The crown jewel of this entire exercise

pgrx/src/callconv.rs

@@ -10,14 +10,15 @@
 #![deny(unsafe_op_in_unsafe_fn)]
 //! Helper implementations for returning sets and tables from `#[pg_extern]`-style functions
 
-use crate::datum::Datum;
 use crate::datum::{
     AnyArray, AnyElement, AnyNumeric, Date, FromDatum, Inet, Internal, Interval, IntoDatum, Json,
     JsonB, Numeric, PgVarlena, Time, TimeWithTimeZone, Timestamp, TimestampWithTimeZone,
     UnboxDatum, Uuid,
 };
+use crate::datum::{BorrowDatum, Datum};
 use crate::datum::{Range, RangeSubType};
 use crate::heap_tuple::PgHeapTuple;
+use crate::layout::PassBy;
 use crate::nullable::Nullable;
 use crate::pg_sys;
 use crate::pgbox::*;
@@ -267,6 +268,19 @@ argue_from_datum! { 'fcx; Inet, Internal, Json, JsonB, Uuid, PgRelation }
 argue_from_datum! { 'fcx; pg_sys::BOX, pg_sys::ItemPointerData, pg_sys::Oid, pg_sys::Point }
 argue_from_datum! { 'fcx; &'fcx str, &'fcx CStr, &'fcx [u8] }
 
+unsafe impl<'fcx, T> ArgAbi<'fcx> for &T
+where
+    T: BorrowDatum,
+{
+    unsafe fn unbox_arg_unchecked(arg: Arg<'_, 'fcx>) -> Self {
+        if let Some(PassBy::Ref) = T::PASS {
+            unsafe { &*T::point_from(arg.2.value.cast_mut_ptr()) }
+        } else {
+            todo!()
+        }
+    }
+}
+
 /// How to return a value from Rust to Postgres
 ///
 /// This bound is necessary to distinguish things which can be returned from a `#[pg_extern] fn`.