Cleanup: Remove unused custom data type

`CD_HAIRLENGTH` is not really a custom data type, it was just used to
specify the hair particle "length" attribute to pass it to EEVEE and
material preview. Because of the "typemap" array in `CustomData`,
it's better not to have this unnecessary type. Instead, use the same
mechanism used to request the active color attribute.

Pull Request: https://projects.blender.org/blender/blender/pulls/109449

source/blender/blenkernel/intern/customdata.cc

@@ -1946,7 +1946,7 @@ static const LayerTypeInfo LAYERTYPEINFO[CD_NUMTYPES] = {
      nullptr,
      nullptr,
      nullptr},
-    /* 51: CD_HAIRLENGTH */
+    /* 51: CD_HAIRLENGTH */ /* DEPRECATED */ /* UNUSED */
     {sizeof(float), "float", 1, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr},
     /* 52: CD_PROP_QUATERNION */
     {sizeof(float[4]),

source/blender/gpu/GPU_material.h

@@ -155,6 +155,10 @@ GPUNodeLink *GPU_attribute(GPUMaterial *mat, eCustomDataType type, const char *n
  * The name, type, and domain are unknown and do not depend on the material.
  */
 GPUNodeLink *GPU_attribute_default_color(GPUMaterial *mat);
+/**
+ * Add a GPU attribute that refers to the approximate length of curves/hairs.
+ */
+GPUNodeLink *GPU_attribute_hair_length(GPUMaterial *mat);
 GPUNodeLink *GPU_attribute_with_default(GPUMaterial *mat,
                                         eCustomDataType type,
                                         const char *name,
@@ -344,6 +348,10 @@ typedef struct GPUMaterialAttribute {
    * valid here.
    */
   bool is_default_color;
+  /**
+   * If true, the attribute is the length of hair particles and curves.
+   */
+  bool is_hair_length;
 } GPUMaterialAttribute;
 
 typedef struct GPUMaterialTexture {

source/blender/gpu/intern/gpu_codegen.cc

@@ -364,25 +364,27 @@ void GPUCodegen::generate_attribs()
     eGPUType input_type, iface_type;
 
     load_ss << "var_attrs." << var_name;
-    switch (attr->type) {
-      case CD_ORCO:
-        /* Need vec4 to detect usage of default attribute. */
-        input_type = GPU_VEC4;
-        iface_type = GPU_VEC3;
-        load_ss << " = attr_load_orco(" << attr_name << ");\n";
-        break;
-      case CD_HAIRLENGTH:
-        iface_type = input_type = GPU_FLOAT;
-        load_ss << " = attr_load_" << input_type << "(" << attr_name << ");\n";
-        break;
-      case CD_TANGENT:
-        iface_type = input_type = GPU_VEC4;
-        load_ss << " = attr_load_tangent(" << attr_name << ");\n";
-        break;
-      default:
-        iface_type = input_type = GPU_VEC4;
-        load_ss << " = attr_load_" << input_type << "(" << attr_name << ");\n";
-        break;
+    if (attr->is_hair_length) {
+      iface_type = input_type = GPU_FLOAT;
+      load_ss << " = attr_load_" << input_type << "(" << attr_name << ");\n";
+    }
+    else {
+      switch (attr->type) {
+        case CD_ORCO:
+          /* Need vec4 to detect usage of default attribute. */
+          input_type = GPU_VEC4;
+          iface_type = GPU_VEC3;
+          load_ss << " = attr_load_orco(" << attr_name << ");\n";
+          break;
+        case CD_TANGENT:
+          iface_type = input_type = GPU_VEC4;
+          load_ss << " = attr_load_tangent(" << attr_name << ");\n";
+          break;
+        default:
+          iface_type = input_type = GPU_VEC4;
+          load_ss << " = attr_load_" << input_type << "(" << attr_name << ");\n";
+          break;
+      }
     }
 
     info.vertex_in(slot--, to_type(input_type), attr_name);

source/blender/gpu/intern/gpu_node_graph.cc

@@ -347,13 +347,14 @@ static char attr_prefix_get(GPUMaterialAttribute *attr)
   if (attr->is_default_color) {
     return 'c';
   }
+  if (attr->is_hair_length) {
+    return 'l';
+  }
   switch (attr->type) {
     case CD_TANGENT:
       return 't';
     case CD_AUTO_FROM_NAME:
       return 'a';
-    case CD_HAIRLENGTH:
-      return 'l';
     default:
       BLI_assert_msg(0, "GPUVertAttr Prefix type not found : This should not happen!");
       return '\0';
@@ -555,6 +556,21 @@ GPUNodeLink *GPU_attribute_default_color(GPUMaterial *mat)
   return link;
 }
 
+GPUNodeLink *GPU_attribute_hair_length(GPUMaterial *mat)
+{
+  GPUNodeGraph *graph = gpu_material_node_graph(mat);
+  GPUMaterialAttribute *attr = gpu_node_graph_add_attribute(graph, CD_AUTO_FROM_NAME, "", true);
+  if (attr == nullptr) {
+    static const float zero_data[GPU_MAX_CONSTANT_DATA] = {0.0f};
+    return GPU_constant(zero_data);
+  }
+  attr->is_hair_length = true;
+  GPUNodeLink *link = gpu_node_link_create();
+  link->link_type = GPU_NODE_LINK_ATTR;
+  link->attr = attr;
+  return link;
+}
+
 GPUNodeLink *GPU_attribute_with_default(GPUMaterial *mat,
                                         const eCustomDataType type,
                                         const char *name,

source/blender/makesdna/DNA_customdata_types.h

@@ -178,7 +178,8 @@ typedef enum eCustomDataType {
   CD_PROP_FLOAT2 = 49,
   CD_PROP_BOOL = 50,
 
-  CD_HAIRLENGTH = 51,
+  /* CD_HAIRLENGTH = 51, */ /* UNUSED */
+
   CD_PROP_QUATERNION = 52,
 
   CD_NUMTYPES = 53,
@@ -224,8 +225,6 @@ typedef enum eCustomDataType {
 #define CD_MASK_PROP_INT32_2D (1ULL << CD_PROP_INT32_2D)
 #define CD_MASK_PROP_QUATERNION (1ULL << CD_PROP_QUATERNION)
 
-#define CD_MASK_HAIRLENGTH (1ULL << CD_HAIRLENGTH)
-
 /** Multi-resolution loop data. */
 #define CD_MASK_MULTIRES_GRIDS (CD_MASK_MDISPS | CD_GRID_PAINT_MASK)
 

source/blender/nodes/shader/nodes/node_shader_hair_info.cc

@@ -24,7 +24,7 @@ static int node_shader_gpu_hair_info(GPUMaterial *mat,
 {
   /* Length: don't request length if not needed. */
   static const float zero = 0;
-  GPUNodeLink *length_link = out[2].hasoutput ? GPU_attribute(mat, CD_HAIRLENGTH, "") :
+  GPUNodeLink *length_link = out[2].hasoutput ? GPU_attribute_hair_length(mat) :
                                                 GPU_constant(&zero);
   return GPU_stack_link(mat, node, "node_hair_info", in, out, length_link);
 }