Catch wrong field names length in serde_test

serde_test/src/assert.rs

@@ -215,7 +215,7 @@ where
 ///
 /// assert_de_tokens_error::<S>(
 ///     &[
-///         Token::Struct { name: "S", len: 1 },
+///         Token::Struct { name: "S", len: 2 },
 ///         Token::Str("x"),
 ///     ],
 ///     "unknown field `x`, expected `a` or `b`",

serde_test/src/de.rs

@@ -352,8 +352,8 @@ impl<'de, 'a> de::Deserializer<'de> for &'a mut Deserializer<'de> {
         V: Visitor<'de>,
     {
         match self.peek_token() {
-            Token::Struct { len: n, .. } => {
-                assert_next_token!(self, Token::Struct { name: name, len: n });
+            Token::Struct { .. } => {
+                assert_next_token!(self, Token::Struct { name: name, len: fields.len() });
                 self.visit_map(Some(fields.len()), Token::StructEnd, visitor)
             }
             Token::Map { .. } => {

serde_test/src/token.rs

@@ -425,6 +425,10 @@ pub enum Token {
 
     /// The header of a struct.
     ///
+    /// When testing deserialization, the `len` field must match the number of
+    /// fields that the struct expects to deserialize. This may be different
+    /// from the number of fields contained in the input tokens.
+    ///
     /// After this header are the fields of the struct, followed by `StructEnd`.
     ///
     /// ```rust
@@ -461,6 +465,10 @@ pub enum Token {
 
     /// The header of a struct variant of an enum.
     ///
+    /// When testing deserialization, the `len` field must match the number of
+    /// fields that the struct variant expects to deserialize. This may be
+    /// different from the number of fields contained in the input tokens.
+    ///
     /// After this header are the fields of the struct variant, followed by
     /// `StructVariantEnd`.
     ///