How to access materials within a specialized mesh pipeline

[HydrogenC]

Hi everyone. I created a specialized mesh pipeline, but different from the official docs, I still want to leverage the material system to calculate the fragment color.

Here's my current version, and my question in the comments:

impl SpecializedMeshPipeline for CustomPipeline {
    type Key = MeshPipelineKey;

    fn specialize(
        &self,
        key: Self::Key,
        layout: &MeshVertexBufferLayoutRef,
    ) -> Result<RenderPipelineDescriptor, SpecializedMeshPipelineError> {
        let mut descriptor = self.mesh_pipeline.specialize(key, layout)?;

        // ...
        
        // What shall I write here to get and set the material bind group layout?
        descriptor
            .layout
            .insert(3, ???);
        
        Ok(descriptor)
    }
}

// ...

// Set the material bind group here
type DrawCustom = (
    SetItemPipeline,
    SetMeshViewBindGroup<0>,
    SetMeshViewBindingArrayBindGroup<1>,
    SetMeshBindGroup<2>,
    SetMaterialBindGroup<3>,
    DrawMeshInstanced,
);