Fix #126872: Cannot assign custom preview to node asset

Caused by 03ae57dd8b
Custom preview operator uses `lib_id_preview_editing_poll`. To enable
this operator in UI, move the new condition to `lib_id_generate_preview_poll`
poll function so adding custom preview to unsupported type would
still be possible.

Co-authored-by: Julian Eisel

Pull Request: https://projects.blender.org/blender/blender/pulls/126874

source/blender/editors/include/ED_render.hh

@@ -75,7 +75,7 @@ void ED_preview_free_dbase();
 /**
  * Check if \a id is supported by the automatic preview render.
  */
-bool ED_preview_id_is_supported(const ID *id);
+bool ED_preview_id_is_supported(const ID *id, const char **r_disabled_hint = nullptr);
 
 void ED_preview_set_visibility(Main *pr_main,
                                Scene *scene,

source/blender/editors/render/render_preview.cc

@@ -27,6 +27,8 @@
 #include "BLI_time.h"
 #include "BLI_utildefines.h"
 
+#include "BLT_translation.hh"
+
 #include "BLO_readfile.hh"
 
 #include "DNA_brush_types.h"
@@ -1892,22 +1894,37 @@ static void icon_preview_free(void *customdata)
   MEM_freeN(ip);
 }
 
-bool ED_preview_id_is_supported(const ID *id)
+bool ED_preview_id_is_supported(const ID *id, const char **r_disabled_hint)
 {
   if (id == nullptr) {
     return false;
   }
-  if (GS(id->name) == ID_NT) {
-    /* Node groups don't support standard preview generation. */
-    return false;
+
+  /* Get both the result and the "potential" disabled hint. After that we can decide if the
+   * disabled hint needs to be returned to the caller. */
+  const auto [result, disabled_hint] = [id]() -> std::pair<bool, const char *> {
+    switch (GS(id->name)) {
+      case ID_NT:
+        return {false, RPT_("Node groups do not support automatic previews")};
+      case ID_OB:
+        return {object_preview_is_type_supported((const Object *)id),
+                RPT_("Object type does not support automatic previews")};
+      case ID_GR:
+        return {
+            collection_preview_contains_geometry_recursive((const Collection *)id),
+            RPT_("Collection does not contain object types that can be rendered for the automatic "
+                 "preview")};
+      default:
+        return {BKE_previewimg_id_get_p(id) != nullptr,
+                RPT_("Data-block type does not support automatic previews")};
+    }
+  }();
+
+  if (result == false && disabled_hint && r_disabled_hint) {
+    *r_disabled_hint = disabled_hint;
   }
-  if (GS(id->name) == ID_OB) {
-    return object_preview_is_type_supported((const Object *)id);
-  }
-  if (GS(id->name) == ID_GR) {
-    return collection_preview_contains_geometry_recursive((const Collection *)id);
-  }
-  return BKE_previewimg_id_get_p(id) != nullptr;
+
+  return result;
 }
 
 void ED_preview_icon_render(

source/blender/editors/util/ed_util_ops.cc

@@ -60,10 +60,6 @@ static bool lib_id_preview_editing_poll(bContext *C)
     CTX_wm_operator_poll_msg_set(C, "Data-block does not support previews");
     return false;
   }
-  if (!ED_preview_id_is_supported(id)) {
-    CTX_wm_operator_poll_msg_set(C, "Object type does not support previews");
-    return false;
-  }
 
   return true;
 }
@@ -147,8 +143,9 @@ static bool lib_id_generate_preview_poll(bContext *C)
 
   const PointerRNA idptr = CTX_data_pointer_get(C, "id");
   const ID *id = (ID *)idptr.data;
-  if (GS(id->name) == ID_NT) {
-    CTX_wm_operator_poll_msg_set(C, "Can't generate automatic preview for node group");
+  const char *disabled_hint = nullptr;
+  if (!ED_preview_id_is_supported(id, &disabled_hint)) {
+    CTX_wm_operator_poll_msg_set(C, disabled_hint);
     return false;
   }
 
@@ -192,7 +189,7 @@ static void ED_OT_lib_id_generate_preview(wmOperatorType *ot)
 
 static bool lib_id_generate_preview_from_object_poll(bContext *C)
 {
-  if (!lib_id_preview_editing_poll(C)) {
+  if (!lib_id_generate_preview_poll(C)) {
     return false;
   }
   if (CTX_data_active_object(C) == nullptr) {