diff --git a/src/de.rs b/src/de.rs
index 9d5a417..9b58606 100644
--- a/src/de.rs
+++ b/src/de.rs
@@ -568,7 +568,8 @@ impl<'de> de::Deserializer<'de> for ValueDeserializer<'de> {
             match &self.value.e {
                 &E::InlineTable(ref values) | &E::DottedTable(ref values) => {
                     let extra_fields = values.iter()
-                        .filter_map(|(ref key, ref _val)| {
+                        .filter_map(|key_value| {
+                            let (ref key, ref _val) = *key_value;
                             if !fields.contains(&&(**key)) {
                                 Some(key.clone())
                             } else {
diff --git a/tests/enum_external_deserialize.rs b/tests/enum_external_deserialize.rs
index e35f06a..3c38580 100644
--- a/tests/enum_external_deserialize.rs
+++ b/tests/enum_external_deserialize.rs
@@ -22,8 +22,7 @@ struct Multi {
 
 #[test]
 fn invalid_variant_returns_error_with_good_message_string() {
-    let error = toml::from_str::<TheEnum>("\"NonExistent\"")
-        .expect_err("Expected deserialization to fail.");
+    let error = toml::from_str::<TheEnum>("\"NonExistent\"").unwrap_err();
 
     assert_eq!(
         error.to_string(),
@@ -33,8 +32,7 @@ fn invalid_variant_returns_error_with_good_message_string() {
 
 #[test]
 fn invalid_variant_returns_error_with_good_message_inline_table() {
-    let error = toml::from_str::<TheEnum>("{ NonExistent = {} }")
-        .expect_err("Expected deserialization to fail.");
+    let error = toml::from_str::<TheEnum>("{ NonExistent = {} }").unwrap_err();
     assert_eq!(
         error.to_string(),
         "unknown variant `NonExistent`, expected one of `Plain`, `Tuple`, `NewType`, `Struct`"
@@ -43,8 +41,7 @@ fn invalid_variant_returns_error_with_good_message_inline_table() {
 
 #[test]
 fn extra_field_returns_expected_empty_table_error() {
-    let error = toml::from_str::<TheEnum>("{ Plain = { extra_field = 404 } }")
-        .expect_err("Expected deserialization to fail.");
+    let error = toml::from_str::<TheEnum>("{ Plain = { extra_field = 404 } }").unwrap_err();
 
     assert_eq!(error.to_string(), "expected empty table");
 }
@@ -52,7 +49,7 @@ fn extra_field_returns_expected_empty_table_error() {
 #[test]
 fn extra_field_returns_expected_empty_table_error_struct_variant() {
     let error = toml::from_str::<TheEnum>("{ Struct = { value = 123, extra_0 = 0, extra_1 = 1 } }")
-        .expect_err("Expected deserialization to fail.");
+        .unwrap_err();
 
     assert_eq!(
         error.to_string(),