fix [#35984] no way to know if a datablock is in editmode

the report explains the issue in detail, but basically you couldn't know if a mesh was in editmode without checking all the objects that use it.
add `is_editmode` readonly property for all datatypes which support editmode.

also make rna fail to build on implicit function declarations.

source/blender/makesrna/intern/CMakeLists.txt

@@ -26,6 +26,11 @@
 # Generated code has some unused vars we can ignore.
 remove_strict_flags()
 
+if(CMAKE_COMPILER_IS_GNUCC)
+	# add here so we fail early.
+	set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Werror=implicit-function-declaration")
+endif()
+
 # message(STATUS "Configuring makesrna")
 
 # files rna_access.c rna_define.c makesrna.c intentionally excluded.

source/blender/makesrna/intern/rna_armature.c

@@ -472,6 +472,12 @@ static void rna_Armature_bones_next(CollectionPropertyIterator *iter)
 	iter->valid = (internal->link != NULL);
 }
 
+static int rna_Armature_is_editmode_get(PointerRNA *ptr)
+{
+	bArmature *arm = (bArmature *)ptr->id.data;
+	return (arm->edbo != NULL);
+}
+
 #else
 
 static int rna_matrix_dimsize_4x4[] = {4, 4};
@@ -1071,6 +1077,12 @@ static void rna_def_armature(BlenderRNA *brna)
 	RNA_def_property_update(prop, 0, "rna_Armature_redraw_data");
 	RNA_def_property_flag(prop, PROP_LIB_EXCEPTION);
 /* XXX deprecated ....... old animviz for armatures only */
+
+
+	prop = RNA_def_property(srna, "is_editmode", PROP_BOOLEAN, PROP_NONE);
+	RNA_def_property_boolean_funcs(prop, "rna_Armature_is_editmode_get", NULL);
+	RNA_def_property_clear_flag(prop, PROP_EDITABLE);
+	RNA_def_property_ui_text(prop, "Is Editmode", "True when used in editmode");
 }
 
 void RNA_def_armature(BlenderRNA *brna)

source/blender/makesrna/intern/rna_curve.c

@@ -683,6 +683,18 @@ static void rna_Curve_splines_begin(CollectionPropertyIterator *iter, PointerRNA
 	rna_iterator_listbase_begin(iter, BKE_curve_nurbs_get(cu), NULL);
 }
 
+static int rna_Curve_is_editmode_get(PointerRNA *ptr)
+{
+	Curve *cu = (Curve *)ptr->id.data;
+	const short type = BKE_curve_type_get(cu);
+	if (type == OB_FONT) {
+		return (cu->editfont != NULL);
+	}
+	else {
+		return (cu->editnurb != NULL);
+	}
+}
+
 #else
 
 static void rna_def_bpoint(BlenderRNA *brna)
@@ -1485,6 +1497,11 @@ static void rna_def_curve(BlenderRNA *brna)
 	RNA_def_property_range(prop, 0, 1.0);
 	RNA_def_property_ui_text(prop, "End Bevel Factor", "Factor that defines to where beveling of spline happens (0=to the very beginning, 1=to the very end)");
 	RNA_def_property_update(prop, 0, "rna_Curve_update_data");
+
+	prop = RNA_def_property(srna, "is_editmode", PROP_BOOLEAN, PROP_NONE);
+	RNA_def_property_boolean_funcs(prop, "rna_Curve_is_editmode_get", NULL);
+	RNA_def_property_clear_flag(prop, PROP_EDITABLE);
+	RNA_def_property_ui_text(prop, "Is Editmode", "True when used in editmode");
 }
 
 static void rna_def_curve_nurb(BlenderRNA *brna)

source/blender/makesrna/intern/rna_lattice.c

@@ -234,6 +234,11 @@ static char *rna_LatticePoint_path(PointerRNA *ptr)
 	return BLI_strdup("");
 }
 
+static int rna_Lattice_is_editmode_get(PointerRNA *ptr)
+{
+	Lattice *lt = (Lattice *)ptr->id.data;
+	return (lt->editlatt != NULL);
+}
 
 #else
 
@@ -350,6 +355,11 @@ static void rna_def_lattice(BlenderRNA *brna)
 	                                  "rna_iterator_array_end", "rna_iterator_array_get", NULL, NULL, NULL, NULL);
 	RNA_def_property_ui_text(prop, "Points", "Points of the lattice");
 
+	prop = RNA_def_property(srna, "is_editmode", PROP_BOOLEAN, PROP_NONE);
+	RNA_def_property_boolean_funcs(prop, "rna_Lattice_is_editmode_get", NULL);
+	RNA_def_property_clear_flag(prop, PROP_EDITABLE);
+	RNA_def_property_ui_text(prop, "Is Editmode", "True when used in editmode");
+
 	/* pointers */
 	rna_def_animdata_common(srna);
 }

source/blender/makesrna/intern/rna_mesh.c

@@ -1556,6 +1556,13 @@ static PointerRNA rna_Mesh_tessface_uv_texture_new(struct Mesh *me, ReportList *
 	return ptr;
 }
 
+
+static int rna_Mesh_is_editmode_get(PointerRNA *ptr)
+{
+	Mesh *me = rna_mesh(ptr);
+	return (me->edit_btmesh != NULL);
+}
+
 /* only to quiet warnings */
 static void UNUSED_FUNCTION(rna_mesh_unused)(void)
 {
@@ -3211,6 +3218,11 @@ static void rna_def_mesh(BlenderRNA *brna)
 	RNA_def_property_ui_text(prop, "Selected Face Total", "Selected face count in editmode");
 	RNA_def_property_clear_flag(prop, PROP_EDITABLE);
 
+	prop = RNA_def_property(srna, "is_editmode", PROP_BOOLEAN, PROP_NONE);
+	RNA_def_property_boolean_funcs(prop, "rna_Mesh_is_editmode_get", NULL);
+	RNA_def_property_clear_flag(prop, PROP_EDITABLE);
+	RNA_def_property_ui_text(prop, "Is Editmode", "True when used in editmode");
+
 	/* pointers */
 	rna_def_animdata_common(srna);
 	rna_def_texmat_common(srna, "rna_Mesh_texspace_editable");

source/blender/makesrna/intern/rna_meta.c

@@ -158,6 +158,12 @@ static void rna_MetaBall_elements_clear(MetaBall *mb)
 	}
 }
 
+static int rna_Meta_is_editmode_get(PointerRNA *ptr)
+{
+	MetaBall *mb = ptr->id.data;
+	return (mb->editelems != NULL);
+}
+
 #else
 
 static void rna_def_metaelement(BlenderRNA *brna)
@@ -350,6 +356,11 @@ static void rna_def_metaball(BlenderRNA *brna)
 	RNA_def_property_srna(prop, "IDMaterials"); /* see rna_ID.c */
 	RNA_def_property_collection_funcs(prop, 0, NULL, NULL, NULL, NULL, NULL, NULL, "rna_IDMaterials_assign_int");
 	
+	prop = RNA_def_property(srna, "is_editmode", PROP_BOOLEAN, PROP_NONE);
+	RNA_def_property_boolean_funcs(prop, "rna_Meta_is_editmode_get", NULL);
+	RNA_def_property_clear_flag(prop, PROP_EDITABLE);
+	RNA_def_property_ui_text(prop, "Is Editmode", "True when used in editmode");
+
 	/* anim */
 	rna_def_animdata_common(srna);