Added comment/explanation

Author: inner-daemons

wgpu-hal/src/gles/device.rs

@@ -507,7 +507,9 @@ impl super::Device {
             for item in stage_items {
                 let source = &shaders[stage_idx].1.module.source;
                 let super::ShaderModuleSource::Naga(naga_module) = source else {
-                    todo!();
+                    // ImmediateItem can only be constructed given a naga module, as it requires a type handle.
+                    // Passthrough shaders will have immediates_items empty
+                    unreachable!();
                 };
                 let type_inner = &naga_module.module.types[item.ty].inner;