Bug 1639533 - Fix a case where we'd allow parsing functional :host incorrectly. r=heycam

This is a missing check I should've introduced in bug 1632647.

Differential Revision: https://phabricator.services.mozilla.com/D76161

layout/style/test/test_selectors.html

@@ -1310,6 +1310,9 @@ function runTests() {
     test_parseable("::-moz-color-swatch:hover");
     test_balanced_unparseable("::-moz-color-swatch:not(.foo)");
     test_balanced_unparseable("::-moz-color-swatch:first-child");
+    test_balanced_unparseable("::-moz-color-swatch:host");
+    test_balanced_unparseable("::-moz-color-swatch:host(div)");
+    test_balanced_unparseable("::-moz-color-swatch:nth-child(1)");
     test_balanced_unparseable("::-moz-color-swatch:hover#foo");
     test_balanced_unparseable(".foo::after:not(.bar) ~ h3");
 

servo/components/selectors/parser.rs

@@ -2207,7 +2207,12 @@ where
         "nth-last-of-type" => return parse_nth_pseudo_class(parser, input, state, Component::NthLastOfType),
         "is" if parser.parse_is_and_where() => return parse_is_or_where(parser, input, state, Component::Is),
         "where" if parser.parse_is_and_where() => return parse_is_or_where(parser, input, state, Component::Where),
-        "host" => return Ok(Component::Host(Some(parse_inner_compound_selector(parser, input, state)?))),
+        "host" => {
+            if !state.allows_tree_structural_pseudo_classes() {
+                return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+            }
+            return Ok(Component::Host(Some(parse_inner_compound_selector(parser, input, state)?)));
+        },
         "not" => {
             if state.intersects(SelectorParsingState::INSIDE_NEGATION) {
                 return Err(input.new_custom_error(

testing/web-platform/tests/css/css-scoping/slotted-parsing.html

@@ -14,6 +14,8 @@
   test_invalid_selector("::slotted(*).class");
   test_invalid_selector("::slotted(*)#id {}");
   test_invalid_selector("::slotted(*)[attr]");
+  test_invalid_selector("::slotted(*):host");
+  test_invalid_selector("::slotted(*):host(div)");
   test_invalid_selector("::slotted(*):hover");
   test_invalid_selector("::slotted(*):read-only");
   test_invalid_selector("::slotted(*)::slotted(*)");