Do we need any kind of constant propagation in WGSL?
This is one of the examples from Metal Shading Language document:

constexpr constant uint MAX_LIGHTS=4;
struct LightDesc{
  uint num_lights;
  float4 light_position[MAX_LIGHTS];
  float4 light_color[MAX_LIGHTS];
  float4 light_attenuation_factors[MAX_LIGHTS];
};

It relies on the MAX_LIGHTS constant to be propagated to define the array sizes.

Related to #572 : specialization constants aren't going to be "real" constants since in MSL they can't be used for array sizes, if I understand correctly.