Change Tuple2as1(Skipped, x) representation: tuple(1) -> newtype

This matches the Tuple1(x) behavior which should be used

Fixed (1):
    tuple_struct::tuple2as1

Changes in generated code (see the file attached to PR):
  Tuple2as1:
  Tuple2as1Default:
  Tuple2as1With:
    *_tuple_struct(1) -> *_newtype_struct

serde_derive/src/de.rs

@@ -505,8 +505,6 @@ fn deserialize_tuple(
     };
     let expecting = cattrs.expecting().unwrap_or(&expecting);
 
-    let nfields = fields.len();
-
     let visit_newtype_struct = match form {
         TupleForm::Tuple if field_count == 1 => {
             let visit_newtype_struct = Stmts(read_fields_in_order(
@@ -564,7 +562,7 @@ fn deserialize_tuple(
         }
     };
     let dispatch = match form {
-        TupleForm::Tuple if field_count != 0 && nfields == 1 => {
+        TupleForm::Tuple if field_count == 1 => {
             let type_name = cattrs.name().deserialize_name();
             quote! {
                 _serde::Deserializer::deserialize_newtype_struct(__deserializer, #type_name, #visitor_expr)
@@ -646,8 +644,6 @@ fn deserialize_tuple_in_place(
     };
     let expecting = cattrs.expecting().unwrap_or(&expecting);
 
-    let nfields = fields.len();
-
     let visit_newtype_struct = if !is_enum && field_count == 1 {
         // We deserialize newtype, so only one field is not skipped
         let index = fields
@@ -660,7 +656,7 @@ fn deserialize_tuple_in_place(
         };
         // Deserialize and write defaults if at least one field is skipped,
         // otherwise only deserialize
-        if nfields > 1 {
+        if fields.len() > 1 {
             let write_defaults = fields.iter().enumerate().filter_map(|(index, field)| {
                 if field.attrs.skip_deserializing() {
                     let index = Index::from(index);
@@ -708,7 +704,7 @@ fn deserialize_tuple_in_place(
         quote!(_serde::Deserializer::deserialize_tuple(#deserializer, #field_count, #visitor_expr))
     } else if is_enum {
         quote!(_serde::de::VariantAccess::tuple_variant(__variant, #field_count, #visitor_expr))
-    } else if field_count != 0 && nfields == 1 {
+    } else if field_count == 1 {
         let type_name = cattrs.name().deserialize_name();
         quote!(_serde::Deserializer::deserialize_newtype_struct(__deserializer, #type_name, #visitor_expr))
     } else {

serde_derive/src/ser.rs

@@ -176,7 +176,7 @@ fn serialize_body(cont: &Container, params: &Parameters) -> Fragment {
                 serialize_tuple_struct(params, fields, &cont.attrs)
             }
             Data::Struct(Style::Newtype, fields) => {
-                serialize_newtype_struct(params, &fields[0], &cont.attrs)
+                serialize_newtype_struct(params, 0, &fields[0], &cont.attrs)
             }
             Data::Struct(Style::Unit, _) => serialize_unit_struct(&cont.attrs),
         }
@@ -225,6 +225,7 @@ fn serialize_unit_struct(cattrs: &attr::Container) -> Fragment {
 
 fn serialize_newtype_struct(
     params: &Parameters,
+    index: usize,
     field: &Field,
     cattrs: &attr::Container,
 ) -> Fragment {
@@ -235,14 +236,7 @@ fn serialize_newtype_struct(
 
     let type_name = cattrs.name().serialize_name();
 
-    let mut field_expr = get_member(
-        params,
-        field,
-        &Member::Unnamed(Index {
-            index: 0,
-            span: Span::call_site(),
-        }),
-    );
+    let mut field_expr = get_member(params, field, &Member::Unnamed(Index::from(index)));
     if let Some(path) = field.attrs.serialize_with() {
         field_expr = wrap_serialize_field_with(params, field.ty, path, &field_expr);
     }
@@ -270,6 +264,14 @@ fn serialize_tuple_struct(
         .filter(|(_, field)| !field.attrs.skip_serializing())
         .peekable();
 
+    match serialized_fields.clone().count() {
+        1 => {
+            let (index, field) = serialized_fields.next().unwrap();
+            return serialize_newtype_struct(params, index, field, cattrs);
+        }
+        _ => {}
+    }
+
     let let_mut = mut_if(serialized_fields.peek().is_some());
 
     let len = serialized_fields