Change Tuple1as0(Skipped) representation: newtype -> tuple(0), similar to Tuple0() struct

A side-effect: Tuple2as1(Skipped, x) now also can be deserialized using visit_newtype_struct

Changes in generated code (see the file attached to PR):
  Tuple1as0:
  Tuple1as0Default:
  Tuple1as0With:
    removed visit_newtype_struct, *_newtype_struct -> *_tuple_struct(0)

  Tuple2as1:
  Tuple2as1Default:
  Tuple2as1With:
    added visit_newtype_struct

This commit fixes compilation error and actually fixes the issue as it was reported in #2105

Author: Mingun

serde_derive/src/de.rs

@@ -511,7 +511,7 @@ fn deserialize_tuple(
     let nfields = fields.len();
 
     let visit_newtype_struct = match form {
-        TupleForm::Tuple if nfields == 1 => {
+        TupleForm::Tuple if field_count == 1 => {
             let visit_newtype_struct = Stmts(read_fields_in_order(
                 &type_path,
                 params,
@@ -570,7 +570,7 @@ fn deserialize_tuple(
         }
     };
     let dispatch = match form {
-        TupleForm::Tuple if nfields == 1 => {
+        TupleForm::Tuple if field_count != 0 && nfields == 1 => {
             let type_name = cattrs.name().deserialize_name();
             quote! {
                 _serde::Deserializer::deserialize_newtype_struct(__deserializer, #type_name, #visitor_expr)
@@ -652,34 +652,27 @@ fn deserialize_tuple_in_place(
 
     let nfields = fields.len();
 
-    let visit_newtype_struct = if nfields == 1 {
+    let visit_newtype_struct = if field_count == 1 {
         // We deserialize newtype, so only one field is not skipped
         let index = fields
             .iter()
             .position(|field| !field.attrs.skip_deserializing())
-            .unwrap_or(0);
-        let index = Index::from(index);
+            .map(Index::from)
+            .unwrap();
         let mut deserialize = quote! {
             _serde::Deserialize::deserialize_in_place(__e, &mut self.place.#index)
         };
-        let write_defaults = fields.iter().enumerate().filter_map(|(index, field)| {
-            if field.attrs.skip_deserializing() {
-                let index = Index::from(index);
-                let default = Expr(expr_is_missing(field, cattrs));
-                return Some(quote!(self.place.#index = #default;));
-            }
-            None
-        });
-        // If there are no deserialized fields, write only defaults
-        if field_count == 0 {
-            deserialize = quote! {
-                #(#write_defaults)*
-                _serde::__private::Ok(())
-            }
-        } else
         // Deserialize and write defaults if at least one field is skipped,
         // otherwise only deserialize
-        if nfields > field_count {
+        if nfields > 1 {
+            let write_defaults = fields.iter().enumerate().filter_map(|(index, field)| {
+                if field.attrs.skip_deserializing() {
+                    let index = Index::from(index);
+                    let default = Expr(expr_is_missing(field, cattrs));
+                    return Some(quote!(self.place.#index = #default;));
+                }
+                None
+            });
             deserialize = quote! {
                 match #deserialize {
                     _serde::__private::Ok(_) => {
@@ -716,7 +709,7 @@ fn deserialize_tuple_in_place(
     };
 
     let type_name = cattrs.name().deserialize_name();
-    let dispatch = if nfields == 1 {
+    let dispatch = if field_count != 0 && nfields == 1 {
         quote!(_serde::Deserializer::deserialize_newtype_struct(__deserializer, #type_name, #visitor_expr))
     } else {
         quote!(_serde::Deserializer::deserialize_tuple_struct(__deserializer, #type_name, #field_count, #visitor_expr))

serde_derive/src/ser.rs

@@ -230,6 +230,11 @@ fn serialize_newtype_struct(
     field: &Field,
     cattrs: &attr::Container,
 ) -> Fragment {
+    // For `struct Tuple1as0(#[serde(skip)] u8);` cases
+    if field.attrs.skip_serializing() {
+        return serialize_tuple_struct(params, &[], cattrs);
+    }
+
     let type_name = cattrs.name().serialize_name();
 
     let mut field_expr = get_member(

test_suite/tests/test_skip.rs

@@ -87,7 +87,6 @@ mod tuple_struct {
         );
     }
 
-    /* FIXME: compilation error: https://github.com/serde-rs/serde/issues/2105
     #[test]
     fn tuple1as0() {
         /// This newtype struct in the serialized form the same as `struct Tuple0();`
@@ -107,7 +106,7 @@ mod tuple_struct {
                 Token::TupleStructEnd,
             ],
         );
-    }*/
+    }
 
     #[test]
     fn tuple2as0() {