Auto merge of #293 - dtolnay:duplicate, r=oli-obk

feat(codegen): Detect repeated struct field when deserializing

Addresses #59. Let me know whether you think we need an escape hatch to opt out of this check.

serde_tests/tests/test_de.rs

@@ -725,4 +725,32 @@ declare_error_tests! {
         ],
         Error::UnexpectedToken(Token::SeqSep),
     }
+    test_duplicate_field_struct<Struct> {
+        vec![
+            Token::MapStart(Some(3)),
+                Token::MapSep,
+                Token::Str("a"),
+                Token::I32(1),
+
+                Token::MapSep,
+                Token::Str("a"),
+                Token::I32(3),
+            Token::MapEnd,
+        ],
+        Error::DuplicateFieldError("a"),
+    }
+    test_duplicate_field_enum<Enum> {
+        vec![
+            Token::EnumMapStart("Enum", "Map", Some(3)),
+                Token::EnumMapSep,
+                Token::Str("a"),
+                Token::I32(1),
+
+                Token::EnumMapSep,
+                Token::Str("a"),
+                Token::I32(3),
+            Token::EnumMapEnd,
+        ],
+        Error::DuplicateFieldError("a"),
+    }
 }

serde_tests/tests/token.rs

@@ -410,6 +410,7 @@ pub enum Error {
     UnknownFieldError(String),
     UnknownVariantError(String),
     MissingFieldError(&'static str),
+    DuplicateFieldError(&'static str),
     InvalidName(&'static str),
     InvalidValue(String),
     UnexpectedToken(Token<'static>),
@@ -429,6 +430,10 @@ impl de::Error for Error {
 
     fn end_of_stream() -> Error { Error::EndOfStreamError }
 
+    fn invalid_value(msg: &str) -> Error {
+        Error::InvalidValue(msg.to_owned())
+    }
+
     fn unknown_field(field: &str) -> Error {
         Error::UnknownFieldError(field.to_owned())
     }
@@ -440,6 +445,10 @@ impl de::Error for Error {
     fn missing_field(field: &'static str) -> Error {
         Error::MissingFieldError(field)
     }
+
+    fn duplicate_field(field: &'static str) -> Error {
+        Error::DuplicateFieldError(field)
+    }
 }
 
 impl fmt::Display for Error {