style: Refactor the selector parser to make implementing ::part() easier.

::slotted() is already weird in the sense that it supports a pseudo-element
afterwards (so ::slotted(*)::before is valid for example).

::part() is weirder because you are supposed to allow stuff like
::part(foo):hover, ::part(foo):hover::before, etc.

In order to avoid making the already-complex parse_compound_selector more
complex, shuffle stuff so that we pass the progress of our current compound
selector around, and is the parsing code for each selector which decides whether
it's ok to parse at the given point.

Differential Revision: https://phabricator.services.mozilla.com/D27158

components/selectors/parser.rs

@@ -30,17 +30,10 @@ pub trait PseudoElement: Sized + ToCss {
 
     /// Whether the pseudo-element supports a given state selector to the right
     /// of it.
-    fn supports_pseudo_class(
-        &self,
-        _pseudo_class: &<Self::Impl as SelectorImpl>::NonTSPseudoClass,
-    ) -> bool {
-        false
-    }
+    fn accepts_state_pseudo_classes(&self) -> bool { false }
 
     /// Whether this pseudo-element is valid after a ::slotted(..) pseudo.
-    fn valid_after_slotted(&self) -> bool {
-        false
-    }
+    fn valid_after_slotted(&self) -> bool { false }
 }
 
 /// A trait that represents a pseudo-class.
@@ -50,6 +43,11 @@ pub trait NonTSPseudoClass: Sized + ToCss {
 
     /// Whether this pseudo-class is :active or :hover.
     fn is_active_or_hover(&self) -> bool;
+
+    /// Whether this pseudo-class belongs to:
+    ///
+    /// https://drafts.csswg.org/selectors-4/#useraction-pseudos
+    fn is_user_action_state(&self) -> bool;
 }
 
 /// Returns a Cow::Borrowed if `s` is already ASCII lowercase, and a
@@ -64,6 +62,55 @@ fn to_ascii_lowercase(s: &str) -> Cow<str> {
     }
 }
 
+bitflags! {
+    /// Flags that indicate at which point of parsing a selector are we.
+    struct SelectorParsingState: u8 {
+        /// Whether we're inside a negation. If we're inside a negation, we're
+        /// not allowed to add another negation or such, for example.
+        const INSIDE_NEGATION = 1 << 0;
+        /// Whether we've parsed an ::slotted() pseudo-element already.
+        ///
+        /// If so, then we can only parse a subset of pseudo-elements, and
+        /// whatever comes after them if so.
+        const AFTER_SLOTTED = 1 << 1;
+        /// Whether we've parsed a pseudo-element (as in, an
+        /// `Impl::PseudoElement` thus not accounting for `::slotted`) already.
+        ///
+        /// If so, then other pseudo-elements and most other selectors are
+        /// disallowed.
+        const AFTER_PSEUDO_ELEMENT = 1 << 2;
+        /// Whether we've parsed a non-stateful pseudo-element (again, as-in
+        /// `Impl::PseudoElement`) already. If so, then other pseudo-classes are
+        /// disallowed. If this flag is set, `AFTER_PSEUDO_ELEMENT` must be set
+        /// as well.
+        const AFTER_NON_STATEFUL_PSEUDO_ELEMENT = 1 << 3;
+        /// Whether we are after any of the pseudo-like things.
+        const AFTER_PSEUDO = Self::AFTER_SLOTTED.bits | Self::AFTER_PSEUDO_ELEMENT.bits;
+    }
+}
+
+impl SelectorParsingState {
+    #[inline]
+    fn allows_functional_pseudo_classes(self) -> bool {
+        !self.intersects(SelectorParsingState::AFTER_PSEUDO)
+    }
+
+    #[inline]
+    fn allows_slotted(self) -> bool {
+        !self.intersects(SelectorParsingState::AFTER_PSEUDO)
+    }
+
+    #[inline]
+    fn allows_non_functional_pseudo_classes(self) -> bool {
+        !self.intersects(SelectorParsingState::AFTER_SLOTTED | SelectorParsingState::AFTER_NON_STATEFUL_PSEUDO_ELEMENT)
+    }
+
+    #[inline]
+    fn allows_tree_structural_pseudo_classes(self) -> bool {
+        !self.intersects(SelectorParsingState::AFTER_PSEUDO)
+    }
+}
+
 pub type SelectorParseError<'i> = ParseError<'i, SelectorParseErrorKind<'i>>;
 
 #[derive(Clone, Debug, PartialEq)]
@@ -76,6 +123,7 @@ pub enum SelectorParseErrorKind<'i> {
     NonCompoundSelector,
     NonPseudoElementAfterSlotted,
     InvalidPseudoElementAfterSlotted,
+    InvalidState,
     UnexpectedTokenInAttributeSelector(Token<'i>),
     PseudoElementExpectedColon(Token<'i>),
     PseudoElementExpectedIdent(Token<'i>),
@@ -1369,9 +1417,9 @@ where
     'outer_loop: loop {
         // Parse a sequence of simple selectors.
         match parse_compound_selector(parser, input, &mut builder)? {
-            Some((has_pseudo, slot)) => {
-                has_pseudo_element = has_pseudo;
-                slotted = slot;
+            Some(state) => {
+                has_pseudo_element = state.intersects(SelectorParsingState::AFTER_PSEUDO_ELEMENT);
+                slotted = state.intersects(SelectorParsingState::AFTER_SLOTTED);
             },
             None => {
                 return Err(input.new_custom_error(if builder.has_combinators() {
@@ -1848,7 +1896,7 @@ where
         Err(e) => return Err(e.into()),
     };
     if !is_type_sel {
-        match parse_one_simple_selector(parser, input, /* inside_negation = */ true)? {
+        match parse_one_simple_selector(parser, input, SelectorParsingState::INSIDE_NEGATION)? {
             Some(SimpleSelectorParseResult::SimpleSelector(s)) => {
                 sequence.push(s);
             },
@@ -1875,14 +1923,11 @@ where
 ///
 /// `Err(())` means invalid selector.
 /// `Ok(None)` is an empty selector
-///
-/// The booleans represent whether a pseudo-element has been parsed, and whether
-/// ::slotted() has been parsed, respectively.
 fn parse_compound_selector<'i, 't, P, Impl>(
     parser: &P,
     input: &mut CssParser<'i, 't>,
     builder: &mut SelectorBuilder<Impl>,
-) -> Result<Option<(bool, bool)>, ParseError<'i, P::Error>>
+) -> Result<Option<SelectorParsingState>, ParseError<'i, P::Error>>
 where
     P: Parser<'i, Impl = Impl>,
     Impl: SelectorImpl,
@@ -1901,122 +1946,44 @@ where
         empty = false;
     }
 
-    let mut pseudo = false;
-    let mut slot = false;
+    let mut state = SelectorParsingState::empty();
     loop {
         let parse_result =
-            match parse_one_simple_selector(parser, input, /* inside_negation = */ false)? {
+            match parse_one_simple_selector(parser, input, state)? {
                 None => break,
                 Some(result) => result,
             };
 
         empty = false;
 
-        let slotted_selector;
-        let pseudo_element;
-
         match parse_result {
             SimpleSelectorParseResult::SimpleSelector(s) => {
                 builder.push_simple_selector(s);
-                continue;
-            },
-            SimpleSelectorParseResult::PseudoElement(p) => {
-                slotted_selector = None;
-                pseudo_element = Some(p);
             },
             SimpleSelectorParseResult::SlottedPseudo(selector) => {
-                slotted_selector = Some(selector);
-                let maybe_pseudo =
-                    parse_one_simple_selector(parser, input, /* inside_negation = */ false)?;
-
-                pseudo_element = match maybe_pseudo {
-                    None => None,
-                    Some(SimpleSelectorParseResult::PseudoElement(pseudo)) => {
-                        if !pseudo.valid_after_slotted() {
-                            return Err(input.new_custom_error(
-                                SelectorParseErrorKind::InvalidPseudoElementAfterSlotted,
-                            ));
-                        }
-                        Some(pseudo)
-                    },
-                    Some(SimpleSelectorParseResult::SimpleSelector(..)) |
-                    Some(SimpleSelectorParseResult::SlottedPseudo(..)) => {
-                        return Err(input.new_custom_error(
-                            SelectorParseErrorKind::NonPseudoElementAfterSlotted,
-                        ));
-                    },
-                };
+                state.insert(SelectorParsingState::AFTER_SLOTTED);
+                if !builder.is_empty() {
+                    builder.push_combinator(Combinator::SlotAssignment);
+                }
+                builder.push_simple_selector(Component::Slotted(selector));
+            },
+            SimpleSelectorParseResult::PseudoElement(p) => {
+                state.insert(SelectorParsingState::AFTER_PSEUDO_ELEMENT);
+                if !p.accepts_state_pseudo_classes() {
+                    state.insert(SelectorParsingState::AFTER_NON_STATEFUL_PSEUDO_ELEMENT);
+                }
+                if !builder.is_empty() {
+                    builder.push_combinator(Combinator::PseudoElement);
+                }
+                builder.push_simple_selector(Component::PseudoElement(p));
             },
         }
-
-        debug_assert!(slotted_selector.is_some() || pseudo_element.is_some());
-        // Try to parse state to the right of the pseudo-element.
-        //
-        // There are only 3 allowable state selectors that can go on
-        // pseudo-elements as of right now.
-        let mut state_selectors = SmallVec::<[Component<Impl>; 3]>::new();
-        if let Some(ref p) = pseudo_element {
-            loop {
-                let location = input.current_source_location();
-                match input.next_including_whitespace() {
-                    Ok(&Token::Colon) => {},
-                    Ok(&Token::WhiteSpace(_)) | Err(_) => break,
-                    Ok(t) => {
-                        let e = SelectorParseErrorKind::PseudoElementExpectedColon(t.clone());
-                        return Err(location.new_custom_error(e));
-                    },
-                }
-
-                let location = input.current_source_location();
-                // TODO(emilio): Functional pseudo-classes too?
-                // We don't need it for now.
-                let name = match input.next_including_whitespace()? {
-                    &Token::Ident(ref name) => name.clone(),
-                    t => {
-                        return Err(location.new_custom_error(
-                            SelectorParseErrorKind::NoIdentForPseudo(t.clone()),
-                        ));
-                    },
-                };
-
-                let pseudo_class = P::parse_non_ts_pseudo_class(parser, location, name.clone())?;
-                if !p.supports_pseudo_class(&pseudo_class) {
-                    return Err(input.new_custom_error(
-                        SelectorParseErrorKind::UnsupportedPseudoClassOrElement(name),
-                    ));
-                }
-                state_selectors.push(Component::NonTSPseudoClass(pseudo_class));
-            }
-        }
-
-        if let Some(slotted) = slotted_selector {
-            slot = true;
-            if !builder.is_empty() {
-                builder.push_combinator(Combinator::SlotAssignment);
-            }
-            builder.push_simple_selector(Component::Slotted(slotted));
-        }
-
-        if let Some(p) = pseudo_element {
-            pseudo = true;
-            if !builder.is_empty() {
-                builder.push_combinator(Combinator::PseudoElement);
-            }
-
-            builder.push_simple_selector(Component::PseudoElement(p));
-
-            for state_selector in state_selectors.drain() {
-                builder.push_simple_selector(state_selector);
-            }
-        }
-
-        break;
     }
     if empty {
         // An empty selector is invalid.
         Ok(None)
     } else {
-        Ok(Some((pseudo, slot)))
+        Ok(Some(state))
     }
 }
 
@@ -2024,12 +1991,18 @@ fn parse_functional_pseudo_class<'i, 't, P, Impl>(
     parser: &P,
     input: &mut CssParser<'i, 't>,
     name: CowRcStr<'i>,
-    inside_negation: bool,
+    state: SelectorParsingState,
 ) -> Result<Component<Impl>, ParseError<'i, P::Error>>
 where
     P: Parser<'i, Impl = Impl>,
     Impl: SelectorImpl,
 {
+    if !state.allows_functional_pseudo_classes() {
+        return Err(input.new_custom_error(
+            SelectorParseErrorKind::InvalidState
+        ));
+    }
+    debug_assert!(state.allows_tree_structural_pseudo_classes());
     match_ignore_ascii_case! { &name,
         "nth-child" => return Ok(parse_nth_pseudo_class(input, Component::NthChild)?),
         "nth-of-type" => return Ok(parse_nth_pseudo_class(input, Component::NthOfType)?),
@@ -2037,11 +2010,12 @@ where
         "nth-last-of-type" => return Ok(parse_nth_pseudo_class(input, Component::NthLastOfType)?),
         "host" => return Ok(Component::Host(Some(parse_inner_compound_selector(parser, input)?))),
         "not" => {
-            if inside_negation {
+            if state.intersects(SelectorParsingState::INSIDE_NEGATION) {
                 return Err(input.new_custom_error(
                     SelectorParseErrorKind::UnexpectedIdent("not".into())
                 ));
             }
+            debug_assert!(state.is_empty());
             return parse_negation(parser, input)
         },
         _ => {}
@@ -2080,37 +2054,52 @@ pub fn is_css2_pseudo_element(name: &str) -> bool {
 fn parse_one_simple_selector<'i, 't, P, Impl>(
     parser: &P,
     input: &mut CssParser<'i, 't>,
-    inside_negation: bool,
+    state: SelectorParsingState,
 ) -> Result<Option<SimpleSelectorParseResult<Impl>>, ParseError<'i, P::Error>>
 where
     P: Parser<'i, Impl = Impl>,
     Impl: SelectorImpl,
 {
     let start = input.state();
-    // FIXME: remove clone() when lifetimes are non-lexical
-    match input.next_including_whitespace().map(|t| t.clone()) {
-        Ok(Token::IDHash(id)) => {
+    let token = match input.next_including_whitespace().map(|t| t.clone()) {
+        Ok(t) => t,
+        Err(..) => {
+            input.reset(&start);
+            return Ok(None);
+        }
+    };
+
+    Ok(Some(match token {
+        Token::IDHash(id) => {
+            if state.intersects(SelectorParsingState::AFTER_PSEUDO) {
+                return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+            }
             let id = Component::ID(id.as_ref().into());
-            Ok(Some(SimpleSelectorParseResult::SimpleSelector(id)))
+            SimpleSelectorParseResult::SimpleSelector(id)
         },
-        Ok(Token::Delim('.')) => {
+        Token::Delim('.') => {
+            if state.intersects(SelectorParsingState::AFTER_PSEUDO) {
+                return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+            }
             let location = input.current_source_location();
-            match *input.next_including_whitespace()? {
-                Token::Ident(ref class) => {
-                    let class = Component::Class(class.as_ref().into());
-                    Ok(Some(SimpleSelectorParseResult::SimpleSelector(class)))
-                },
+            let class = match *input.next_including_whitespace()? {
+                Token::Ident(ref class) => class,
                 ref t => {
                     let e = SelectorParseErrorKind::ClassNeedsIdent(t.clone());
-                    Err(location.new_custom_error(e))
+                    return Err(location.new_custom_error(e))
                 },
+            };
+            let class = Component::Class(class.as_ref().into());
+            SimpleSelectorParseResult::SimpleSelector(class)
+        },
+        Token::SquareBracketBlock => {
+            if state.intersects(SelectorParsingState::AFTER_PSEUDO) {
+                return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
             }
-        },
-        Ok(Token::SquareBracketBlock) => {
             let attr = input.parse_nested_block(|input| parse_attribute_selector(parser, input))?;
-            Ok(Some(SimpleSelectorParseResult::SimpleSelector(attr)))
+            SimpleSelectorParseResult::SimpleSelector(attr)
         },
-        Ok(Token::Colon) => {
+        Token::Colon => {
             let location = input.current_source_location();
             let (is_single_colon, next_token) = match input.next_including_whitespace()?.clone() {
                 Token::Colon => (false, input.next_including_whitespace()?.clone()),
@@ -2127,69 +2116,83 @@ where
             let is_pseudo_element =
                 !is_single_colon || P::pseudo_element_allows_single_colon(&name);
             if is_pseudo_element {
-                let parse_result = if is_functional {
+                if state.intersects(SelectorParsingState::AFTER_PSEUDO_ELEMENT) {
+                    return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+                }
+                let pseudo_element = if is_functional {
                     if P::parse_slotted(parser) && name.eq_ignore_ascii_case("slotted") {
+                        if !state.allows_slotted() {
+                            return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+                        }
                         let selector = input.parse_nested_block(|input| {
                             parse_inner_compound_selector(parser, input)
                         })?;
-                        SimpleSelectorParseResult::SlottedPseudo(selector)
-                    } else {
-                        let selector = input.parse_nested_block(|input| {
-                            P::parse_functional_pseudo_element(parser, name, input)
-                        })?;
-                        SimpleSelectorParseResult::PseudoElement(selector)
+                        return Ok(Some(SimpleSelectorParseResult::SlottedPseudo(selector)));
                     }
+                    input.parse_nested_block(|input| {
+                        P::parse_functional_pseudo_element(parser, name, input)
+                    })?
                 } else {
-                    SimpleSelectorParseResult::PseudoElement(P::parse_pseudo_element(
-                        parser, location, name,
-                    )?)
+                    P::parse_pseudo_element(parser, location, name)?
                 };
-                Ok(Some(parse_result))
+
+                if state.intersects(SelectorParsingState::AFTER_SLOTTED) && !pseudo_element.valid_after_slotted() {
+                    return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+                }
+                SimpleSelectorParseResult::PseudoElement(pseudo_element)
             } else {
                 let pseudo_class = if is_functional {
                     input.parse_nested_block(|input| {
-                        parse_functional_pseudo_class(parser, input, name, inside_negation)
+                        parse_functional_pseudo_class(parser, input, name, state)
                     })?
                 } else {
-                    parse_simple_pseudo_class(parser, location, name)?
+                    parse_simple_pseudo_class(parser, location, name, state)?
                 };
-                Ok(Some(SimpleSelectorParseResult::SimpleSelector(
-                    pseudo_class,
-                )))
+                SimpleSelectorParseResult::SimpleSelector(pseudo_class)
             }
         },
         _ => {
             input.reset(&start);
-            Ok(None)
+            return Ok(None)
         },
-    }
+    }))
 }
 
 fn parse_simple_pseudo_class<'i, P, Impl>(
     parser: &P,
     location: SourceLocation,
     name: CowRcStr<'i>,
+    state: SelectorParsingState,
 ) -> Result<Component<Impl>, ParseError<'i, P::Error>>
 where
     P: Parser<'i, Impl = Impl>,
     Impl: SelectorImpl,
 {
-    (match_ignore_ascii_case! { &name,
-        "first-child" => Ok(Component::FirstChild),
-        "last-child"  => Ok(Component::LastChild),
-        "only-child"  => Ok(Component::OnlyChild),
-        "root" => Ok(Component::Root),
-        "empty" => Ok(Component::Empty),
-        "scope" => Ok(Component::Scope),
-        "host" if P::parse_host(parser) => Ok(Component::Host(None)),
-        "first-of-type" => Ok(Component::FirstOfType),
-        "last-of-type" => Ok(Component::LastOfType),
-        "only-of-type" => Ok(Component::OnlyOfType),
-        _ => Err(())
-    })
-    .or_else(|()| {
-        P::parse_non_ts_pseudo_class(parser, location, name).map(Component::NonTSPseudoClass)
-    })
+    if !state.allows_non_functional_pseudo_classes() {
+        return Err(location.new_custom_error(SelectorParseErrorKind::InvalidState));
+    }
+
+    if state.allows_tree_structural_pseudo_classes() {
+        match_ignore_ascii_case! { &name,
+            "first-child" => return Ok(Component::FirstChild),
+            "last-child" => return Ok(Component::LastChild),
+            "only-child" => return Ok(Component::OnlyChild),
+            "root" => return Ok(Component::Root),
+            "empty" => return Ok(Component::Empty),
+            "scope" => return Ok(Component::Scope),
+            "host" if P::parse_host(parser) => return Ok(Component::Host(None)),
+            "first-of-type" => return Ok(Component::FirstOfType),
+            "last-of-type" => return Ok(Component::LastOfType),
+            "only-of-type" => return Ok(Component::OnlyOfType),
+            _ => {},
+        }
+    }
+
+    let pseudo_class = P::parse_non_ts_pseudo_class(parser, location, name)?;
+    if state.intersects(SelectorParsingState::AFTER_PSEUDO_ELEMENT) && !pseudo_class.is_user_action_state() {
+        return Err(location.new_custom_error(SelectorParseErrorKind::InvalidState));
+    }
+    Ok(Component::NonTSPseudoClass(pseudo_class))
 }
 
 // NB: pub module in order to access the DummyParser
@@ -2218,16 +2221,9 @@ pub mod tests {
     impl parser::PseudoElement for PseudoElement {
         type Impl = DummySelectorImpl;
 
-        fn supports_pseudo_class(&self, pc: &PseudoClass) -> bool {
-            match *pc {
-                PseudoClass::Hover => true,
-                PseudoClass::Active | PseudoClass::Lang(..) => false,
-            }
-        }
+        fn accepts_state_pseudo_classes(&self) -> bool { true }
 
-        fn valid_after_slotted(&self) -> bool {
-            true
-        }
+        fn valid_after_slotted(&self) -> bool { true }
     }
 
     impl parser::NonTSPseudoClass for PseudoClass {
@@ -2237,6 +2233,11 @@ pub mod tests {
         fn is_active_or_hover(&self) -> bool {
             matches!(*self, PseudoClass::Active | PseudoClass::Hover)
         }
+
+        #[inline]
+        fn is_user_action_state(&self) -> bool {
+            self.is_active_or_hover()
+        }
     }
 
     impl ToCss for PseudoClass {
@@ -2789,11 +2790,11 @@ pub mod tests {
                 specificity(0, 2, 1) | HAS_PSEUDO_BIT,
             )]))
         );
-        assert!(parse("::before:hover:active").is_err());
+        assert!(parse("::before:hover:lang(foo)").is_err());
         assert!(parse("::before:hover .foo").is_err());
         assert!(parse("::before .foo").is_err());
         assert!(parse("::before ~ bar").is_err());
-        assert!(parse("::before:active").is_err());
+        assert!(parse("::before:active").is_ok());
 
         // https://github.com/servo/servo/issues/15335
         assert!(parse(":: before").is_err());