Error when deserializing struct if encountering unknown fields.

Issue #225

src/de.rs

@@ -178,6 +178,16 @@ enum ErrorKind {
     /// Dotted key attempted to extend something that is not a table.
     DottedKeyInvalidType,
 
+    /// An unexpected key was encountered.
+    ///
+    /// Used when deserializing a struct with a limited set of fields.
+    UnexpectedKeys {
+        /// The unexpected keys.
+        keys: Vec<String>,
+        /// Keys that may be specified.
+        available: &'static [&'static str],
+    },
+
     #[doc(hidden)]
     __Nonexhaustive,
 }
@@ -547,6 +557,28 @@ impl<'de> de::Deserializer<'de> for ValueDeserializer<'de> {
             }
         }
 
+        match &self.value.e {
+            E::InlineTable(values) | E::DottedTable(values) => {
+                let extra_fields = values.iter()
+                    .filter_map(|(key, _val)| {
+                        if !fields.contains(&&(**key)) {
+                            Some(key.clone())
+                        } else {
+                            None
+                        }
+                    })
+                    .collect::<Vec<Cow<'de, str>>>();
+
+                if !extra_fields.is_empty() {
+                    return Err(Error::from_kind(ErrorKind::UnexpectedKeys {
+                        keys: extra_fields.iter().map(|k| k.to_string()).collect::<Vec<_>>(),
+                        available: fields,
+                    }));
+                }
+            }
+            _ => {}
+        }
+
         if name == spanned::NAME && fields == &[spanned::START, spanned::END, spanned::VALUE] {
             let start = self.value.start;
             let end = self.value.end;
@@ -1653,6 +1685,14 @@ impl fmt::Display for Error {
             ErrorKind::DottedKeyInvalidType => {
                 "dotted key attempted to extend non-table type".fmt(f)?
             }
+            ErrorKind::UnexpectedKeys { ref keys, available } => {
+                write!(
+                    f,
+                    "unexpected keys in table: `{:?}`, available keys: `{:?}`",
+                    keys,
+                    available
+                )?
+            }
             ErrorKind::__Nonexhaustive => panic!(),
         }
 
@@ -1700,6 +1740,7 @@ impl error::Error for Error {
             ErrorKind::ExpectedTupleIndex { .. } => "expected table key",
             ErrorKind::ExpectedEmptyTable => "expected empty table",
             ErrorKind::DottedKeyInvalidType => "dotted key invalid type",
+            ErrorKind::UnexpectedKeys { .. } => "unexpected keys in table",
             ErrorKind::__Nonexhaustive => panic!(),
         }
     }

tests/enum_external_deserialize.rs

@@ -20,6 +20,46 @@ struct Multi {
     enums: Vec<TheEnum>,
 }
 
+#[test]
+fn invalid_variant_returns_error_with_good_message_string() {
+    let error = toml::from_str::<TheEnum>("\"NonExistent\"")
+        .expect_err("Expected deserialization to fail.");
+
+    assert_eq!(
+        error.to_string(),
+        "unknown variant `NonExistent`, expected one of `Plain`, `Tuple`, `NewType`, `Struct`"
+    );
+}
+
+#[test]
+fn invalid_variant_returns_error_with_good_message_inline_table() {
+    let error = toml::from_str::<TheEnum>("{ NonExistent = {} }")
+        .expect_err("Expected deserialization to fail.");
+    assert_eq!(
+        error.to_string(),
+        "unknown variant `NonExistent`, expected one of `Plain`, `Tuple`, `NewType`, `Struct`"
+    );
+}
+
+#[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.");
+
+    assert_eq!(error.to_string(), "expected empty table");
+}
+
+#[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.");
+
+    assert_eq!(
+        error.to_string(),
+        r#"unexpected keys in table: `["extra_0", "extra_1"]`, available keys: `["value"]`"#
+    );
+}
+
 mod enum_unit {
     use super::*;