implement manual_partial_ord_impl

first try at this

tests/ui/manual_partial_ord_impl.rs

@@ -0,0 +1,57 @@
+#![allow(unused)]
+#![warn(clippy::manual_partial_ord_impl)]
+#![no_main]
+
+use std::cmp::Ordering;
+
+// lint
+
+#[derive(Eq, PartialEq)]
+struct A(u32);
+
+impl Ord for A {
+    fn cmp(&self, other: &Self) -> Ordering {
+        todo!();
+    }
+}
+
+impl PartialOrd for A {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        todo!();
+    }
+}
+
+// do not lint
+
+#[derive(Eq, PartialEq)]
+struct B(u32);
+
+impl Ord for B {
+    fn cmp(&self, other: &Self) -> Ordering {
+        todo!();
+    }
+}
+
+impl PartialOrd for B {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+// lint, but we cannot give a suggestion since &Self is not named
+
+#[derive(Eq, PartialEq)]
+struct C(u32);
+
+impl Ord for C {
+    fn cmp(&self, other: &Self) -> Ordering {
+        todo!();
+    }
+}
+
+impl PartialOrd for C {
+    fn partial_cmp(&self, _: &Self) -> Option<Ordering> {
+        todo!();
+    }
+}
+

tests/ui/manual_partial_ord_impl.stderr

@@ -0,0 +1,28 @@
+error: manual implementation of `PartialOrd` when `Ord` is already implemented
+  --> $DIR/manual_partial_ord_impl.rs:18:1
+   |
+LL | /  impl PartialOrd for A {
+LL | |      fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+   | | _____________________________________________________________-
+LL | ||         todo!();
+LL | ||     }
+   | ||_____- help: change this to: `{ Some(self.cmp(other)) }`
+LL | |  }
+   | |__^
+   |
+   = note: `-D clippy::manual-partial-ord-impl` implied by `-D warnings`
+
+error: manual implementation of `PartialOrd` when `Ord` is already implemented
+  --> $DIR/manual_partial_ord_impl.rs:52:1
+   |
+LL | / impl PartialOrd for C {
+LL | |     fn partial_cmp(&self, _: &Self) -> Option<Ordering> {
+LL | |         todo!();
+LL | |     }
+LL | | }
+   | |_^
+   |
+   = help: return the value of `self.cmp` wrapped in `Some` instead
+
+error: aborting due to 2 previous errors
+