Distinguish serial index from mask (#21287)

Marlin/src/core/serial_base.h

@@ -22,12 +22,29 @@
 #pragma once
 
 #include "../inc/MarlinConfigPre.h"
-#include "macros.h"
 
 #if ENABLED(EMERGENCY_PARSER)
   #include "../feature/e_parser.h"
 #endif
 
+// Used in multiple places
+// You can build it but not manipulate it.
+// There are only few places where it's required to access the underlying member: GCodeQueue, SerialMask and MultiSerial
+struct serial_index_t {
+  // A signed index, where -1 is a special case meaning no action (neither output or input)
+  int8_t  index;
+
+  // Check if the index is within the range [a ... b]
+  constexpr inline bool within(const int8_t a, const int8_t b) const { return WITHIN(index, a, b); }
+  constexpr inline bool valid() const { return WITHIN(index, 0, 7); } // At most, 8 bits
+
+  // Construction is either from an index
+  constexpr serial_index_t(const int8_t index) : index(index) {}
+
+  // Default to "no index"
+  constexpr serial_index_t() : index(-1) {}
+};
+
 // flushTX is not implemented in all HAL, so use SFINAE to call the method where it is.
 CALL_IF_EXISTS_IMPL(void, flushTX);
 CALL_IF_EXISTS_IMPL(bool, connected, true);
@@ -79,10 +96,10 @@ struct SerialBase {
   void end()                        { static_cast<Child*>(this)->end(); }
   /** Check for available data from the port
       @param index  The port index, usually 0 */
-  int available(uint8_t index = 0)  { return static_cast<Child*>(this)->available(index); }
+  int available(serial_index_t index = 0)  { return static_cast<Child*>(this)->available(index); }
   /** Read a value from the port
       @param index  The port index, usually 0 */
-  int  read(uint8_t index = 0)      { return static_cast<Child*>(this)->read(index); }
+  int  read(serial_index_t index = 0)      { return static_cast<Child*>(this)->read(index); }
   // Check if the serial port is connected (usually bypassed)
   bool connected()                  { return static_cast<Child*>(this)->connected(); }
   // Redirect flush