MAX_DIRECTIONAL_LIGHTS must be defined to have the Lights struct

[mockersf]

Objective

• Fixes Shaderdef values break shader imports #6996, fixes Importing bevy_pbr::mesh_view_types imples defining MAX_DIRECTIONAL_LIGHTS #6799
• Do not fail shader compilation when MAX_DIRECTIONAL_LIGHTS is not defined

Solution

• Lights struct only have the directional_lights field when MAX_DIRECTIONAL_LIGHTS is defined

[IceSentry]

When running the post_processing example with this PR I get.

2022-12-21T17:16:06.229004Z ERROR bevy_render::render_resource::pipeline_cache: failed to process shader:
error: unknown type: 'Lights'
   ┌─ wgsl:68:22
   │
68 │ var<uniform> lights: Lights;
   │                      ^^^^^^ unknown type

So this needs to be added anywhere that Lights is used for this to work correctly.

[mockersf]

oh right that example was already crashing... changed to not have the directional_lights field then, so the struct is still present.
the Lights struct is used as a binding, not sure what to replace it with when MAX_DIRECTIONAL_LIGHTS is not available

[aevyrie]

This also fixes #6799, turns out my issue was a duplicate.

[IceSentry]

not sure what to replace it with when MAX_DIRECTIONAL_LIGHTS is not available

Maybe just hardcode it to 0?

Just tested the latest commit and it works correctly for the post_processing example

[Elabajaba]

This could probably be updated now that cascaded shadow maps has landed, since it's the same error/fix for that. (assuming we won't get #5703 in before next release)

Getting failures on both shader_prepass and post_processing now.

ERROR bevy_render::render_resource::pipeline_cache: failed to process shader:
error: expected expression, found '#'
   ┌─ wgsl:38:41
   │
38 │     cascades: array<DirectionalCascade, #{MAX_CASCADES_PER_LIGHT}>,
   │                                         ^ expected expression

                                  ^ expected expression

[superdump]

Does this work? It’s not obvious to me that it would but if it does, ok I guess. :) As was mentioned, the cascade also need fixing, could you do that too, please?

[superdump]

Suggested change

	#ifdef MAX_DIRECTIONAL_LIGHTS
	#ifdef MAX_DIRECTIONAL_LIGHTS

[superdump]

Suggested change

	#endif
	#endif

[Elabajaba]

Made a PR to fix the issue with MAX_CASCADES_PER_LIGHT being undefined sometimes so it doesn't need to be added to this one. #7380

[superdump]

I feel like this approach will only work if the binding isn't used. Because we comment out a member of the struct, we change its data layout. So if anything is bound, it will be misinterpreted. Same for #7380. I'm not sure how to solve it at this moment. I feel like the shader failing to compile is an indication that the use of the import should be fixed.

[superdump]

I implemented what I think is a better fix in #7419