Refactor BoxRet to be easier to impl before showing it (#1776)

People need to implement some of the code in this module, but I want to
make sure it is easier to actually do so. To that end, refactor BoxRet
to have a more user-friendly interface, ish, and then expose it.

While doing so, adjust some safety requirements here and there.

pgrx-examples/custom_types/src/hexint.rs

@@ -104,8 +104,8 @@ where
 }
 
 unsafe impl BoxRet for HexInt {
-    unsafe fn box_in_fcinfo(self, _fcinfo: pg_sys::FunctionCallInfo) -> pg_sys::Datum {
-        Datum::from(self.value)
+    unsafe fn box_into<'fcx>(self, fcinfo: &mut pgrx::callconv::FcInfo<'fcx>) -> pgrx::Datum<'fcx> {
+        unsafe { fcinfo.return_raw_datum(Datum::from(self.value)) }
     }
 }
 

pgrx-macros/src/lib.rs

@@ -732,8 +732,11 @@ fn impl_postgres_enum(ast: DeriveInput) -> syn::Result<proc_macro2::TokenStream>
         }
 
         unsafe impl ::pgrx::callconv::BoxRet for #enum_ident {
-            unsafe fn box_in_fcinfo(self, fcinfo: ::pgrx::pg_sys::FunctionCallInfo) -> ::pgrx::pg_sys::Datum {
-                ::pgrx::datum::IntoDatum::into_datum(self).unwrap()
+            unsafe fn box_into<'fcx>(self, fcinfo: &mut ::pgrx::callconv::FcInfo<'fcx>) -> ::pgrx::datum::Datum<'fcx> {
+                match ::pgrx::datum::IntoDatum::into_datum(self) {
+                    None => fcinfo.return_null(),
+                    Some(datum) => unsafe { fcinfo.return_raw_datum(datum) },
+                }
             }
         }
     });
@@ -859,10 +862,10 @@ fn impl_postgres_type(ast: DeriveInput) -> syn::Result<proc_macro2::TokenStream>
                 }
 
                 unsafe impl #generics ::pgrx::callconv::BoxRet for #name #generics {
-                    unsafe fn box_in_fcinfo(self, fcinfo: ::pgrx::pg_sys::FunctionCallInfo) -> ::pgrx::pg_sys::Datum {
+                    unsafe fn box_into<'fcx>(self, fcinfo: &mut ::pgrx::callconv::FcInfo<'fcx>) -> ::pgrx::datum::Datum<'fcx> {
                         match ::pgrx::datum::IntoDatum::into_datum(self) {
-                            None => ::pgrx::fcinfo::pg_return_null(fcinfo),
-                            Some(datum) => datum,
+                            None => fcinfo.return_null(),
+                            Some(datum) => unsafe { fcinfo.return_raw_datum(datum) },
                         }
                     }
                 }