From 6d1807bb4a2a2dd9811c2c954810ee7e8dd32f7f Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Sat, 21 Apr 2018 11:41:37 -0700
Subject: [PATCH] Improve Serialize of adjacently tagged newtype variants

The existing implementation was unnecessarily complicated.

    struct __AdjacentlyTagged<'__a> {
        data: (&'__a String,),
        phantom: _serde::export::PhantomData<AdjacentlyTagged>,
    }
    impl<'__a> _serde::Serialize for __AdjacentlyTagged<'__a> {
        fn serialize<__S>(
            &self,
            __serializer: __S,
        ) -> _serde::export::Result<__S::Ok, __S::Error>
        where
            __S: _serde::Serializer,
        {
            let (__field0,) = self.data;
            _serde::Serialize::serialize(__field0, __serializer)
        }
    }
    _serde::ser::SerializeStruct::serialize_field(
        &mut __struct,
        "content",
        &__AdjacentlyTagged {
            data: (__field0,),
            phantom: _serde::export::PhantomData::<AdjacentlyTagged>,
        },
    )?;

Instead the new implementation does simply:

    _serde::ser::SerializeStruct::serialize_field(
        &mut __struct,
        "content",
        __field0,
    )?;
---
 serde_derive/src/ser.rs | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/serde_derive/src/ser.rs b/serde_derive/src/ser.rs
index e5c1b000..802aaf5c 100644
--- a/serde_derive/src/ser.rs
+++ b/serde_derive/src/ser.rs
@@ -596,9 +596,15 @@ fn serialize_adjacently_tagged_variant(
                     field_expr = wrap_serialize_field_with(params, field.ty, path, &field_expr);
                 }
 
-                quote_expr! {
-                    _serde::Serialize::serialize(#field_expr, __serializer)
-                }
+                return quote_block! {
+                    let mut __struct = try!(_serde::Serializer::serialize_struct(
+                        __serializer, #type_name, 2));
+                    try!(_serde::ser::SerializeStruct::serialize_field(
+                        &mut __struct, #tag, #variant_name));
+                    try!(_serde::ser::SerializeStruct::serialize_field(
+                        &mut __struct, #content, #field_expr));
+                    _serde::ser::SerializeStruct::end(__struct)
+                };
             }
             Style::Tuple => {
                 serialize_tuple_variant(TupleVariant::Untagged, params, &variant.fields)