Allow Self::cmp(self, other) as a correct impl

tests/ui/incorrect_partial_ord_impl_on_ord_type.fixed

@@ -1,5 +1,4 @@
 //@run-rustfix
-#![allow(unused)]
 #![no_main]
 
 use std::cmp::Ordering;
@@ -112,3 +111,35 @@ impl PartialOrd<u32> for F {
         todo!();
     }
 }
+
+// #11178, do not lint
+
+#[derive(Eq, PartialEq)]
+struct G(u32);
+
+impl Ord for G {
+    fn cmp(&self, other: &Self) -> Ordering {
+        todo!();
+    }
+}
+
+impl PartialOrd for G {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(Self::cmp(self, other))
+    }
+}
+
+#[derive(Eq, PartialEq)]
+struct H(u32);
+
+impl Ord for H {
+    fn cmp(&self, other: &Self) -> Ordering {
+        todo!();
+    }
+}
+
+impl PartialOrd for H {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(Ord::cmp(self, other))
+    }
+}

tests/ui/incorrect_partial_ord_impl_on_ord_type.rs

@@ -1,5 +1,4 @@
 //@run-rustfix
-#![allow(unused)]
 #![no_main]
 
 use std::cmp::Ordering;
@@ -116,3 +115,35 @@ impl PartialOrd<u32> for F {
         todo!();
     }
 }
+
+// #11178, do not lint
+
+#[derive(Eq, PartialEq)]
+struct G(u32);
+
+impl Ord for G {
+    fn cmp(&self, other: &Self) -> Ordering {
+        todo!();
+    }
+}
+
+impl PartialOrd for G {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(Self::cmp(self, other))
+    }
+}
+
+#[derive(Eq, PartialEq)]
+struct H(u32);
+
+impl Ord for H {
+    fn cmp(&self, other: &Self) -> Ordering {
+        todo!();
+    }
+}
+
+impl PartialOrd for H {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(Ord::cmp(self, other))
+    }
+}

tests/ui/incorrect_partial_ord_impl_on_ord_type.stderr

@@ -1,5 +1,5 @@
 error: incorrect implementation of `partial_cmp` on an `Ord` type
-  --> $DIR/incorrect_partial_ord_impl_on_ord_type.rs:18:1
+  --> $DIR/incorrect_partial_ord_impl_on_ord_type.rs:17:1
    |
 LL | /  impl PartialOrd for A {
 LL | |      fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
@@ -13,7 +13,7 @@ LL | |  }
    = note: `#[deny(clippy::incorrect_partial_ord_impl_on_ord_type)]` on by default
 
 error: incorrect implementation of `partial_cmp` on an `Ord` type
-  --> $DIR/incorrect_partial_ord_impl_on_ord_type.rs:52:1
+  --> $DIR/incorrect_partial_ord_impl_on_ord_type.rs:51:1
    |
 LL | / impl PartialOrd for C {
 LL | |     fn partial_cmp(&self, _: &Self) -> Option<Ordering> {

tests/ui/incorrect_partial_ord_impl_on_ord_type_fully_qual.rs

@@ -0,0 +1,51 @@
+// This test's filename is... a bit verbose. But it ensures we suggest the correct code when `Ord`
+// is not in scope.
+#![no_main]
+#![no_implicit_prelude]
+
+extern crate std;
+
+use std::cmp::{self, Eq, Ordering, PartialEq, PartialOrd};
+use std::option::Option::{self, Some};
+use std::todo;
+
+// lint
+
+#[derive(Eq, PartialEq)]
+struct A(u32);
+
+impl cmp::Ord for A {
+    fn cmp(&self, other: &Self) -> Ordering {
+        todo!();
+    }
+}
+
+impl PartialOrd for A {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        // NOTE: This suggestion is wrong, as `Ord` is not in scope. But this should be fine as it isn't
+        // automatically applied
+        todo!();
+    }
+}
+
+#[derive(Eq, PartialEq)]
+struct B(u32);
+
+impl B {
+    fn cmp(&self, other: &Self) -> Ordering {
+        todo!();
+    }
+}
+
+impl cmp::Ord for B {
+    fn cmp(&self, other: &Self) -> Ordering {
+        todo!();
+    }
+}
+
+impl PartialOrd for B {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        // This calls `B.cmp`, not `Ord::cmp`!
+        Some(self.cmp(other))
+    }
+}

tests/ui/incorrect_partial_ord_impl_on_ord_type_fully_qual.stderr

@@ -0,0 +1,31 @@
+error: incorrect implementation of `partial_cmp` on an `Ord` type
+  --> $DIR/incorrect_partial_ord_impl_on_ord_type_fully_qual.rs:23:1
+   |
+LL | /  impl PartialOrd for A {
+LL | |      fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+   | | _____________________________________________________________-
+LL | ||         // NOTE: This suggestion is wrong, as `Ord` is not in scope. But this should be fine as it isn't
+LL | ||         // automatically applied
+LL | ||         todo!();
+LL | ||     }
+   | ||_____- help: change this to: `{ Some(self.cmp(other)) }`
+LL | |  }
+   | |__^
+   |
+   = note: `#[deny(clippy::incorrect_partial_ord_impl_on_ord_type)]` on by default
+
+error: incorrect implementation of `partial_cmp` on an `Ord` type
+  --> $DIR/incorrect_partial_ord_impl_on_ord_type_fully_qual.rs:46:1
+   |
+LL | /  impl PartialOrd for B {
+LL | |      fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+   | | _____________________________________________________________-
+LL | ||         // This calls `B.cmp`, not `Ord::cmp`!
+LL | ||         Some(self.cmp(other))
+LL | ||     }
+   | ||_____- help: change this to: `{ Some(std::cmp::Ord::cmp(self, other)) }`
+LL | |  }
+   | |__^
+
+error: aborting due to 2 previous errors
+