proper error messages in (mixin|function)-exists

src/builtin/meta.rs

@@ -107,19 +107,22 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
         "mixin-exists".to_owned(),
         Box::new(|args, scope| {
             max_args!(args, 1);
-            let value = arg!(args, 0, "name");
+            match arg!(args, 0, "name") {
-            Ok(Value::bool(scope.mixin_exists(&value.to_string())))
+                Value::Ident(s, _) => Ok(Value::bool(scope.mixin_exists(&s))),
+                v => Err(format!("$name: {} is not a string.", v).into()),
+            }
         }),
     );
     f.insert(
         "function-exists".to_owned(),
         Box::new(|args, scope| {
             max_args!(args, 1);
-            let value = arg!(args, 0, "name");
+            match arg!(args, 0, "name") {
-            let s = value.unquote().to_string();
+                Value::Ident(s, _) => Ok(Value::bool(
-            Ok(Value::bool(
                     scope.fn_exists(&s) || GLOBAL_FUNCTIONS.contains_key(&s),
-            ))
+                )),
+                v => Err(format!("$name: {} is not a string.", v).into()),
+            }
         }),
     );
     f.insert("call".to_owned(), Box::new(|_args, _scope| {

src/value/mod.rs

@@ -111,7 +111,9 @@ impl Value {
     pub fn is_true(&self) -> SassResult<bool> {
         match self {
             Value::Null | Value::False => Ok(false),
-            Self::BinaryOp(..) | Self::Paren(..) | Self::UnaryOp(..) => self.clone().eval()?.is_true(),
+            Self::BinaryOp(..) | Self::Paren(..) | Self::UnaryOp(..) => {
+                self.clone().eval()?.is_true()
+            }
             _ => Ok(true),
         }
     }

tests/meta.rs

@@ -278,6 +278,11 @@ test!(
     "@function a(){} a {\n  color: function-exists(a)\n}\n",
     "a {\n  color: true;\n}\n"
 );
+test!(
+    builtin_function_does_exist,
+    "a {\n  color: function-exists(function-exists)\n}\n",
+    "a {\n  color: true;\n}\n"
+);
 test!(
     function_does_not_exist,
     "a {\n  color: function-exists(a)\n}\n",
@@ -288,6 +293,14 @@ test!(
     "@function a(){} a {\n  color: function-exists($name: a)\n}\n",
     "a {\n  color: true;\n}\n"
 );
+error!(
+    function_exists_non_string,
+    "a {color:function-exists(12px)}", "Error: $name: 12px is not a string."
+);
+error!(
+    mixin_exists_non_string,
+    "a {color:mixin-exists(12px)}", "Error: $name: 12px is not a string."
+);
 // test!(
 //     inspect_empty_list,
 //     "a {\n  color: inspect(())\n}\n",