fmt: fix ForInStmt

vlib/v/fmt/fmt.v

@@ -178,9 +178,15 @@ fn (f mut Fmt) stmt(node ast.Stmt) {
 			f.writeln('}\n')
 		}
 		ast.ForInStmt {
-			f.write('for $it.key_var')
+			f.write('for ')
+			if it.key_var != '' {
+				f.write(it.key_var)
+			}
 			if it.val_var != '' {
-				f.write(', $it.val_var')
+				if it.key_var != '' {
+					f.write(', ')
+				}
+				f.write(it.val_var)
 			}
 			f.write(' in ')
 			f.expr(it.cond)

vlib/v/fmt/tests/loops_expected.vv

@@ -0,0 +1,17 @@
+fn for_in_loop() {
+	for item in arr {
+		println(item)
+	}
+}
+
+fn for_in_loop_with_counter() {
+	for i, item in arr {
+		println(item)
+	}
+}
+
+fn for_in_loop_with_index_expr() {
+	for i in 0 .. 10 {
+		println(i)
+	}
+}

vlib/v/fmt/tests/loops_input.vv

@@ -0,0 +1,17 @@
+fn for_in_loop() {
+	for item in arr {
+		println(item)
+	}
+}
+
+fn for_in_loop_with_counter() {
+	for i, item in arr {
+		println(item)
+	}
+}
+
+fn for_in_loop_with_index_expr() {
+	for i in 0..10 {
+		println(i)
+	}
+}