Read BambuStudio Nil Slots as Not Set Instead of Aborting the Load

BambuStudio and its forks save a nozzle variant that matches the parent
preset as "nil", including in keys OrcaSlicer can't leave empty, such as
retraction, z-hop and nozzle temperature. Reading one threw, which ended
the rest of the settings file: a project kept only the keys before it
alphabetically, and an embedded or user preset was dropped.

load_from_json now reads such a slot as not set when the caller opts in
(project settings, embedded presets, user presets). If every slot is nil,
the key is left out. Otherwise the slot holds the option default and is
recorded, and update_diff_values_to_child_config gives it the parent
preset's value. Other loaders still reject nil.

tests/libslic3r/test_config.cpp

@@ -585,6 +585,81 @@ TEST_CASE("A BambuStudio project config loads its renamed settings without subst
     CHECK(config.validate().count("raft_first_layer_expansion") == 0);
 }
 
+TEST_CASE("load_from_json reads a BambuStudio nil slot as not set", "[Config]") {
+    ScopedTemporaryFile tmp(".json");
+    {
+        boost::nowide::ofstream ofs(tmp.string());
+        // Keys after retraction_length in file order must still load.
+        ofs << R"({"layer_height":"0.2","retraction_length":["0.8","nil"],"wall_loops":"3","z_hop":["nil","nil"],)"
+               R"("z_hop_types":["Spiral Lift","nil"]})";
+    }
+    std::map<std::string, std::string> key_values;
+    std::string reason;
+
+    SECTION("a loader that doesn't merge onto a parent still rejects nil") {
+        DynamicPrintConfig config;
+        ConfigSubstitutionContext context(ForwardCompatibilitySubstitutionRule::Enable);
+        config.load_from_json(tmp.string(), context, true, key_values, reason);
+        CHECK_FALSE(reason.empty());
+    }
+
+    SECTION("a loader that opts in reads it as not set") {
+        DynamicPrintConfig config;
+        ConfigSubstitutionContext context(ForwardCompatibilitySubstitutionRule::Enable);
+        context.accept_nil = true;
+        REQUIRE(config.load_from_json(tmp.string(), context, true, key_values, reason) == 0);
+        CHECK(reason.empty());
+
+        const auto *default_length = static_cast<const ConfigOptionFloats*>(print_config_def.get("retraction_length")->default_value.get());
+        const auto &retraction_length = config.option<ConfigOptionFloats>("retraction_length")->values;
+        REQUIRE(retraction_length.size() == 2);
+        CHECK_THAT(retraction_length[0], Catch::Matchers::WithinAbs(0.8, 1e-9));
+        CHECK_THAT(retraction_length[1], Catch::Matchers::WithinAbs(default_length->get_at(1), 1e-9));
+
+        std::unique_ptr<ConfigOption> default_types(print_config_def.get("z_hop_types")->create_default_option());
+        const std::vector<std::string> type_defaults = static_cast<const ConfigOptionVectorBase*>(default_types.get())->vserialize();
+        CHECK(static_cast<const ConfigOptionVectorBase*>(config.option("z_hop_types"))->vserialize() ==
+              std::vector<std::string>{"Spiral Lift", type_defaults[1 % type_defaults.size()]});
+
+        CHECK(context.nil_slots == std::map<std::string, std::vector<size_t>>{{"retraction_length", {1}}, {"z_hop_types", {1}}});
+        CHECK_FALSE(config.has("z_hop"));
+        CHECK(config.opt_int("wall_loops") == 3);
+    }
+}
+
+TEST_CASE("A nozzle variant that was nil keeps the parent preset's value", "[Config]") {
+    auto printer = [](std::vector<double> nozzle_diameter, std::vector<double> retraction_length, std::vector<double> max_speed_x) {
+        DynamicPrintConfig config;
+        config.set_key_value("printer_extruder_variant", new ConfigOptionStrings({"Direct Drive Standard", "Direct Drive High Flow"}));
+        config.set_key_value("printer_extruder_id", new ConfigOptionInts({1, 1}));
+        // Not a per-variant key, so the merge copies it whole.
+        config.set_key_value("nozzle_diameter", new ConfigOptionFloats(nozzle_diameter));
+        config.set_key_value("retraction_length", new ConfigOptionFloats(retraction_length));
+        // Two values per variant: normal and silent mode.
+        config.set_key_value("machine_max_speed_x", new ConfigOptionFloats(max_speed_x));
+        return config;
+    };
+    auto check_values = [](const DynamicPrintConfig &config, const char *key, const std::vector<double> &expected) {
+        const std::vector<double> &values = config.option<ConfigOptionFloats>(key)->values;
+        INFO(key);
+        REQUIRE(values.size() == expected.size());
+        for (size_t i = 0; i < expected.size(); ++i)
+            CHECK_THAT(values[i], Catch::Matchers::WithinAbs(expected[i], 1e-9));
+    };
+    DynamicPrintConfig parent = printer({0.4, 0.6}, {0.6, 0.5}, {500, 200, 400, 100});
+    // The nil slots of the child hold the option default after loading.
+    DynamicPrintConfig child = printer({0.2, 0.4}, {0.8, 0.4}, {500, 200, 300, 90});
+    const std::map<std::string, std::vector<size_t>> nil_slots{{"nozzle_diameter", {1}}, {"retraction_length", {1}}, {"machine_max_speed_x", {3}}};
+
+    parent.update_diff_values_to_child_config(child, "printer_extruder_id", "printer_extruder_variant",
+                                              printer_options_with_variant_1, printer_options_with_variant_2, &nil_slots);
+
+    check_values(parent, "nozzle_diameter", {0.2, 0.6});
+    check_values(parent, "retraction_length", {0.8, 0.5});
+    // Only the silent-mode value of the second variant was nil.
+    check_values(parent, "machine_max_speed_x", {500, 200, 300, 100});
+}
+
 TEST_CASE("save_to_json writes the same document to a stream as to a file", "[Config]") {
     DynamicPrintConfig config;
     config.set_key_value("layer_height", new ConfigOptionFloat(0.2));