Demand Eq for derive(PostgresEq)

Previously, PartialEq was enough for a derivation of PostgresEq.
However, the way that we already annotate these operators implies
heavy usage of an assumption that there is an absolute equality.
Partially-ordered sets need not apply.

pgrx-macros/src/operators.rs

@@ -65,6 +65,9 @@ pub(crate) fn deriving_postgres_hash(ast: DeriveInput) -> syn::Result<proc_macro
 pub fn derive_pg_eq(type_name: &Ident, type_path: &proc_macro2::TokenStream) -> proc_macro2::TokenStream {
     let pg_name = Ident::new(&format!("{}_eq", type_name).to_lowercase(), type_name.span());
     quote! {
+        #[doc(hidden)]
+        impl ::pgrx::deriving::PostgresEqRequiresTotalEq for #type_name {}
+
         #[allow(non_snake_case)]
         #[::pgrx::pgrx_macros::pg_operator(immutable, parallel_safe)]
         #[::pgrx::pgrx_macros::opname(=)]

pgrx-tests/tests/ui/total_eq_for_postgres_eq.rs

@@ -0,0 +1,8 @@
+use pgrx::prelude::*;
+
+#[derive(PartialEq, PostgresEq)]
+struct BrokenType {
+    int: i32,
+}
+
+fn main() {}

pgrx-tests/tests/ui/total_eq_for_postgres_eq.stderr

@@ -0,0 +1,16 @@
+error[E0277]: the trait bound `BrokenType: std::cmp::Eq` is not satisfied
+ --> tests/ui/total_eq_for_postgres_eq.rs:4:8
+  |
+4 | struct BrokenType {
+  |        ^^^^^^^^^^ the trait `std::cmp::Eq` is not implemented for `BrokenType`
+  |
+note: required by a bound in `PostgresEqRequiresTotalEq`
+ --> $WORKSPACE/pgrx/src/deriving.rs
+  |
+  | pub trait PostgresEqRequiresTotalEq: Eq {}
+  |                                      ^^ required by this bound in `PostgresEqRequiresTotalEq`
+help: consider annotating `BrokenType` with `#[derive(Eq)]`
+  |
+4 + #[derive(Eq)]
+5 | struct BrokenType {
+  |

pgrx/src/deriving.rs

@@ -0,0 +1,3 @@
+#![doc(hidden)]
+
+pub trait PostgresEqRequiresTotalEq: Eq {}

pgrx/src/lib.rs

@@ -42,6 +42,7 @@ pub mod array;
 pub mod atomics;
 pub mod bgworkers;
 pub mod callbacks;
+pub mod deriving;
 pub mod datum;
 pub mod enum_helper;
 pub mod fcinfo;