implement builtin function index

src/builtin/list.rs

@@ -193,4 +193,20 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
             }))
         }),
     );
+    f.insert(
+        "index".to_owned(),
+        Box::new(|mut args, _| {
+            max_args!(args, 2);
+            let list = match arg!(args, 0, "list") {
+                Value::List(v, ..) => v,
+                v => vec![v],
+            };
+            let value = arg!(args, 1, "value");
+            let index = match list.into_iter().position(|v| v == value) {
+                Some(v) => Number::from(v + 1),
+                None => return Ok(Value::Null),
+            };
+            Ok(Value::Dimension(index, Unit::None))
+        }),
+    );
 }

tests/list.rs

@@ -189,3 +189,18 @@ test!(
     deeply_nested_square_braces,
     "a {\n  color: [[[[[[a]]]]]];\n}\n"
 );
+test!(
+    index_found_space_separated,
+    "a {\n  color: index(1px solid red, solid);\n}\n",
+    "a {\n  color: 2;\n}\n"
+);
+test!(
+    index_not_found_space_separated,
+    "a {\n  color: index(1px solid red, dashed);\n}\n",
+    ""
+);
+// test!(
+//     index_found_map,
+//     "a {\n  color: index((width: 10px, height: 20px), (height 20px));\n}\n",
+//     "a {\n  color: 2;\n}\n"
+// );