related to tfjs issue: tensorflow/tfjs#6842 (comment)

[SparkyPotato]

Note that a must be declared const a = 256; in accordance with the spec.
(Naga does not support this yet)

[teoxoy]

This will be fixed as part of #1829