ShaderLab: #define macro not expanded in swizzle position

[GuoLei1990]

Description

ShaderLab's compiler does not expand #define macros when used in swizzle position (after .). This is valid in standard GLSL but fails in ShaderLab.

Reproduction

#define OCCLUSION_CHANNEL r
#define ROUGHNESS_CHANNEL g
#define METALLIC_CHANNEL b

vec4 res = texture2D(pbrMap, v.v_uv);
pbr.x *= res.OCCLUSION_CHANNEL;   // ❌ CompilationError: Unexpected token OCCLUSION_CHANNEL
pbr.y *= res.ROUGHNESS_CHANNEL;
pbr.z *= res.METALLIC_CHANNEL;

Expected behavior

The preprocessor should expand OCCLUSION_CHANNEL to r before the compiler parses the swizzle, resulting in res.r — which is valid GLSL.

Actual behavior

CompilationError: Unexpected token OCCLUSION_CHANNEL

The compiler encounters OCCLUSION_CHANNEL as an unknown identifier in swizzle position because macro expansion did not occur before swizzle parsing.

Workaround

Replace macro-based swizzle with literal channel names:

pbr.x *= res.r;   // occlusion
pbr.y *= res.g;   // roughness
pbr.z *= res.b;   // metallic

Context

Discovered during Cocos Creator → Galacean ShaderLab migration. Cocos shaders commonly use #define to abstract channel selection (e.g., #pragma define-meta ALPHA_TEST_CHANNEL options([a, r, g, b])), which works in standard GLSL but breaks in ShaderLab.