style: Allow :is() and :where() to have better error recovery.

Adjust is-where-parsing.html to work with both the new and old behavior,
and add a test for the new behavior.

Depends on D90049

Differential Revision: https://phabricator.services.mozilla.com/D90050

components/selectors/parser.rs

@@ -251,6 +251,11 @@ pub trait Parser<'i> {
         false
     }
 
+    /// The error recovery that selector lists inside :is() and :where() have.
+    fn is_and_where_error_recovery(&self) -> ParseErrorRecovery {
+        ParseErrorRecovery::IgnoreInvalidSelector
+    }
+
     /// Whether the given function name is an alias for the `:is()` function.
     fn is_is_alias(&self, _name: &str) -> bool {
         false
@@ -329,6 +334,17 @@ pub struct SelectorList<Impl: SelectorImpl>(
     #[shmem(field_bound)] pub SmallVec<[Selector<Impl>; 1]>,
 );
 
+/// How to treat invalid selectors in a selector list.
+pub enum ParseErrorRecovery {
+    /// Discard the entire selector list, this is the default behavior for
+    /// almost all of CSS.
+    DiscardList,
+    /// Ignore invalid selectors, potentially creating an empty selector list.
+    ///
+    /// This is the error recovery mode of :is() and :where()
+    IgnoreInvalidSelector,
+}
+
 impl<Impl: SelectorImpl> SelectorList<Impl> {
     /// Parse a comma-separated list of Selectors.
     /// <https://drafts.csswg.org/selectors/#grouping>
@@ -341,26 +357,42 @@ impl<Impl: SelectorImpl> SelectorList<Impl> {
     where
         P: Parser<'i, Impl = Impl>,
     {
-        Self::parse_with_state(parser, input, SelectorParsingState::empty())
+        Self::parse_with_state(parser, input, SelectorParsingState::empty(), ParseErrorRecovery::DiscardList)
     }
 
+    #[inline]
     fn parse_with_state<'i, 't, P>(
         parser: &P,
         input: &mut CssParser<'i, 't>,
         state: SelectorParsingState,
+        recovery: ParseErrorRecovery,
     ) -> Result<Self, ParseError<'i, P::Error>>
     where
         P: Parser<'i, Impl = Impl>,
     {
         let mut values = SmallVec::new();
         loop {
-            values.push(input.parse_until_before(Delimiter::Comma, |input| {
+            let selector = input.parse_until_before(Delimiter::Comma, |input| {
                 parse_selector(parser, input, state)
-            })?);
-            match input.next() {
-                Err(_) => return Ok(SelectorList(values)),
-                Ok(&Token::Comma) => continue,
-                Ok(_) => unreachable!(),
+            });
+
+            let was_ok = selector.is_ok();
+            match selector {
+                Ok(selector) => values.push(selector),
+                Err(err) => match recovery {
+                    ParseErrorRecovery::DiscardList => return Err(err),
+                    ParseErrorRecovery::IgnoreInvalidSelector => {},
+                },
+            }
+
+            loop {
+                match input.next() {
+                    Err(_) => return Ok(SelectorList(values)),
+                    Ok(&Token::Comma) => break,
+                    Ok(_) => {
+                        debug_assert!(!was_ok, "Shouldn't have got a selector if getting here");
+                    }
+                }
             }
         }
     }
@@ -1251,18 +1283,18 @@ impl<Impl: SelectorImpl> Debug for LocalName<Impl> {
     }
 }
 
-fn serialize_selector_list<'a, Impl, I, W>(mut iter: I, dest: &mut W) -> fmt::Result
+fn serialize_selector_list<'a, Impl, I, W>(iter: I, dest: &mut W) -> fmt::Result
 where
     Impl: SelectorImpl,
     I: Iterator<Item = &'a Selector<Impl>>,
     W: fmt::Write,
 {
-    let first = iter
-        .next()
-        .expect("Empty SelectorList, should contain at least one selector");
-    first.to_css(dest)?;
+    let mut first = true;
     for selector in iter {
-        dest.write_str(", ")?;
+        if !first {
+            dest.write_str(", ")?;
+        }
+        first = false;
         selector.to_css(dest)?;
     }
     Ok(())
@@ -2266,6 +2298,7 @@ where
         parser,
         input,
         state | SelectorParsingState::DISALLOW_PSEUDOS,
+        parser.is_and_where_error_recovery(),
     )?;
     Ok(component(inner.0.into_vec().into_boxed_slice()))
 }