Allow to deserialize unit structs from the empty sequences and maps

Otherwise the following tests will fail:
- test_internally_tagged_newtype_variant_containing_unit_struct
- test_internally_tagged_struct_variant_containing_unit_variant

This reverts commit 1986c17.

failures (2):
    newtype_variant_containing_unit
    struct_variant_containing_unit_variant

serde_derive/src/de.rs

@@ -441,6 +441,43 @@ fn deserialize_unit_struct(params: &Parameters, cattrs: &attr::Container) -> Fra
  {
  _serde::__private::Ok(#this_value)
  }
+
+ #[inline]
+ fn visit_seq<__A>(self, mut __seq: __A) -> _serde::__private::Result<Self::Value, __A::Error>
+ where
+ __A: _serde::de::SeqAccess<'de>,
+ {
+ // Allow to deserialize unit from empty sequence
+ if _serde::de::SeqAccess::next_element::<
+ _serde::de::IgnoredAny,
+ >(&mut __seq)?.is_some() {
+ _serde::__private::Err(_serde::de::Error::invalid_type(
+ _serde::de::Unexpected::Seq,
+ &self
+ ))
+ } else {
+ _serde::__private::Ok(#this_value)
+ }
+ }
+
+ #[inline]
+ fn visit_map<__A>(self, mut __map: __A) -> _serde::__private::Result<Self::Value, __A::Error>
+ where
+ __A: _serde::de::MapAccess<'de>,
+ {
+ // Allow to deserialize unit from empty map
+ if _serde::de::MapAccess::next_entry::<
+ _serde::de::IgnoredAny,
+ _serde::de::IgnoredAny,
+ >(&mut __map)?.is_some() {
+ _serde::__private::Err(_serde::de::Error::invalid_type(
+ _serde::de::Unexpected::Map,
+ &self
+ ))
+ } else {
+ _serde::__private::Ok(#this_value)
+ }
+ }
  }
 
  _serde::Deserializer::deserialize_unit_struct(

test_suite/tests/test_de.rs

@@ -903,6 +903,20 @@ fn test_unit() {
 fn test_unit_struct() {
  test(UnitStruct, &[Token::Unit]);
  test(UnitStruct, &[Token::UnitStruct { name: "UnitStruct" }]);
+ test(UnitStruct, &[Token::Seq { len: Some(0) }, Token::SeqEnd]);
+ test(UnitStruct, &[Token::Seq { len: None }, Token::SeqEnd]);
+ test(UnitStruct, &[Token::Map { len: Some(0) }, Token::MapEnd]);
+ test(UnitStruct, &[Token::Map { len: None }, Token::MapEnd]);
+ test(
+ UnitStruct,
+ &[
+ Token::Struct {
+ name: "ZeroStruct",
+ len: 0,
+ },
+ Token::StructEnd,
+ ],
+ );
 }
 
 #[test]

test_suite/tests/test_de_error.rs

@@ -1446,14 +1446,6 @@ fn test_nan_no_decimal_point() {
  );
 }
 
-#[test]
-fn test_unit_struct_from_seq() {
- assert_de_tokens_error::<UnitStruct>(
- &[Token::Seq { len: Some(0) }, Token::SeqEnd],
- "invalid type: sequence, expected unit struct UnitStruct",
- );
-}
-
 #[test]
 fn test_wrapping_overflow() {
  assert_de_tokens_error::<Wrapping<u16>>(