proper error message for non-string to feature-exists

src/builtin/meta.rs

@@ -22,7 +22,8 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
         "feature-exists".to_owned(),
         Box::new(|args, _| {
             max_args!(args, 1);
-            match arg!(args, 0, "feature").unquote().to_string().as_str() {
+            match arg!(args, 0, "feature") {
+                Value::Ident(s, _) => match s.as_str() {
                     // A local variable will shadow a global variable unless
                     // `!global` is used.
                     "global-variable-shadowing" => Ok(Value::False),
@@ -39,6 +40,8 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
                     // interpolation treated as SassScript.
                     "custom-property" => Ok(Value::False),
                     _ => Ok(Value::False),
+                },
+                v => Err(format!("$feature: {} is not a string.", v).into()),
             }
         }),
     );