Fix mixing with "unload all" compile

Marlin/src/feature/fwretract.cpp

  */
 void FWRetract::retract(const bool retracting
   #if EXTRUDERS > 1
-    , bool swapping /* =false */
+    , bool swapping/*=false*/
   #endif
 ) {
   // Prevent two retracts or recovers in a row
     SERIAL_ECHOLNPAIR("current_hop ", current_hop);
   //*/
 
-  const float base_retract = (
-                (swapping ? settings.swap_retract_length : settings.retract_length)
-                #if ENABLED(RETRACT_SYNC_MIXING)
-                  * (MIXING_STEPPERS)
-                #endif
-              );
+  const float base_retract = TERN(RETRACT_SYNC_MIXING, MIXING_STEPPERS, 1)
+                * (swapping ? settings.swap_retract_length : settings.retract_length);
 
   // The current position will be the destination for E and Z moves
   destination = current_position;
     // Retract by moving from a faux E position back to the current E position
     current_retract[active_extruder] = base_retract;
     prepare_internal_move_to_destination(                 // set current to destination
-      settings.retract_feedrate_mm_s
-      #if ENABLED(RETRACT_SYNC_MIXING)
-        * (MIXING_STEPPERS)
-      #endif
+      settings.retract_feedrate_mm_s * TERN(RETRACT_SYNC_MIXING, MIXING_STEPPERS, 1)
     );
 
     // Is a Z hop set, and has the hop not yet been done?
 
     current_retract[active_extruder] = 0;
 
-    const feedRate_t fr_mm_s = (
-      (swapping ? settings.swap_retract_recover_feedrate_mm_s : settings.retract_recover_feedrate_mm_s)
-      #if ENABLED(RETRACT_SYNC_MIXING)
-        * (MIXING_STEPPERS)
-      #endif
-    );
+    const feedRate_t fr_mm_s = TERN(RETRACT_SYNC_MIXING, MIXING_STEPPERS, 1)
+      * (swapping ? settings.swap_retract_recover_feedrate_mm_s : settings.retract_recover_feedrate_mm_s);
     prepare_internal_move_to_destination(fr_mm_s);        // Recover E, set_current_to_destination
   }
 
-  #if ENABLED(RETRACT_SYNC_MIXING)
-    mixer.T(old_mixing_tool);                             // Restore original mixing tool
-  #endif
+  TERN_(RETRACT_SYNC_MIXING, mixer.T(old_mixing_tool));   // Restore original mixing tool
 
   retracted[active_extruder] = retracting;                // Active extruder now retracted / recovered
 

Marlin/src/feature/mixing.cpp

 
   reset_vtools();
 
-  #if ENABLED(RETRACT_SYNC_MIXING)
+  #if HAS_MIXER_SYNC_CHANNEL
     // AUTORETRACT_TOOL gets the same amount of all filaments
     MIXER_STEPPER_LOOP(i)
       color[MIXER_AUTORETRACT_TOOL][i] = COLOR_A_MASK;

Marlin/src/feature/mixing.h

   LAST_USER_VIRTUAL_TOOL = MIXING_VIRTUAL_TOOLS - 1,
   NR_USER_VIRTUAL_TOOLS,
   MIXER_DIRECT_SET_TOOL = NR_USER_VIRTUAL_TOOLS,
-  #if ENABLED(RETRACT_SYNC_MIXING)
+  #if HAS_MIXER_SYNC_CHANNEL
     MIXER_AUTORETRACT_TOOL,
   #endif
   NR_MIXING_VIRTUAL_TOOLS
 };
 
-#if ENABLED(RETRACT_SYNC_MIXING)
-  #define MAX_VTOOLS 254
-#else
-  #define MAX_VTOOLS 255
-#endif
+#define MAX_VTOOLS TERN(HAS_MIXER_SYNC_CHANNEL, 254, 255)
 static_assert(NR_MIXING_VIRTUAL_TOOLS <= MAX_VTOOLS, "MIXING_VIRTUAL_TOOLS must be <= " STRINGIFY(MAX_VTOOLS) "!");
 
-#define MIXER_STEPPER_LOOP(VAR) \
-  for (uint_fast8_t VAR = 0; VAR < MIXING_STEPPERS; VAR++)
-
 #define MIXER_BLOCK_FIELD       mixer_comp_t b_color[MIXING_STEPPERS]
 #define MIXER_POPULATE_BLOCK()  mixer.populate_block(block->b_color)
 #define MIXER_STEPPER_SETUP()   mixer.stepper_setup(current_block->b_color)
+#define MIXER_STEPPER_LOOP(VAR) for (uint_fast8_t VAR = 0; VAR < MIXING_STEPPERS; VAR++)
 
 #if ENABLED(GRADIENT_MIX)
 

Marlin/src/gcode/feature/pause/M701_M702.cpp

 
     #if ENABLED(FILAMENT_UNLOAD_ALL_EXTRUDERS)
       float mix_multiplier = 1.0;
-      if (!parser.seenval('T')) {
+      const bool seenT = parser.seenval('T');
+      if (!seenT) {
         mixer.T(MIXER_AUTORETRACT_TOOL);
         mix_multiplier = MIXING_STEPPERS;
       }
-      else
+    #else
+      constexpr bool seenT = true;
     #endif
-    {
+
+    if (seenT) {
       const int8_t target_e_stepper = get_target_e_stepper_from_command();
       if (target_e_stepper < 0) return;
-
       mixer.T(MIXER_DIRECT_SET_TOOL);
       MIXER_STEPPER_LOOP(i) mixer.set_collector(i, (i == (uint8_t)target_e_stepper) ? 1.0 : 0.0);
       mixer.normalize();

Marlin/src/inc/Conditionals_adv.h

   #undef SHOW_TEMP_ADC_VALUES
 #endif
 
+#if ENABLED(MIXING_EXTRUDER) && (ENABLED(RETRACT_SYNC_MIXING) || BOTH(FILAMENT_LOAD_UNLOAD_GCODES, FILAMENT_UNLOAD_ALL_EXTRUDERS))
+  #define HAS_MIXER_SYNC_CHANNEL 1
+#endif
+
 #if EITHER(DUAL_X_CARRIAGE, MULTI_NOZZLE_DUPLICATION)
   #define HAS_DUPLICATION_MODE 1
 #endif

Marlin/src/module/planner.cpp

   #if EXTRUDERS
     {
       current_speed.e = steps_dist_mm.e * inverse_secs;
-      #if BOTH(MIXING_EXTRUDER, RETRACT_SYNC_MIXING)
+      #if HAS_MIXER_SYNC_CHANNEL
         // Move all mixing extruders at the specified rate
         if (mixer.get_current_vtool() == MIXER_AUTORETRACT_TOOL)
           current_speed.e *= MIXING_STEPPERS;
       #endif
       const feedRate_t cs = ABS(current_speed.e),
-                   max_fr = (settings.max_feedrate_mm_s[E_AXIS_N(extruder)]
-                              #if BOTH(MIXING_EXTRUDER, RETRACT_SYNC_MIXING)
-                                * MIXING_STEPPERS
-                              #endif
-                            );
+                   max_fr = settings.max_feedrate_mm_s[E_AXIS_N(extruder)]
+                            * TERN(HAS_MIXER_SYNC_CHANNEL, MIXING_STEPPERS, 1);
       if (cs > max_fr) NOMORE(speed_factor, max_fr / cs);
     }
   #endif