LibOverrides: Refactor how diffing of RNA collections is handled.

Original implementation was a quick prototype which should have never
landed as-is in master. It had very limiting constraints and did not
allow for any real further development.

This commit fixes the internal implementation to make more sensible,
maintainable and evolutive.

NOTE: This commit introduces another forward-incompatibility in the
Blender file format: Files saved after this commit won't open properly
in older versions of blender regarding local inserted constraints or
modifiers into overrides of linked data.

NOTE: Technical details: The 'anchor' item name/index is now stored in
`subitem_reference_` members, and the actual 'source' item name/index is
stored in `subitem_local_` members of the override property operation
data.
Previously, only the `subitem_local_` members were used, storing the
anchor item name/index, and assuming the 'source' item was always the
next in the list.

Milestone I of T82160.

Maniphest Tasks: T82160

Differential Revision: https://developer.blender.org/D13282

1 files changed, 8 insertions, 4 deletions

diff --git a/source/blender/makesdna/DNA_ID.h b/source/blender/makesdna/DNA_ID.h
index d829d707a71..2c04d0b06ef 100644
--- a/source/blender/makesdna/DNA_ID.h
+++ b/source/blender/makesdna/DNA_ID.h
@@ -211,11 +211,15 @@ typedef struct IDOverrideLibraryPropertyOperation {
   char _pad0[2];
 
   /* Sub-item references, if needed (for arrays or collections only).
-   * We need both reference and local values to allow e.g. insertion into collections
+   * We need both reference and local values to allow e.g. insertion into RNA collections
    * (constraints, modifiers...).
-   * In collection case, if names are defined, they are used in priority.
-   * Names are pointers (instead of char[64]) to save some space, NULL when unset.
-   * Indices are -1 when unset. */
+   * In RNA collection case, if names are defined, they are used in priority.
+   * Names are pointers (instead of char[64]) to save some space, NULL or empty string when unset.
+   * Indices are -1 when unset.
+   *
+   * NOTE: For insertion operations in RNA collections, reference may not actually exist in the
+   * linked reference data. It is used to identify the anchor of the insertion operation (i.e. the
+   * item after or before which the new local item should be inserted), in the local override. */
   char *subitem_reference_name;
   char *subitem_local_name;
   int subitem_reference_index;