gen: optimize `str == ''` to `str.len == 0` for performance (#6527)

vlib/v/gen/cgen.v

@@ -2390,7 +2390,16 @@ fn (mut g Gen) infix_expr(node ast.InfixExpr) {
 		g.write(', ')
 		g.expr(node.right)
 		g.write(')')
-	} else if left_type == table.string_type_idx && node.op != .key_in && node.op != .not_in {
+	} else if left_type == table.string_type_idx && node.op !in [.key_in, .not_in] {
+		// `str == ''` -> `str.len == 0` optimization
+		if node.op in [.eq, .ne] &&
+			node.right is ast.StringLiteral && (node.right as ast.StringLiteral).val == '' {
+			arrow := if left_type.is_ptr() { '->' } else { '.' }
+			g.write('(')
+			g.expr(node.left)
+			g.write(')')
+			g.write('${arrow}len $node.op 0')
+		} else {
 			fn_name := match node.op {
 				.plus {
 					'string_add('
@@ -2423,6 +2432,7 @@ fn (mut g Gen) infix_expr(node ast.InfixExpr) {
 			g.write(', ')
 			g.expr(node.right)
 			g.write(')')
+		}
 	} else if node.op in [.eq, .ne] && left_sym.kind == .array && right_sym.kind == .array {
 		ptr_typ := g.gen_array_equality_fn(left_type)
 		if node.op == .eq {
@@ -2723,11 +2733,15 @@ fn (mut g Gen) match_expr(node ast.MatchExpr) {
 					}
 					g.expr(expr)
 				} else if type_sym.kind == .string {
+					if (expr as ast.StringLiteral).val == '' {
+						g.write('${cond_var}.len == 0')
+					} else {
 						g.write('string_eq(')
 						g.write(cond_var)
 						g.write(', ')
 						g.expr(expr)
 						g.write(')')
+					}
 				} else if expr is ast.RangeExpr {
 					// if type is unsigned and low is 0, check is unneeded
 					mut skip_low := false