Demand Ord for PostgresOrd (#1262)

Previously, the `left.cmp(&right)` call could resolve to `Iterator::cmp`.
A malicious impl could even force it to use `fn(&mut self, rhs: &Self)`!
Code exploiting this bug is unlikely, but it could hit other errors,
so this way the error emitted is much nicer.

Also makes the previous trybuild test for PostgresEq more realistic.

pgrx-macros/src/operators.rs

@@ -208,7 +208,7 @@ pub fn derive_pg_cmp(name: &Ident, path: &proc_macro2::TokenStream) -> proc_macr
         #[allow(non_snake_case)]
         #[::pgrx::pgrx_macros::pg_extern(immutable, parallel_safe)]
         fn #pg_name(left: #path, right: #path) -> i32 {
-            left.cmp(&right) as i32
+            ::core::cmp::Ord::cmp(&left, &right) as i32
         }
     }
 }

pgrx-tests/tests/ui/total_eq_for_postgres_eq.rs

@@ -1,7 +1,8 @@
 use pgrx::prelude::*;
+use serde::{Serialize, Deserialize};
 
-#[derive(PartialEq, PostgresEq)]
-struct BrokenType {
+#[derive(Serialize, Deserialize, PartialEq, PostgresType, PostgresEq)]
+pub struct BrokenType {
     int: i32,
 }
 

pgrx-tests/tests/ui/total_eq_for_postgres_eq.stderr

@@ -1,8 +1,8 @@
 error[E0277]: the trait bound `BrokenType: std::cmp::Eq` is not satisfied
- --> tests/ui/total_eq_for_postgres_eq.rs:4:8
+ --> tests/ui/total_eq_for_postgres_eq.rs:5:12
   |
-4 | struct BrokenType {
-  |        ^^^^^^^^^^ the trait `std::cmp::Eq` is not implemented for `BrokenType`
+5 | pub struct BrokenType {
+  |            ^^^^^^^^^^ the trait `std::cmp::Eq` is not implemented for `BrokenType`
   |
 note: required by a bound in `PostgresEqRequiresTotalEq`
  --> $WORKSPACE/pgrx/src/deriving.rs
@@ -11,6 +11,6 @@ note: required by a bound in `PostgresEqRequiresTotalEq`
   |                                      ^^ required by this bound in `PostgresEqRequiresTotalEq`
 help: consider annotating `BrokenType` with `#[derive(Eq)]`
   |
-4 + #[derive(Eq)]
-5 | struct BrokenType {
+5 + #[derive(Eq)]
+6 | pub struct BrokenType {
   |

pgrx-tests/tests/ui/total_ord_for_postgres_ord.rs

@@ -0,0 +1,20 @@
+use pgrx::prelude::*;
+use serde::{Serialize, Deserialize};
+
+#[derive(Serialize, Deserialize, PartialEq, PartialOrd, PostgresType, PostgresOrd)]
+pub struct BrokenType {
+    int: i32,
+}
+
+impl BrokenType {
+    fn cmp(&mut self, _other: &Self) -> Anything {
+        Anything::Whatever
+    }
+}
+
+#[repr(u8)]
+enum Anything {
+    Whatever = 0,
+}
+
+fn main() {}

pgrx-tests/tests/ui/total_ord_for_postgres_ord.stderr

@@ -0,0 +1,12 @@
+error[E0277]: the trait bound `BrokenType: std::cmp::Ord` is not satisfied
+ --> tests/ui/total_ord_for_postgres_ord.rs:4:71
+  |
+4 | #[derive(Serialize, Deserialize, PartialEq, PartialOrd, PostgresType, PostgresOrd)]
+  |                                                                       ^^^^^^^^^^^ the trait `std::cmp::Ord` is not implemented for `BrokenType`
+  |
+  = note: this error originates in the derive macro `PostgresOrd` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider annotating `BrokenType` with `#[derive(Ord)]`
+  |
+5 + #[derive(Ord)]
+6 | pub struct BrokenType {
+  |