Mesh: Move selection flags to generic attributes

Using the attribute name semantics from T97452, this patch moves the
selection status of mesh elements from the `SELECT` of vertices, and
edges, and the `ME_FACE_SEL` of faces to generic boolean attribute
Storing this data as generic attributes can significantly simplify and
improve code, as described in T95965.

The attributes are called `.select_vert`, `.select_edge`, and
`.select_poly`. The `.` prefix means they are "UI attributes",so they
still contain original data edited by users, but they aren't meant to
be accessed procedurally by the user in arbitrary situations. They are
also be hidden in the spreadsheet and the attribute list.

Until 4.0, the attributes are still written to and read from the mesh
in the old way, so neither forward nor backward compatibility are
affected. This means memory requirements will be increased by one byte
per element when selection is used. When the flags are removed
completely, requirements will decrease.

Further notes:
* The `MVert` flag is empty at runtime now, so it can be ignored.
* `BMesh` is unchanged, otherwise the change would be much larger.
* Many tests have slightly different results, since the selection
  attribute uses more generic propagation. Previously you couldn't
  really rely on edit mode selections being propagated procedurally.
  Now it mostly works as expected.

Similar to 2480b55f216c
Ref T95965

Differential Revision: https://developer.blender.org/D15795

1 files changed, 22 insertions, 8 deletions

diff --git a/source/blender/blenkernel/intern/mesh.cc b/source/blender/blenkernel/intern/mesh.cc
index 34cebeaa5d4..6df6cd31cf4 100644
--- a/source/blender/blenkernel/intern/mesh.cc
+++ b/source/blender/blenkernel/intern/mesh.cc
@@ -68,6 +68,7 @@ using blender::BitVector;
 using blender::float3;
 using blender::MutableSpan;
 using blender::Span;
+using blender::StringRef;
 using blender::VArray;
 using blender::Vector;
 
@@ -248,12 +249,19 @@ static void mesh_blend_write(BlendWriter *writer, ID *id, const void *id_address
     Set<std::string> names_to_skip;
     if (!BLO_write_is_undo(writer)) {
       BKE_mesh_legacy_convert_hide_layers_to_flags(mesh);
+      BKE_mesh_legacy_convert_selection_layers_to_flags(mesh);
       BKE_mesh_legacy_convert_material_indices_to_mpoly(mesh);
       BKE_mesh_legacy_bevel_weight_from_layers(mesh);
       BKE_mesh_legacy_face_set_from_generic(mesh);
       BKE_mesh_legacy_edge_crease_from_layers(mesh);
       /* When converting to the old mesh format, don't save redundant attributes. */
-      names_to_skip.add_multiple_new({".hide_vert", ".hide_edge", ".hide_poly", "material_index"});
+      names_to_skip.add_multiple_new({".hide_vert",
+                                      ".hide_edge",
+                                      ".hide_poly",
+                                      "material_index",
+                                      ".select_vert",
+                                      ".select_edge",
+                                      ".select_poly"});
 
       /* Set deprecated mesh data pointers for forward compatibility. */
       mesh->mvert = const_cast<MVert *>(mesh->verts().data());
@@ -689,7 +697,6 @@ static int customdata_compare(
         case CD_PROP_BOOL: {
           const bool *l1_data = (bool *)l1->data;
           const bool *l2_data = (bool *)l2->data;
-
           for (int i = 0; i < total_length; i++) {
             if (l1_data[i] != l2_data[i]) {
               return MESHCMP_ATTRIBUTE_VALUE_MISMATCH;
@@ -1638,39 +1645,46 @@ void BKE_mesh_mselect_clear(Mesh *me)
 
 void BKE_mesh_mselect_validate(Mesh *me)
 {
+  using namespace blender;
+  using namespace blender::bke;
   MSelect *mselect_src, *mselect_dst;
   int i_src, i_dst;
 
   if (me->totselect == 0) {
     return;
   }
-  const Span<MVert> verts = me->verts();
-  const Span<MEdge> edges = me->edges();
-  const Span<MPoly> polys = me->polys();
 
   mselect_src = me->mselect;
   mselect_dst = (MSelect *)MEM_malloc_arrayN(
       (me->totselect), sizeof(MSelect), "Mesh selection history");
 
+  const AttributeAccessor attributes = me->attributes();
+  const VArray<bool> select_vert = attributes.lookup_or_default<bool>(
+      ".select_vert", ATTR_DOMAIN_POINT, false);
+  const VArray<bool> select_edge = attributes.lookup_or_default<bool>(
+      ".select_edge", ATTR_DOMAIN_EDGE, false);
+  const VArray<bool> select_poly = attributes.lookup_or_default<bool>(
+      ".select_poly", ATTR_DOMAIN_FACE, false);
+
   for (i_src = 0, i_dst = 0; i_src < me->totselect; i_src++) {
     int index = mselect_src[i_src].index;
     switch (mselect_src[i_src].type) {
       case ME_VSEL: {
-        if (verts[index].flag & SELECT) {
+        if (select_vert[index]) {
           mselect_dst[i_dst] = mselect_src[i_src];
           i_dst++;
         }
         break;
       }
       case ME_ESEL: {
-        if (edges[index].flag & SELECT) {
+        if (select_edge[index]) {
           mselect_dst[i_dst] = mselect_src[i_src];
           i_dst++;
         }
         break;
       }
       case ME_FSEL: {
-        if (polys[index].flag & SELECT) {
+        if (select_poly[index]) {
           mselect_dst[i_dst] = mselect_src[i_src];
           i_dst++;
         }