Reinstate macro arg scan distance

Used to be broken, so it was removed, but doing so prevents escaping them.
So it was instead put back in, but with corrected behavior

src/asm/lexer.c

@@ -278,6 +278,7 @@ struct LexerState {
 	char *captureBuf; /* Buffer to send the captured text to if non-NULL */
 	size_t captureCapacity; /* Size of the buffer above */
 
+	size_t macroArgScanDistance; /* Max distance already scanned for macro args */
 	bool expandStrings;
 	struct Expansion *expansions;
 	size_t expansionOfs; /* Offset into the current top-level expansion (negative = before) */
@@ -295,6 +296,7 @@ static void initState(struct LexerState *state)
 	state->capturing = false;
 	state->captureBuf = NULL;
 
+	state->macroArgScanDistance = 0;
 	state->expandStrings = true;
 	state->expansions = NULL;
 	state->expansionOfs = 0;
@@ -716,18 +718,31 @@ static int peek(uint8_t distance)
 {
 	int c = peekInternal(distance);
 
+	if (distance >= lexerState->macroArgScanDistance) {
+		lexerState->macroArgScanDistance = distance + 1; /* Do not consider again */
 		/* If not capturing and character is a backslash, check for a macro arg */
 		if (!lexerState->capturing && c == '\\') {
 			distance++;
+			lexerState->macroArgScanDistance++;
 			c = peekInternal(distance);
-		if (c == '@' || (c >= '0' && c <= '9')) {
+			if (c == '@' || (c >= '1' && c <= '9')) {
 				/* Expand the argument and return its first character */
-			c = expandMacroArg(c, distance - 1)[0];
+				char const *str = expandMacroArg(c, distance - 1);
+
+				/*
+				 * Assuming macro args can't be recursive (I'll be damned if a way
+				 * is found...), then we mark the entire macro arg as scanned;
+				 * however, the two macro arg characters (\1) will be ignored,
+				 * so they shouldn't be counted in the scan distance!
+				 */
+				lexerState->macroArgScanDistance += strlen(str) - 2;
 				/* WARNING: this assumes macro args can't be empty!! */
+				c = str[0];
 			} else {
 				c = '\\';
 			}
 		}
+	}
 	return c;
 }
 
@@ -745,6 +760,8 @@ static void shiftChars(uint8_t distance)
 		}
 	}
 
+	lexerState->macroArgScanDistance -= distance;
+
 	/* FIXME: this may not be too great, as only the top level is considered... */
 
 	/*