str-len is a count of characters, not bytes

src/builtin/string.rs

@@ -35,7 +35,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
         Box::new(|args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "string") {
-                Value::Ident(i, _) => Ok(Value::Dimension(Number::from(i.len()), Unit::None)),
+                Value::Ident(i, _) => Ok(Value::Dimension(Number::from(i.chars().count()), Unit::None)),
                 v => Err(format!("$string: {} is not a string.", v).into()),
             }
         }),

tests/strings.rs

@@ -103,6 +103,16 @@ test!(
     "a {\n  color: str-length(\"foo bar\");\n}\n",
     "a {\n  color: 7;\n}\n"
 );
+test!(
+    str_len_double_wide,
+    "a {\n  color: str-length(\"👭\");\n}\n",
+    "@charset \"UTF-8\";\na {\n  color: 1;\n}\n"
+);
+test!(
+    str_len_combining,
+    "a {\n  color: str-length(\"c\\0308\");\n}\n",
+    "a {\n  color: 2;\n}\n"
+);
 test!(
     str_index_char,
     "a {\n  color: str-index(abcd, a);\n}\n",