Use correct tokens in test to represent an enum variant

Unit variant of externally tagged enum cannot be deserialized from the string
token by itself. It is ContentDeserializer + serde_test::Deserializer that makes
this possible, because serde_test::Deserializer produces Content::Str() from
Token::BorrowedStr() and ContentDeserializer produces unit variant from Content::Str().

The following tokens all produces Content::String(variant):
- Token::String(variant)
- Token::Str(variant)
- Token::UnitVariant { variant, .. }

Token::BorrowedStr(variant) produces Content::Str(variant) that was the real purpose to
use it in test in #933. This actually makes this test testing `Content` rather than type itself.

Correct way to represent enum one of:
- [xxxVariant { .. }]
- [Enum { .. }, xxxVariant { variant, .. }]
- [Enum { .. }, String(variant), <variant content>]
- [Enum { .. }, Str(variant), <variant content>]
- [Enum { .. }, BorrowedStr(variant), <variant content>]

serde/src/private/de.rs

@@ -313,6 +313,8 @@ mod content {
         }
     }
 
+    /// Used to capture data in [`Content`] from other deserializers.
+    /// Cannot capture externally tagged enums, `i128` and `u128`.
     struct ContentVisitor<'de> {
         value: PhantomData<Content<'de>>,
     }
@@ -528,6 +530,8 @@ mod content {
         Content(Content<'de>),
     }
 
+    /// Serves as a seed for deserializing a key of internally tagged enum.
+    /// Cannot capture externally tagged enums, `i128` and `u128`.
     struct TagOrContentVisitor<'de> {
         name: &'static str,
         value: PhantomData<TagOrContent<'de>>,
@@ -814,6 +818,9 @@ mod content {
 
     /// Used by generated code to deserialize an internally tagged enum.
     ///
+    /// Captures map or sequence from the original deserializer and searches
+    /// a tag in it (in case of sequence, tag is the first element of sequence).
+    ///
     /// Not public API.
     pub struct TaggedContentVisitor<T> {
         tag_name: &'static str,

test_suite/tests/test_macros.rs

@@ -997,7 +997,9 @@ fn test_internally_tagged_struct_variant_containing_unit_variant() {
             Token::Str("action"),
             Token::Str("Log"),
             Token::Str("level"),
+            Token::Enum { name: "Level" },
             Token::BorrowedStr("Info"),
+            Token::Unit,
             Token::StructEnd,
         ],
     );
@@ -1009,7 +1011,9 @@ fn test_internally_tagged_struct_variant_containing_unit_variant() {
             Token::Str("action"),
             Token::Str("Log"),
             Token::Str("level"),
+            Token::Enum { name: "Level" },
             Token::BorrowedStr("Info"),
+            Token::Unit,
             Token::MapEnd,
         ],
     );
@@ -1019,7 +1023,9 @@ fn test_internally_tagged_struct_variant_containing_unit_variant() {
         &[
             Token::Seq { len: Some(2) },
             Token::Str("Log"),
+            Token::Enum { name: "Level" },
             Token::BorrowedStr("Info"),
+            Token::Unit,
             Token::SeqEnd,
         ],
     );