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cc1: Improve escape handling in string()

The code in string() was a bit convoluted due to the semantic
of disescape. This patch improves the code reducing the number
of conditions and adding a comment explaining the logic
between esc and disescape.

--- a/src/cmd/cc/cc1/lex.c

+++ b/src/cmd/cc/cc1/lex.c

@@ -539,6 +539,14 @@

 	return CONSTANT;

 }

+/*

+ * string() parses a constant string, and convert all the

+ * escape sequences into single characters. This behaviour

+ * is correct except when we parse a #define, where we want

+ * to preserve the literal content of the string. In that

+ * case cpp.c:/^define( sets the variable disescape to

+ * disable converting escape sequences into characters.

+ */

 static int

 string(void)

 {

@@ -557,14 +565,15 @@

 			errorp("missing terminating '\"' character");

 			break;

 		}

-		if (c != '\\') {

+

+		if (c == '\\' && !esc && disescape)

+			esc = 1;

+		else

 			esc = 0;

-		} else {

-			if (!disescape)

-				c = escape();

-			else

-				esc = 1;

-		}

+

+		if (c == '\\' && !esc)

+			c = escape();

+

 		if (bp == &yytext[STRINGSIZ+1]) {

 			for (++input->p; *input->p != '"'; ++input->p) {

 				if (*input->p == '\\')