selectors: Remove custom attribute-affecting logic and sibling-affecting logic from rust-selectors.

MozReview-Commit-ID: BjZY6TjJbcb
Signed-off-by: Emilio Cobos Álvarez <emilio@crisal.io>

components/selectors/visitor.rs

@@ -0,0 +1,35 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//! Visitor traits for selectors.
+
+#![deny(missing_docs)]
+
+use parser::{AttrSelector, ComplexSelector, SelectorImpl, SimpleSelector};
+
+/// A trait to visit selector properties.
+///
+/// All the `visit_foo` methods return a boolean indicating whether the
+/// traversal should continue or not.
+pub trait SelectorVisitor {
+    /// The selector implementation this visitor wants to visit.
+    type Impl: SelectorImpl;
+
+    /// Visit an attribute selector that may match (there are other selectors
+    /// that may never match, like those containing whitespace or the empty
+    /// string).
+    fn visit_attribute_selector(&mut self, _: &AttrSelector<Self::Impl>) -> bool {
+        true
+    }
+
+    /// Visits a complex selector.
+    fn visit_complex_selector(&mut self, _: &ComplexSelector<Self::Impl>) -> bool {
+        true
+    }
+
+    /// Visits a simple selector.
+    fn visit_simple_selector(&mut self, _: &SimpleSelector<Self::Impl>) -> bool {
+        true
+    }
+}