Fix incorrect count of fields passed to tuple deserialization methods

This count should mean the number of fields expected in the serialized form,
so if some fields are skipped, they shouldn't be counted

Methods affected:
- Deserializer::deserialize_tuple
- Deserializer::deserialize_tuple_struct
- VariantAccess::tuple_variant

serde_derive/src/de.rs

@@ -445,14 +445,19 @@ fn deserialize_tuple(
     cattrs: &attr::Container,
     deserializer: Option<TokenStream>,
 ) -> Fragment {
+    assert!(!cattrs.has_flatten());
+
+    let field_count = fields
+        .iter()
+        .filter(|field| !field.attrs.skip_deserializing())
+        .count();
+
     let this_type = &params.this_type;
     let this_value = &params.this_value;
     let (de_impl_generics, de_ty_generics, ty_generics, where_clause) =
         split_with_de_lifetime(params);
     let delife = params.borrowed.de_lifetime();
 
-    assert!(!cattrs.has_flatten());
-
     // If there are getters (implying private fields), construct the local type
     // and use an `Into` conversion to get the remote type. If there are no
     // getters then construct the target type directly.
@@ -493,19 +498,18 @@ fn deserialize_tuple(
         }
     };
     let dispatch = if let Some(deserializer) = deserializer {
-        quote!(_serde::Deserializer::deserialize_tuple(#deserializer, #nfields, #visitor_expr))
+        quote!(_serde::Deserializer::deserialize_tuple(#deserializer, #field_count, #visitor_expr))
     } else if is_enum {
-        quote!(_serde::de::VariantAccess::tuple_variant(__variant, #nfields, #visitor_expr))
+        quote!(_serde::de::VariantAccess::tuple_variant(__variant, #field_count, #visitor_expr))
     } else if nfields == 1 {
         let type_name = cattrs.name().deserialize_name();
         quote!(_serde::Deserializer::deserialize_newtype_struct(__deserializer, #type_name, #visitor_expr))
     } else {
         let type_name = cattrs.name().deserialize_name();
-        quote!(_serde::Deserializer::deserialize_tuple_struct(__deserializer, #type_name, #nfields, #visitor_expr))
+        quote!(_serde::Deserializer::deserialize_tuple_struct(__deserializer, #type_name, #field_count, #visitor_expr))
     };
 
-    let all_skipped = fields.iter().all(|field| field.attrs.skip_deserializing());
-    let visitor_var = if all_skipped {
+    let visitor_var = if field_count == 0 {
         quote!(_)
     } else {
         quote!(mut __seq)
@@ -548,13 +552,18 @@ fn deserialize_tuple_in_place(
     cattrs: &attr::Container,
     deserializer: Option<TokenStream>,
 ) -> Fragment {
+    assert!(!cattrs.has_flatten());
+
+    let field_count = fields
+        .iter()
+        .filter(|field| !field.attrs.skip_deserializing())
+        .count();
+
     let this_type = &params.this_type;
     let (de_impl_generics, de_ty_generics, ty_generics, where_clause) =
         split_with_de_lifetime(params);
     let delife = params.borrowed.de_lifetime();
 
-    assert!(!cattrs.has_flatten());
-
     let is_enum = variant_ident.is_some();
     let expecting = match variant_ident {
         Some(variant_ident) => format!("tuple variant {}::{}", params.type_name(), variant_ident),
@@ -580,19 +589,18 @@ fn deserialize_tuple_in_place(
     };
 
     let dispatch = if let Some(deserializer) = deserializer {
-        quote!(_serde::Deserializer::deserialize_tuple(#deserializer, #nfields, #visitor_expr))
+        quote!(_serde::Deserializer::deserialize_tuple(#deserializer, #field_count, #visitor_expr))
     } else if is_enum {
-        quote!(_serde::de::VariantAccess::tuple_variant(__variant, #nfields, #visitor_expr))
+        quote!(_serde::de::VariantAccess::tuple_variant(__variant, #field_count, #visitor_expr))
     } else if nfields == 1 {
         let type_name = cattrs.name().deserialize_name();
         quote!(_serde::Deserializer::deserialize_newtype_struct(__deserializer, #type_name, #visitor_expr))
     } else {
         let type_name = cattrs.name().deserialize_name();
-        quote!(_serde::Deserializer::deserialize_tuple_struct(__deserializer, #type_name, #nfields, #visitor_expr))
+        quote!(_serde::Deserializer::deserialize_tuple_struct(__deserializer, #type_name, #field_count, #visitor_expr))
     };
 
-    let all_skipped = fields.iter().all(|field| field.attrs.skip_deserializing());
-    let visitor_var = if all_skipped {
+    let visitor_var = if field_count == 0 {
         quote!(_)
     } else {
         quote!(mut __seq)

test_suite/tests/test_annotations.rs

@@ -1533,7 +1533,7 @@ fn test_invalid_length_enum() {
             Token::TupleVariant {
                 name: "InvalidLengthEnum",
                 variant: "B",
-                len: 3,
+                len: 2,
             },
             Token::I32(1),
             Token::TupleVariantEnd,