Add a lint to check needless Waker clones

tests/ui/waker_clone_wake.fixed

@@ -0,0 +1,22 @@
+#[derive(Clone)]
+pub struct Custom;
+
+impl Custom {
+    pub fn wake(self) {}
+}
+
+pub fn wake(cx: &mut std::task::Context) {
+    cx.waker().wake_by_ref();
+
+    // We don't do that for now
+    let w = cx.waker().clone();
+    w.wake();
+
+    cx.waker().clone().wake_by_ref();
+}
+
+pub fn no_lint(c: &Custom) {
+    c.clone().wake()
+}
+
+fn main() {}

tests/ui/waker_clone_wake.rs

@@ -0,0 +1,22 @@
+#[derive(Clone)]
+pub struct Custom;
+
+impl Custom {
+    pub fn wake(self) {}
+}
+
+pub fn wake(cx: &mut std::task::Context) {
+    cx.waker().clone().wake();
+
+    // We don't do that for now
+    let w = cx.waker().clone();
+    w.wake();
+
+    cx.waker().clone().wake_by_ref();
+}
+
+pub fn no_lint(c: &Custom) {
+    c.clone().wake()
+}
+
+fn main() {}

tests/ui/waker_clone_wake.stderr

@@ -0,0 +1,11 @@
+error: cloning a `Waker` only to wake it
+  --> $DIR/waker_clone_wake.rs:9:5
+   |
+LL |     cx.waker().clone().wake();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `cx.waker().wake_by_ref()`
+   |
+   = note: `-D clippy::waker-clone-wake` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::waker_clone_wake)]`
+
+error: aborting due to previous error
+