feat(de): Add an Error::unknown_variant error.

Closes #169

serde_tests/tests/test_de.rs

@@ -8,7 +8,13 @@ use num::rational::Ratio;
 
 use serde::de::{Deserializer, Visitor};
 
-use token::{Token, assert_de_tokens, assert_de_tokens_ignore};
+use token::{
+    Error,
+    Token,
+    assert_de_tokens,
+    assert_de_tokens_ignore,
+    assert_de_tokens_error,
+};
 
 //////////////////////////////////////////////////////////////////////////
 
@@ -600,3 +606,13 @@ declare_tests! {
         ],
     }
 }
+
+#[test]
+fn test_enum_error() {
+    assert_de_tokens_error::<Enum>(
+        vec![
+            Token::EnumUnit("Enum", "Foo"),
+        ],
+        Error::UnknownVariantError("Foo".to_owned()),
+    )
+}

serde_tests/tests/token.rs

@@ -371,6 +371,7 @@ pub enum Error {
     SyntaxError,
     EndOfStreamError,
     UnknownFieldError(String),
+    UnknownVariantError(String),
     MissingFieldError(&'static str),
     InvalidName(&'static str),
     InvalidValue(String),
@@ -395,6 +396,10 @@ impl de::Error for Error {
         Error::UnknownFieldError(field.to_owned())
     }
 
+    fn unknown_variant(variant: &str) -> Error {
+        Error::UnknownVariantError(variant.to_owned())
+    }
+
     fn missing_field(field: &'static str) -> Error {
         Error::MissingFieldError(field)
     }