Move util::vec::ForgetfulSink to style::sink and simplify it

components/style/sink.rs

@@ -0,0 +1,40 @@
+/* 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/. */
+
+use smallvec::{Array, SmallVec};
+use std::marker::PhantomData;
+
+pub trait Push<T> {
+    fn push(&mut self, value: T);
+}
+
+impl<T> Push<T> for Vec<T> {
+    fn push(&mut self, value: T) {
+        Vec::push(self, value);
+    }
+}
+
+impl<A: Array> Push<A::Item> for SmallVec<A> {
+    fn push(&mut self, value: A::Item) {
+        SmallVec::push(self, value);
+    }
+}
+
+pub struct ForgetfulSink<T>(bool, PhantomData<T>);
+
+impl<T> ForgetfulSink<T> {
+    pub fn new() -> Self {
+        ForgetfulSink(true, PhantomData)
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.0
+    }
+}
+
+impl<T> Push<T> for ForgetfulSink<T> {
+    fn push(&mut self, _value: T) {
+        self.0 = false;
+    }
+}