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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

--- a/src/asm/lexer.c

+++ b/src/asm/lexer.c

@@ -278,6 +278,7 @@

 	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 @@

 	state->capturing = false;

 	state->captureBuf = NULL;

+	state->macroArgScanDistance = 0;

 	state->expandStrings = true;

 	state->expansions = NULL;

 	state->expansionOfs = 0;

@@ -716,16 +718,29 @@

 {

 	int c = peekInternal(distance);

-	/* If not capturing and character is a backslash, check for a macro arg */

-	if (!lexerState->capturing && c == '\\') {

-		distance++;

-		c = peekInternal(distance);

-		if (c == '@' || (c >= '0' && c <= '9')) {

-			/* Expand the argument and return its first character */

-			c = expandMacroArg(c, distance - 1)[0];

-			/* WARNING: this assumes macro args can't be empty!! */

-		} else {

-			c = '\\';

+	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 >= '1' && c <= '9')) {

+				/* Expand the argument and return its first character */

+				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;

@@ -744,6 +759,8 @@

 			lexerState->captureSize += distance;

 		}

 	}

+

+	lexerState->macroArgScanDistance -= distance;

 	/* FIXME: this may not be too great, as only the top level is considered... */