Belt validation: refuse a brim next to a purge tower object

The purge plan moves objects onto a common layer grid after the brim bands are
built, so the two cannot share a print. The prime tower setting alone still does
not block a brim. The missing-prism warning now counts the filaments the objects
use, as the GUI does.

src/libslic3r/GCode/ToolOrdering.cpp

@@ -1066,8 +1066,8 @@ void ToolOrdering::fill_wipe_tower_partitions(const PrintConfig &config, coordf_
     // The `print_z < object_bottom_z` clause reads "below the object" as "raft
     // gap".  On a belt printer that is wrong: the brim apron legitimately prints
     // below the object's first layer, and treating those layers as raft would put a
-    // wipe tower at negative Z.  Belt brim and the prime tower are mutually
-    // exclusive (rejected in Print::validate()), so simply drop the clause there.
+    // wipe tower at negative Z.  A belt printer never prints the classic
+    // prime tower (Print::has_wipe_tower()), so simply drop the clause there.
     //
     // Gate on config.belt_printer, NOT on has_belt_brim: every layer below the
     // object bottom on a belt printer is legitimately a sub-object stream - brim

src/libslic3r/Print.cpp

@@ -1420,10 +1420,14 @@ StringObjectException Print::validate(std::vector<StringObjectException> *warnin
             return { L("Draft shield is not compatible with belt printer mode.") };
 
         // Belt brim spans many layers and owns the layers below the object, which
-        // spiral vase cannot share. The prime tower setting is no obstacle: belt
-        // printers never print the classic tower, and the belt purge prism is an
-        // ordinary object that never takes a brim.
+        // neither the belt purge tower nor spiral vase can share (the purge plan moves
+        // objects onto a common layer grid after the brim bands are built). The prime
+        // tower setting alone is no obstacle: it stays on for any multi-filament
+        // project, and belt printers never print the classic tower.
         if (this->has_belt_brim()) {
+            if (this->has_belt_purge_tower())
+                return { L("Brim is not compatible with the belt purge tower. "
+                           "Disable one of them.") };
             if (m_config.spiral_mode.value)
                 return { L("Brim is not compatible with spiral vase mode on a belt printer. "
                            "Disable one of them.") };
@@ -1558,7 +1562,7 @@ StringObjectException Print::validate(std::vector<StringObjectException> *warnin
     // the tower was generated) changes filament with nowhere to purge.
     if (m_config.belt_printer.value && m_config.enable_belt_purge_tower.value
         && m_config.print_sequence != PrintSequence::ByObject
-        && ! m_config.spiral_mode.value && extruders.size() > 1 && ! this->has_belt_purge_tower()) {
+        && ! m_config.spiral_mode.value && this->object_extruders().size() > 1 && ! this->has_belt_purge_tower()) {
         StringObjectException warningtemp;
         warningtemp.string     = L("The belt purge tower is enabled but the project has no purge tower object; "
                                    "filament changes will not be purged. Open the project in the application "

tests/fff_print/test_skirt_brim.cpp

@@ -930,6 +930,28 @@ TEST_CASE("Belt inner-only leading brim does not reject the prime tower or spira
         CHECK(print.validate().string.empty());
         CHECK_FALSE(gcode(print).empty());
     }
+    // An actual purge tower object is different: the purge plan moves objects onto a common
+    // layer grid after the brim bands are built, so the two are still refused together.
+    SECTION("a brim is refused next to a belt purge tower object") {
+        DynamicPrintConfig config = belt_brim_multifilament_config(2, {
+            { "brim_type",               "outer_only" },
+            { "brim_width",              4 },
+            { "brim_object_gap",         0 },
+            { "enable_belt_purge_tower", 1 },
+        });
+        const std::vector<std::vector<Slic3r::ConfigBase::SetDeserializeItem>> overrides {
+            { { "extruder", 1 } }, { { "extruder", 2 } },
+        };
+        Print print;
+        Model model;
+        init_print({ cube(20), cube(20) }, print, model, config, &overrides);
+        CHECK(print.validate().string.empty());
+
+        model.objects.back()->config.set_key_value("belt_purge_tower_object", new ConfigOptionBool(true));
+        print.apply(model, config);
+        REQUIRE(print.has_belt_purge_tower());
+        CHECK_FALSE(print.validate().string.empty());
+    }
 }
 
 TEST_CASE("Belt brim spans many layers instead of one", "[SkirtBrim][belt]")