disallow selectors and styles in functions

src/parse/mod.rs

@@ -245,6 +245,13 @@ impl<'a> Parser<'a> {
                 // dart-sass seems to special-case the error message here?
                 '!' | '{' => return Err(("expected \"}\".", *pos).into()),
                 _ => {
+                    if self.flags.in_function() {
+                        return Err((
+                        "Functions can only contain variable declarations and control directives.",
+                        self.span_before
+                    )
+                        .into());
+                    }
                     if self.flags.in_keyframes() {
                         match self.is_selector_or_style()? {
                             SelectorOrStyle::Style(property, value) => {

tests/functions.rs

@@ -305,3 +305,16 @@ error!(
     }",
     "Error: This at-rule is not allowed here."
 );
+error!(
+    disallows_selectors,
+    "@function foo($a) {
+        functiona {
+            @return $a;
+        }
+    }
+
+    a {
+        color: foo(nul);
+    }",
+    "Error: Functions can only contain variable declarations and control directives."
+);