Implement ++ operator for string concatenation (#1698)

src/asm/lexer.cpp

@@ -1774,12 +1774,17 @@ static Token yylex_NORMAL() {
 
 			// Handle ambiguous 1- or 2-char tokens
 
-		case '+': // Either += or ADD
-			if (peek() == '=') {
+		case '+': // Either +=, ADD, or CAT
+			switch (peek()) {
+			case '=':
 				shiftChar();
 				return Token(T_(POP_ADDEQ));
+			case '+':
+				shiftChar();
+				return Token(T_(OP_CAT));
+			default:
+				return Token(T_(OP_ADD));
 			}
-			return Token(T_(OP_ADD));
 
 		case '-': // Either -= or SUB
 			if (peek() == '=') {

src/asm/parser.y

@@ -125,6 +125,9 @@
 %token OP_MUL "*" OP_DIV "/" OP_MOD "%"
 %token OP_EXP "**"
 
+// String operators
+%token OP_CAT "++"
+
 // Comparison operators
 %token OP_LOGICEQU "==" OP_LOGICNE "!="
 %token OP_LOGICLT "<" OP_LOGICGT ">"
@@ -147,6 +150,7 @@
 %left OP_AND OP_OR OP_XOR
 %left OP_SHL OP_SHR OP_USHR
 %left OP_MUL OP_DIV OP_MOD
+%left OP_CAT
 %precedence NEG // applies to unary OP_LOGICNOT, OP_ADD, OP_SUB, OP_NOT
 %right OP_EXP
 
@@ -1613,6 +1617,10 @@ string_literal:
 	STRING {
 		$$ = std::move($1);
 	}
+	| string OP_CAT string {
+		$$ = std::move($1);
+		$$.append($3);
+	}
 	| OP_STRSLICE LPAREN string COMMA iconst COMMA iconst RPAREN {
 		size_t len = strlenUTF8($3, false);
 		uint32_t start = adjustNegativeIndex($5, len, "STRSLICE");