UI/Nodes: Improve feedback when adding node fails (e.g. on drag & drop)

This is especially useful when trying to add a node group instance, e.g. via
drag & drop from the Outliner or Asset Browser.
Previously this would just silently fail, with no information why. This is a
source of confusion, e.g. earlier, it took me a moment to realize I was
dragging a node group into itself, which failed of course.
Blender should always try to help the user with useful error messages.

Adds error messages like: "Nesting a node group inside of itself is not
allowed", "Not a compositor node tree", etc.

Adds a disabled hint return argument to node and node tree polling functions.
On error the hint is reported, or could even be shown in advance (e.g. if
checked via an operator poll option).

Differential Revision: https://developer.blender.org/D10422

Reviewed by: Jacques Lucke

4 files changed, 40 insertions, 17 deletions

diff --git a/source/blender/nodes/composite/node_composite_util.c b/source/blender/nodes/composite/node_composite_util.c
index b6cbffea413..6cc17d8c272 100644
--- a/source/blender/nodes/composite/node_composite_util.c
+++ b/source/blender/nodes/composite/node_composite_util.c
@@ -23,9 +23,15 @@
 
 #include "node_composite_util.h"
 
-bool cmp_node_poll_default(bNodeType *UNUSED(ntype), bNodeTree *ntree)
+bool cmp_node_poll_default(bNodeType *UNUSED(ntype),
+                           bNodeTree *ntree,
+                           const char **r_disabled_hint)
 {
-  return STREQ(ntree->idname, "CompositorNodeTree");
+  if (!STREQ(ntree->idname, "CompositorNodeTree")) {
+    *r_disabled_hint = "Not a compositor node tree";
+    return false;
+  }
+  return true;
 }
 
 void cmp_node_update_default(bNodeTree *UNUSED(ntree), bNode *node)
diff --git a/source/blender/nodes/composite/node_composite_util.h b/source/blender/nodes/composite/node_composite_util.h
index 800c55df4d6..4fcccbb79f0 100644
--- a/source/blender/nodes/composite/node_composite_util.h
+++ b/source/blender/nodes/composite/node_composite_util.h
@@ -54,7 +54,9 @@ extern "C" {
 
 #define CMP_SCALE_MAX 12000
 
-bool cmp_node_poll_default(struct bNodeType *ntype, struct bNodeTree *ntree);
+bool cmp_node_poll_default(struct bNodeType *ntype,
+                           struct bNodeTree *ntree,
+                           const char **r_disabled_info);
 void cmp_node_update_default(struct bNodeTree *ntree, struct bNode *node);
 void cmp_node_type_base(
     struct bNodeType *ntype, int type, const char *name, short nclass, short flag);
diff --git a/source/blender/nodes/composite/nodes/node_composite_cryptomatte.cc b/source/blender/nodes/composite/nodes/node_composite_cryptomatte.cc
index c5135482ec2..d9b36924516 100644
--- a/source/blender/nodes/composite/nodes/node_composite_cryptomatte.cc
+++ b/source/blender/nodes/composite/nodes/node_composite_cryptomatte.cc
@@ -263,7 +263,9 @@ static void node_copy_cryptomatte(bNodeTree *UNUSED(dest_ntree),
   dest_node->storage = dest_nc;
 }
 
-static bool node_poll_cryptomatte(bNodeType *UNUSED(ntype), bNodeTree *ntree)
+static bool node_poll_cryptomatte(bNodeType *UNUSED(ntype),
+                                  bNodeTree *ntree,
+                                  const char **r_disabled_hint)
 {
   if (STREQ(ntree->idname, "CompositorNodeTree")) {
     Scene *scene;
@@ -276,8 +278,13 @@ static bool node_poll_cryptomatte(bNodeType *UNUSED(ntype), bNodeTree *ntree)
       }
     }
 
+    if (scene == nullptr) {
+      *r_disabled_hint =
+          "The node tree must be the compositing node tree of any scene in the file";
+    }
     return scene != nullptr;
   }
+  *r_disabled_hint = "Not a compositor node tree";
   return false;
 }
 
diff --git a/source/blender/nodes/composite/nodes/node_composite_image.c b/source/blender/nodes/composite/nodes/node_composite_image.c
index 53ea02ff8a7..243300b0a44 100644
--- a/source/blender/nodes/composite/nodes/node_composite_image.c
+++ b/source/blender/nodes/composite/nodes/node_composite_image.c
@@ -526,24 +526,32 @@ static void node_composit_init_rlayers(const bContext *C, PointerRNA *ptr)
   }
 }
 
-static bool node_composit_poll_rlayers(bNodeType *UNUSED(ntype), bNodeTree *ntree)
+static bool node_composit_poll_rlayers(bNodeType *UNUSED(ntype),
+                                       bNodeTree *ntree,
+                                       const char **r_disabled_hint)
 {
-  if (STREQ(ntree->idname, "CompositorNodeTree")) {
-    Scene *scene;
+  if (!STREQ(ntree->idname, "CompositorNodeTree")) {
+    *r_disabled_hint = "Not a compositor node tree";
+    return false;
+  }
 
-    /* XXX ugly: check if ntree is a local scene node tree.
-     * Render layers node can only be used in local scene->nodetree,
-     * since it directly links to the scene.
-     */
-    for (scene = G.main->scenes.first; scene; scene = scene->id.next) {
-      if (scene->nodetree == ntree) {
-        break;
-      }
+  Scene *scene;
+
+  /* XXX ugly: check if ntree is a local scene node tree.
+   * Render layers node can only be used in local scene->nodetree,
+   * since it directly links to the scene.
+   */
+  for (scene = G.main->scenes.first; scene; scene = scene->id.next) {
+    if (scene->nodetree == ntree) {
+      break;
     }
+  }
 
-    return (scene != NULL);
+  if (scene == NULL) {
+    *r_disabled_hint = "The node tree must be the compositing node tree of any scene in the file";
+    return false;
   }
-  return false;
+  return true;
 }
 
 static void node_composit_free_rlayers(bNode *node)