PlaceholderParser on G-code export:

1) added "num_extruders" placeholder
2) changed "is_extruder_used" vector to be always minimum 256 elements long
   with the non-existent extruder values set to zero.
   This is wasteful, but necessary until we change the PlaceholderParser
   handling of vector values to not substitute missing elements
   with the zero'th value.

src/libslic3r/GCode.cpp

@@ -1296,8 +1296,12 @@ void GCode::_do_export(Print& print, GCodeOutputStream &file, ThumbnailsGenerato
         this->placeholder_parser().set("first_layer_print_min",  new ConfigOptionFloats({ bbox.min.x(), bbox.min.y() }));
         this->placeholder_parser().set("first_layer_print_max",  new ConfigOptionFloats({ bbox.max.x(), bbox.max.y() }));
         this->placeholder_parser().set("first_layer_print_size", new ConfigOptionFloats({ bbox.size().x(), bbox.size().y() }));
-
+        this->placeholder_parser().set("num_extruders", int(print.config().nozzle_diameter.values.size()));
-        std::vector<unsigned char> is_extruder_used(print.config().nozzle_diameter.size(), 0);
+        // PlaceholderParser currently substitues non-existent vector values with the zero'th value, which is harmful in the case of "is_extruder_used[]"
+        // as Slicer may lie about availability of such non-existent extruder.
+        // We rather sacrifice 256B of memory before we change the behavior of the PlaceholderParser, which should really only fill in the non-existent
+        // vector elements for filament parameters.
+        std::vector<unsigned char> is_extruder_used(std::max(size_t(255), print.config().nozzle_diameter.size()), 0);
         for (unsigned int extruder_id : tool_ordering.all_extruders())
             is_extruder_used[extruder_id] = true;
         this->placeholder_parser().set("is_extruder_used", new ConfigOptionBools(is_extruder_used));

src/libslic3r/PlaceholderParser.cpp

@@ -1090,6 +1090,7 @@ namespace client
 
         static void scalar_variable_assign_scalar(const MyContext *ctx, OptWithPos &lhs, const expr &rhs)
         {
+            assert(! ctx->skipping());
             assert(lhs.opt->is_scalar());
             check_writable(ctx, lhs);
             ConfigOption *wropt = const_cast<ConfigOption*>(lhs.opt);
@@ -1121,6 +1122,7 @@ namespace client
 
         static void vector_variable_element_assign_scalar(const MyContext *ctx, OptWithPos &lhs, const expr &rhs)
         {
+            assert(! ctx->skipping());
             assert(lhs.opt->is_vector());
             check_writable(ctx, lhs);
             if (! lhs.has_index())
@@ -1158,6 +1160,7 @@ namespace client
 
         static void vector_variable_assign_expr_with_count(const MyContext *ctx, OptWithPos &lhs, const expr &rhs_count, const expr &rhs_value)
         {
+            assert(! ctx->skipping());
             size_t count = evaluate_count(rhs_count);
             auto *opt = const_cast<ConfigOption*>(lhs.opt);
             switch (lhs.opt->type()) {