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>>(