Problem: pgx::datum::Serializer trait scope

It takes care of both serialization and deserialization. However, some
types can potentially be only deserialized, or serialized. Besides, the
naming of the trait itself showed that it is not a perfect match.

Solution: split it into Serializer and Deserializer, respectively

pgx-macros/src/lib.rs

@@ -781,7 +781,8 @@ fn impl_postgres_type(ast: DeriveInput) -> syn::Result<proc_macro2::TokenStream>
         de.bounds = bounds;
         lt_generics.params.insert(0, GenericParam::Lifetime(de));
         stream.extend(quote! {
-            impl #lt_generics ::pgx::datum::Serializer<'de> for #name #generics { }
+            impl #generics ::pgx::datum::Serializer for #name #generics { }
+            impl #lt_generics ::pgx::datum::Deserializer<'de> for #name #generics { }
         });
     }
 

pgx-tests/src/tests/postgres_type_tests.rs

@@ -8,7 +8,7 @@ Use of this source code is governed by the MIT license that can be found in the
 */
 use core::ffi::CStr;
 use pgx::prelude::*;
-use pgx::{InOutFuncs, PgVarlena, PgVarlenaInOutFuncs, Serializer, StringInfo};
+use pgx::{Deserializer, InOutFuncs, PgVarlena, PgVarlenaInOutFuncs, Serializer, StringInfo};
 use serde::{Deserialize, Serialize};
 use std::io::Write;
 use std::str::FromStr;
@@ -157,11 +157,13 @@ pub enum JsonEnumType {
 #[custom_serializer]
 pub struct CustomSerialized;
 
-impl<'de> Serializer<'de> for CustomSerialized {
+impl Serializer for CustomSerialized {
     fn to_writer<W: Write>(&self, mut writer: W) {
         writer.write(&[1]).expect("can't write");
     }
+}
 
+impl<'de> Deserializer<'de> for CustomSerialized {
     fn from_slice(slice: &'de [u8]) -> Self {
         if slice != &[1] {
             panic!("wrong type")
@@ -171,33 +173,15 @@ impl<'de> Serializer<'de> for CustomSerialized {
     }
 }
 
-#[derive(Serialize, Deserialize, PostgresType)]
-#[custom_serializer]
-pub struct AnotherCustomSerialized;
-
-impl<'de> Serializer<'de> for AnotherCustomSerialized {
-    fn to_writer<W: Write>(&self, mut writer: W) {
-        writer.write(&[0]).expect("can't write");
-    }
-
-    fn from_slice(slice: &'de [u8]) -> Self {
-        if slice != &[0] {
-            panic!("wrong type")
-        } else {
-            AnotherCustomSerialized
-        }
-    }
-}
-
 #[cfg(any(test, feature = "pg_test"))]
 #[pgx::pg_schema]
 mod tests {
     #[allow(unused_imports)]
     use crate as pgx_tests;
 
     use crate::tests::postgres_type_tests::{
-        AnotherCustomSerialized, CustomSerialized, CustomTextFormatSerializedEnumType,
-        CustomTextFormatSerializedType, JsonEnumType, JsonType, VarlenaEnumType, VarlenaType,
+        CustomSerialized, CustomTextFormatSerializedEnumType, CustomTextFormatSerializedType,
+        JsonEnumType, JsonType, VarlenaEnumType, VarlenaType,
     };
     use pgx::prelude::*;
     use pgx::PgVarlena;
@@ -300,14 +284,4 @@ mod tests {
         )
         .unwrap();
     }
-
-    #[pg_test(error = "wrong type")]
-    fn custom_serializer_wrong_type() {
-        let s = CustomSerialized;
-        let _ = Spi::get_one_with_args::<AnotherCustomSerialized>(
-            r#"SELECT $1"#,
-            vec![(PgOid::Custom(CustomSerialized::type_oid()), s.into_datum())],
-        )
-        .unwrap();
-    }
 }

pgx/src/datum/mod.rs

@@ -65,10 +65,10 @@ use pgx_sql_entity_graph::RustSqlMapping;
 /// Implemented automatically by `#[derive(PostgresType)]`
 pub trait PostgresType {}
 
-/// Serializing to and from datum
+/// Serializing to datum
 ///
 /// Default implementation uses CBOR and Varlena
-pub trait Serializer<'de>: Serialize + Deserialize<'de> {
+pub trait Serializer: Serialize {
     /// Serializes the value to Datum
     ///
     /// Default implementation wraps the output of `Self::to_writer` into a Varlena
@@ -93,7 +93,12 @@ pub trait Serializer<'de>: Serialize + Deserialize<'de> {
     fn to_writer<W: std::io::Write>(&self, writer: W) {
         serde_cbor::to_writer(writer, &self).expect("failed to encode as CBOR");
     }
+}
 
+/// Deserializing from datum
+///
+/// Default implementation uses CBOR and Varlena
+pub trait Deserializer<'de>: Deserialize<'de> {
     /// Deserializes datum into a value
     ///
     /// Default implementation assumes datum to be a varlena and uses `Self::from_slice`
@@ -121,7 +126,7 @@ pub trait Serializer<'de>: Serialize + Deserialize<'de> {
                 let input = datum.cast_mut_ptr();
                 // this gets the varlena Datum copied into this memory context
                 let varlena = pg_sys::pg_detoast_datum_copy(input as *mut pg_sys::varlena);
-                <Self as Serializer<'de>>::deserialize(varlena.into())
+                <Self as Deserializer<'de>>::deserialize(varlena.into())
             })
         }
     }
@@ -134,9 +139,9 @@ pub trait Serializer<'de>: Serialize + Deserialize<'de> {
     }
 }
 
-impl<'de, T> IntoDatum for T
+impl<T> IntoDatum for T
 where
-    T: PostgresType + Serializer<'de>,
+    T: PostgresType + Serializer,
 {
     fn into_datum(self) -> Option<pg_sys::Datum> {
         Some(Serializer::serialize(&self))
@@ -149,7 +154,7 @@ where
 
 impl<'de, T> FromDatum for T
 where
-    T: PostgresType + Serializer<'de>,
+    T: PostgresType + Deserializer<'de>,
 {
     unsafe fn from_polymorphic_datum(
         datum: pg_sys::Datum,
@@ -159,7 +164,7 @@ where
         if is_null {
             None
         } else {
-            Some(<T as Serializer>::deserialize(datum))
+            Some(<T as Deserializer>::deserialize(datum))
         }
     }