Initial implementation of placeholder selectors

src/css.rs

@@ -72,7 +72,11 @@ impl Css {
                 super_selector,
                 rules,
             }) => {
-                let mut vals = vec![Toplevel::new_rule(super_selector.zip(&selector))];
+                let selector = super_selector.zip(&selector).remove_placeholders();
+                if selector.is_empty() {
+                    return Vec::new();
+                }
+                let mut vals = vec![Toplevel::new_rule(selector)];
                 for rule in rules {
                     match rule {
                         Stmt::RuleSet(_) => vals.extend(self.parse_stmt(rule)),

src/selector.rs

@@ -351,6 +351,34 @@ impl Selector {
         }
         Selector(rules)
     }
+
+    pub fn remove_placeholders(self) -> Selector {
+        let mut selectors = Vec::with_capacity(self.0.len());
+        let mut temp_sels = Vec::new();
+        let mut found_placeholder = false;
+        for sel in self.0 {
+            match sel {
+                SelectorKind::Placeholder => found_placeholder = true,
+                SelectorKind::Multiple => {
+                    temp_sels.push(SelectorKind::Multiple);
+                    if !found_placeholder {
+                        selectors.extend(temp_sels.clone());
+                    }
+                    temp_sels.clear();
+                    found_placeholder = false;
+                }
+                _ => temp_sels.push(sel),
+            }
+        }
+        if !found_placeholder {
+            selectors.extend(temp_sels);
+        }
+        Selector(selectors)
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
 }
 
 #[derive(Clone, Debug, Eq, PartialEq)]

tests/selectors.rs

@@ -277,3 +277,19 @@ test!(
 //     "a,, {\n  b {\n    color: /**/red;\n  }\n}\n",
 //     "a b {\n  color: red;\n}\n"
 // );
+test!(simple_placeholder, "%a {\n  color: red;\n}\n", "");
+test!(
+    placeholder_first,
+    "%a, b {\n  color: red;\n}\n",
+    "b {\n  color: red;\n}\n"
+);
+test!(
+    placeholder_last,
+    "a, %b {\n  color: red;\n}\n",
+    "a {\n  color: red;\n}\n"
+);
+test!(
+    placeholder_middle,
+    "a, %b, c {\n  color: red;\n}\n",
+    "a, c {\n  color: red;\n}\n"
+);