Merge non-adjacent @layer rules

Closes #414

src/lib.rs

@@ -22609,6 +22609,32 @@ mod tests {
       "@import 'test.css' layer(foo, bar) {};",
       ParserError::UnexpectedToken(Token::Comma),
     );
+    minify_test(
+      r#"
+      @layer one {
+        body {
+          background: red;
+        }
+      }
+
+      body {
+        background: red;
+      }
+
+      @layer two {
+        body {
+          background: green;
+        }
+      }
+
+      @layer one {
+        body {
+          background: yellow;
+        }
+      }
+      "#,
+      "@layer one{body{background:#ff0}}body{background:red}@layer two{body{background:green}}",
+    );
   }
 
   #[test]

src/rules/layer.rs

@@ -15,7 +15,7 @@ use smallvec::SmallVec;
 /// a `@layer` or `@import` rule.
 ///
 /// Nested layers are represented using a list of identifiers. In CSS syntax, these are dot-separated.
-#[derive(Debug, Clone, PartialEq)]
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
 #[cfg_attr(feature = "into_owned", derive(lightningcss_derive::IntoOwned))]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize), serde(transparent))]
 #[cfg_attr(feature = "jsonschema", derive(schemars::JsonSchema))]

src/rules/mod.rs

@@ -430,6 +430,7 @@ impl<'i, T> CssRuleList<'i, T> {
     parent_is_unused: bool,
   ) -> Result<(), MinifyError> {
     let mut keyframe_rules = HashMap::new();
+    let mut layer_rules = HashMap::new();
     let mut rules = Vec::new();
     for mut rule in self.0.drain(..) {
       match &mut rule {
@@ -520,12 +521,18 @@ impl<'i, T> CssRuleList<'i, T> {
           }
         }
         CssRule::LayerBlock(layer) => {
-          if let Some(CssRule::LayerBlock(last_rule)) = rules.last_mut() {
-            if last_rule.name == layer.name {
-              last_rule.rules.0.extend(layer.rules.0.drain(..));
-              last_rule.minify(context, parent_is_unused)?;
-              continue;
+          // Merging non-adjacent layer rules is safe because they are applied
+          // in the order they are first defined.
+          if let Some(name) = &layer.name {
+            if let Some(idx) = layer_rules.get(name) {
+              if let Some(CssRule::LayerBlock(last_rule)) = rules.get_mut(*idx) {
+                last_rule.rules.0.extend(layer.rules.0.drain(..));
+                last_rule.minify(context, parent_is_unused)?;
+                continue;
+              }
             }
+
+            layer_rules.insert(name.clone(), rules.len());
           }
           if layer.minify(context, parent_is_unused)? {
             continue;