Auto merge of #18344 - bradwerth:selectorSerialize, r=SimonSapin

Change selector::to_css function to handle combinators in between universal selectors.

https://bugzilla.mozilla.org/show_bug.cgi?id=1391169
https://reviewboard.mozilla.org/r/172368/

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components/selectors/parser.rs

@@ -787,8 +787,8 @@ impl<Impl: SelectorImpl> ToCss for Selector<Impl> {
                 // something like `... > ::before`, because we store `>` and `::`
                 // both as combinators internally.
                 //
-                // If we are in this case, we continue to the next iteration of the
+                // If we are in this case, after we have serialized the universal
-                // `for compound in compound_selectors` loop.
+                // selector, we skip Step 2 and continue with the algorithm.
                 let (can_elide_namespace, first_non_namespace) = match &compound[0] {
                     &Component::ExplicitAnyNamespace |
                     &Component::ExplicitNoNamespace |
@@ -796,6 +796,7 @@ impl<Impl: SelectorImpl> ToCss for Selector<Impl> {
                     &Component::DefaultNamespace(_) => (true, 1),
                     _ => (true, 0),
                 };
+                let mut perform_step_2 = true;
                 if first_non_namespace == compound.len() - 1 {
                     match (combinators.peek(), &compound[first_non_namespace]) {
                         // We have to be careful here, because if there is a pseudo
@@ -811,7 +812,8 @@ impl<Impl: SelectorImpl> ToCss for Selector<Impl> {
                             for simple in compound.iter() {
                                 simple.to_css(dest)?;
                             }
-                            continue
+                            // Skip step 2, which is an "otherwise".
+                            perform_step_2 = false;
                         }
                         (_, _) => (),
                     }
@@ -826,6 +828,7 @@ impl<Impl: SelectorImpl> ToCss for Selector<Impl> {
                 // proposing to change this to match up with the behavior asserted
                 // in cssom/serialize-namespaced-type-selectors.html, which the
                 // following code tries to match.
+                if perform_step_2 {
                     for simple in compound.iter() {
                         if let Component::ExplicitUniversalType = *simple {
                             // Can't have a namespace followed by a pseudo-element
@@ -839,6 +842,7 @@ impl<Impl: SelectorImpl> ToCss for Selector<Impl> {
                         simple.to_css(dest)?;
                     }
                 }
+            }
 
             // 3. If this is not the last part of the chain of the selector
             //    append a single SPACE (U+0020), followed by the combinator

tests/unit/style/parsing/selectors.rs

@@ -25,4 +25,6 @@ fn test_selectors() {
     assert_roundtrip!(parse_selector, "svg|circle");
     assert_roundtrip!(parse_selector, "p:before", "p::before");
     assert_roundtrip!(parse_selector, "[border=\"0\"]:-servo-nonzero-border ~ ::-servo-details-summary");
+    assert_roundtrip!(parse_selector, "* > *");
+    assert_roundtrip!(parse_selector, "*|* + *", "* + *");
 }