Ported test_gcodewriter from upstream Slic3r, thanks @lordofhyphens.

The format for G1 Fxxx was changed to fixed three decimal digits.

src/libslic3r/GCodeWriter.cpp

@@ -269,7 +269,7 @@ std::string GCodeWriter::set_speed(double F, const std::string &comment, const s
     assert(F > 0.);
     assert(F < 100000.);
     std::ostringstream gcode;
-    gcode << "G1 F" << F;
+    gcode << "G1 F" << XYZF_NUM(F);
     COMMENT(comment);
     gcode << cooling_marker;
     gcode << "\n";

tests/data/fff_print_tests/test_gcodewriter/config_lift_unlift.ini

@@ -0,0 +1,30 @@
+before_layer_gcode = 
+between_objects_gcode = 
+end_filament_gcode = "; Filament-specific end gcode \n;END gcode for filament\n"
+end_gcode = M104 S0 ; turn off temperature\nG28 X0  ; home X axis\nM84     ; disable motors\n
+extrusion_axis = E
+extrusion_multiplier = 1
+filament_cost = 0
+filament_density = 0
+filament_diameter = 3
+filament_max_volumetric_speed = 0
+gcode_comments = 0
+gcode_flavor = reprap
+layer_gcode = 
+max_print_speed = 80
+max_volumetric_speed = 0
+retract_length = 2
+retract_length_toolchange = 10
+retract_lift = 1.5
+retract_lift_above = 0
+retract_lift_below = 0
+retract_restart_extra = 0
+retract_restart_extra_toolchange = 0
+retract_speed = 40
+start_filament_gcode = "; Filament gcode\n"
+start_gcode = G28 ; home all axes\nG1 Z5 F5000 ; lift nozzle\n
+toolchange_gcode = 
+travel_speed = 130
+use_firmware_retraction = 0
+use_relative_e_distances = 0
+use_volumetric_e = 0

tests/fff_print/CMakeLists.txt

@@ -4,6 +4,7 @@ add_executable(${_TEST_NAME}_tests
 	test_data.cpp
 	test_data.hpp
 	test_flow.cpp
+	test_gcodewriter.cpp
 	test_skirt_brim.cpp
 	test_trianglemesh.cpp
 	)

tests/fff_print/test_data.cpp

@@ -11,7 +11,6 @@
 #include <boost/nowide/cstdio.hpp>
 #include <boost/filesystem.hpp>
 
-using namespace std::string_literals;
 using namespace std;
 
 namespace Slic3r { namespace Test {

tests/fff_print/test_gcodewriter.cpp

@@ -0,0 +1,124 @@
+#include <catch2/catch.hpp>
+
+#include <memory>
+
+#include "libslic3r/GCodeWriter.hpp"
+
+using namespace Slic3r;
+
+SCENARIO("lift() and unlift() behavior with large values of Z", "[!shouldfail]") {
+    GIVEN("A config from a file and a single extruder.") {
+        GCodeWriter writer;
+        GCodeConfig &config = writer.config;
+        config.load(std::string(TEST_DATA_DIR) + "/fff_print_tests/test_gcodewriter/config_lift_unlift.ini");
+
+        std::vector<unsigned int> extruder_ids {0};
+        writer.set_extruders(extruder_ids);
+        writer.set_extruder(0);
+
+        WHEN("Z is set to 9007199254740992") {
+            double trouble_Z = 9007199254740992;
+            writer.travel_to_z(trouble_Z);
+            AND_WHEN("GcodeWriter::Lift() is called") {
+                REQUIRE(writer.lift().size() > 0);
+                AND_WHEN("Z is moved post-lift to the same delta as the config Z lift") {
+                    REQUIRE(writer.travel_to_z(trouble_Z + config.retract_lift.values[0]).size() == 0);
+                    AND_WHEN("GCodeWriter::Unlift() is called") {
+                        REQUIRE(writer.unlift().size() == 0); // we're the same height so no additional move happens.
+                        THEN("GCodeWriter::Lift() emits gcode.") {
+                            REQUIRE(writer.lift().size() > 0);
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+
+SCENARIO("lift() is not ignored after unlift() at normal values of Z") {
+    GIVEN("A config from a file and a single extruder.") {
+        GCodeWriter writer;
+        GCodeConfig &config = writer.config;
+        config.load(std::string(TEST_DATA_DIR) + "/fff_print_tests/test_gcodewriter/config_lift_unlift.ini");
+
+        std::vector<unsigned int> extruder_ids {0};
+        writer.set_extruders(extruder_ids);
+        writer.set_extruder(0);
+
+        WHEN("Z is set to 203") {
+            double trouble_Z = 203;
+            writer.travel_to_z(trouble_Z);
+            AND_WHEN("GcodeWriter::Lift() is called") {
+                REQUIRE(writer.lift().size() > 0);
+                AND_WHEN("Z is moved post-lift to the same delta as the config Z lift") {
+                    REQUIRE(writer.travel_to_z(trouble_Z + config.retract_lift.values[0]).size() == 0);
+                    AND_WHEN("GCodeWriter::Unlift() is called") {
+                        REQUIRE(writer.unlift().size() == 0); // we're the same height so no additional move happens.
+                        THEN("GCodeWriter::Lift() emits gcode.") {
+                            REQUIRE(writer.lift().size() > 0);
+                        }
+                    }
+                }
+            }
+        }
+        WHEN("Z is set to 500003") {
+            double trouble_Z = 500003;
+            writer.travel_to_z(trouble_Z);
+            AND_WHEN("GcodeWriter::Lift() is called") {
+                REQUIRE(writer.lift().size() > 0);
+                AND_WHEN("Z is moved post-lift to the same delta as the config Z lift") {
+                    REQUIRE(writer.travel_to_z(trouble_Z + config.retract_lift.values[0]).size() == 0);
+                    AND_WHEN("GCodeWriter::Unlift() is called") {
+                        REQUIRE(writer.unlift().size() == 0); // we're the same height so no additional move happens.
+                        THEN("GCodeWriter::Lift() emits gcode.") {
+                            REQUIRE(writer.lift().size() > 0);
+                        }
+                    }
+                }
+            }
+        }
+        WHEN("Z is set to 10.3") {
+            double trouble_Z = 10.3;
+            writer.travel_to_z(trouble_Z);
+            AND_WHEN("GcodeWriter::Lift() is called") {
+                REQUIRE(writer.lift().size() > 0);
+                AND_WHEN("Z is moved post-lift to the same delta as the config Z lift") {
+                    REQUIRE(writer.travel_to_z(trouble_Z + config.retract_lift.values[0]).size() == 0);
+                    AND_WHEN("GCodeWriter::Unlift() is called") {
+                        REQUIRE(writer.unlift().size() == 0); // we're the same height so no additional move happens.
+                        THEN("GCodeWriter::Lift() emits gcode.") {
+                            REQUIRE(writer.lift().size() > 0);
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+
+SCENARIO("set_speed emits values with fixed-point output.") {
+
+    GIVEN("GCodeWriter instance") {
+        GCodeWriter writer;
+        WHEN("set_speed is called to set speed to 99999.123") {
+            THEN("Output string is G1 F99999.123") {
+                REQUIRE_THAT(writer.set_speed(99999.123), Catch::Equals("G1 F99999.123\n"));
+            }
+        }
+        WHEN("set_speed is called to set speed to 1") {
+            THEN("Output string is G1 F1.000") {
+                REQUIRE_THAT(writer.set_speed(1.0), Catch::Equals("G1 F1.000\n"));
+            }
+        }
+        WHEN("set_speed is called to set speed to 203.200022") {
+            THEN("Output string is G1 F203.200") {
+                REQUIRE_THAT(writer.set_speed(203.200022), Catch::Equals("G1 F203.200\n"));
+            }
+        }
+        WHEN("set_speed is called to set speed to 203.200522") {
+            THEN("Output string is G1 F203.201") {
+                REQUIRE_THAT(writer.set_speed(203.200522), Catch::Equals("G1 F203.201\n"));
+            }
+        }
+    }
+}