The identifier var should be fine in Yul

libyul/AsmParser.cpp

@@ -324,6 +324,7 @@ Parser::ElementaryOperation Parser::parseElementaryOperation()
 	case Token::Byte:
 	case Token::Bool:
 	case Token::Address:
+	case Token::Var:
 	{
 		YulString literal{currentLiteral()};
 		if (m_dialect.builtin(literal))
@@ -513,6 +514,7 @@ YulString Parser::expectAsmIdentifier()
 	case Token::Address:
 	case Token::Bool:
 	case Token::Identifier:
+	case Token::Var:
 		break;
 	default:
 		expectToken(Token::Identifier);

test/libsolidity/semanticTests/viaYul/mapping_string_key.sol

@@ -0,0 +1,10 @@
+contract C {
+	mapping (string => uint) map;
+	function set(string memory s) public {
+		map[s];
+	}
+}
+// ====
+// compileViaYul: true
+// ----
+// set(string): 0x20, 32, "01234567890123456789012345678901" ->