Test special cases that reaches SeqRefDeserializer::deserialize_any len==0 condition

failures (2):
    newtype_enum::empty_struct_from_seq
    newtype_enum::tuple0

serde/src/private/de.rs

@@ -2188,6 +2188,7 @@ mod content {
                 //      test_partially_untagged_enum
                 //      test_partially_untagged_enum_desugared
                 // Covered by tests/test_enum_untagged.rs
+                //      newtype_enum::tuple0
                 //      newtype_enum::tuple2
                 Some(Content::Seq(v)) => {
                     de::Deserializer::deserialize_any(SeqRefDeserializer::new(v), visitor)
@@ -2219,6 +2220,7 @@ mod content {
                 }
                 // Covered by tests/test_enum_untagged.rs
                 //      newtype_enum::struct_from_seq
+                //      newtype_enum::empty_struct_from_seq
                 Some(Content::Seq(v)) => {
                     de::Deserializer::deserialize_any(SeqRefDeserializer::new(v), visitor)
                 }

test_suite/tests/test_enum_untagged.rs

@@ -147,8 +147,10 @@ mod newtype_enum {
     enum Inner {
         Unit,
         Newtype(u8),
+        Tuple0(),
         Tuple2(u8, u8),
         Struct { f: u8 },
+        EmptyStruct {},
     }
 
     // Reaches crate::private::de::content::VariantRefDeserializer::unit_variant
@@ -178,6 +180,22 @@ mod newtype_enum {
         );
     }
 
+    // Reaches crate::private::de::content::VariantRefDeserializer::tuple_variant
+    #[test]
+    fn tuple0() {
+        assert_tokens(
+            &Outer::Inner(Inner::Tuple0()),
+            &[
+                Token::TupleVariant {
+                    name: "Inner",
+                    variant: "Tuple0",
+                    len: 0,
+                },
+                Token::TupleVariantEnd,
+            ],
+        );
+    }
+
     // Reaches crate::private::de::content::VariantRefDeserializer::tuple_variant
     #[test]
     fn tuple2() {
@@ -233,6 +251,44 @@ mod newtype_enum {
             ],
         );
     }
+
+    // Reaches crate::private::de::content::VariantRefDeserializer::struct_variant
+    // Content::Map case
+    // Special case - empty map
+    #[test]
+    fn empty_struct_from_map() {
+        assert_de_tokens(
+            &Outer::Inner(Inner::EmptyStruct {}),
+            &[
+                Token::Map { len: Some(1) },
+                // tag
+                Token::Str("EmptyStruct"),
+                // content
+                Token::Map { len: Some(0) },
+                Token::MapEnd,
+                Token::MapEnd,
+            ],
+        );
+    }
+
+    // Reaches crate::private::de::content::VariantRefDeserializer::struct_variant
+    // Content::Seq case
+    // Special case - empty seq
+    #[test]
+    fn empty_struct_from_seq() {
+        assert_de_tokens(
+            &Outer::Inner(Inner::EmptyStruct {}),
+            &[
+                Token::Map { len: Some(1) },
+                // tag
+                Token::Str("EmptyStruct"),
+                // content
+                Token::Seq { len: Some(0) },
+                Token::SeqEnd,
+                Token::MapEnd,
+            ],
+        );
+    }
 }
 
 // Reaches crate::private::de::content::ContentRefDeserializer::deserialize_option