style: Fix insertRule with layer statements before imports

We need to do a bit more nuanced check because @layer statements might
go before imports.

Differential Revision: https://phabricator.services.mozilla.com/D144996

components/style/stylesheets/mod.rs

@@ -385,16 +385,6 @@ impl CssRule {
         }
     }
 
-    fn rule_state(&self) -> State {
-        match *self {
-            // CssRule::Charset(..) => State::Start,
-            CssRule::Import(..) => State::Imports,
-            CssRule::Namespace(..) => State::Namespaces,
-            // TODO(emilio): Do we need something for EarlyLayers?
-            _ => State::Body,
-        }
-    }
-
     /// Parse a CSS rule.
     ///
     /// Returns a parsed CSS rule and the final state of the parser.

components/style/stylesheets/rule_list.rs

@@ -153,21 +153,17 @@ impl CssRulesHelpers for RawOffsetArc<Locked<CssRules>> {
             }
 
             // Computes the parser state at the given index
+            let insert_rule_context = InsertRuleContext {
+                rule_list: &rules.0,
+                index,
+            };
+
             let state = if nested {
                 State::Body
             } else if index == 0 {
                 State::Start
             } else {
-                rules
-                    .0
-                    .get(index - 1)
-                    .map(CssRule::rule_state)
-                    .unwrap_or(State::Body)
-            };
-
-            let insert_rule_context = InsertRuleContext {
-                rule_list: &rules.0,
-                index,
+                insert_rule_context.max_rule_state_at_index(index - 1)
             };
 
             // Steps 3, 4, 5, 6

components/style/stylesheets/rule_parser.rs

@@ -48,6 +48,36 @@ pub struct InsertRuleContext<'a> {
     pub index: usize,
 }
 
+impl<'a> InsertRuleContext<'a> {
+    /// Returns the max rule state allowable for insertion at a given index in
+    /// the rule list.
+    pub fn max_rule_state_at_index(&self, index: usize) -> State {
+        let rule = match self.rule_list.get(index) {
+            Some(rule) => rule,
+            None => return State::Body,
+        };
+        match rule {
+            CssRule::Import(..) => State::Imports,
+            CssRule::Namespace(..) => State::Namespaces,
+            CssRule::LayerStatement(..) => {
+                // If there are @import / @namespace after this layer, then
+                // we're in the early-layers phase, otherwise we're in the body
+                // and everything is fair game.
+                let next_non_layer_statement_rule = self.rule_list[index + 1..]
+                    .iter()
+                    .find(|r| !matches!(*r, CssRule::LayerStatement(..)));
+                if let Some(non_layer) = next_non_layer_statement_rule {
+                    if matches!(*non_layer, CssRule::Import(..) | CssRule::Namespace(..)) {
+                        return State::EarlyLayers;
+                    }
+                }
+                State::Body
+            }
+            _ => State::Body,
+        }
+    }
+}
+
 /// The parser for the top-level rules in a stylesheet.
 pub struct TopLevelRuleParser<'a> {
     /// A reference to the lock we need to use to create rules.
@@ -105,12 +135,8 @@ impl<'b> TopLevelRuleParser<'b> {
             None => return true,
         };
 
-        let next_rule_state = match ctx.rule_list.get(ctx.index) {
-            None => return true,
-            Some(rule) => rule.rule_state(),
-        };
-
-        if new_state > next_rule_state {
+        let max_rule_state = ctx.max_rule_state_at_index(ctx.index);
+        if new_state > max_rule_state {
             self.dom_error = Some(RulesMutateError::HierarchyRequest);
             return false;
         }
@@ -267,11 +293,23 @@ impl<'a, 'i> AtRuleParser<'i> for TopLevelRuleParser<'a> {
                 self.dom_error = Some(RulesMutateError::HierarchyRequest);
                 return Err(input.new_custom_error(StyleParseErrorKind::UnexpectedCharsetRule))
             },
-            _ => {}
-        }
-
-        if !self.check_state(State::Body) {
-            return Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError));
+            "layer" => {
+                let state_to_check = if self.state <= State::EarlyLayers {
+                    // The real state depends on whether there's a block or not.
+                    // We don't know that yet, but the parse_block check deals
+                    // with that.
+                    State::EarlyLayers
+                } else {
+                    State::Body
+                };
+                if !self.check_state(state_to_check) {
+                    return Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError));
+                }
+            },
+            _ => {
+                // All other rules have blocks, so we do this check early in
+                // parse_block instead.
+            }
         }
 
         AtRuleParser::parse_prelude(&mut self.nested(), name, input)
@@ -284,6 +322,9 @@ impl<'a, 'i> AtRuleParser<'i> for TopLevelRuleParser<'a> {
         start: &ParserState,
         input: &mut Parser<'i, 't>,
     ) -> Result<Self::AtRule, ParseError<'i>> {
+        if !self.check_state(State::Body) {
+            return Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError));
+        }
         let rule = AtRuleParser::parse_block(&mut self.nested(), prelude, start, input)?;
         self.state = State::Body;
         Ok((start.position(), rule))