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/src/de/mod.rs

@@ -46,6 +46,12 @@ pub trait Error: Sized + error::Error {
     fn missing_field(field: &'static str) -> Self {
         Error::custom(format!("Missing field `{}`", field))
     }
+
+    /// Raised when a `Deserialize` struct type received more than one of the
+    /// same struct field.
+    fn duplicate_field(field: &'static str) -> Self {
+        Error::custom(format!("Duplicate field `{}`", field))
+    }
 }
 
 /// `Type` represents all the primitive types that can be deserialized. This is used by

serde_codegen/src/de.rs

@@ -1141,6 +1141,9 @@ fn deserialize_map(
     let value_arms = fields_attrs_names.iter()
         .filter(|&&(_, ref attrs, _)| !attrs.skip_deserializing_field())
         .map(|&(field, ref attrs, name)| {
+            let deser_name = attrs.name().deserialize_name();
+            let name_str = builder.expr().lit().str(deser_name);
+
             let visit = match attrs.deserialize_with() {
                 None => {
                     let field_ty = &field.ty;
@@ -1158,6 +1161,9 @@ fn deserialize_map(
             };
             quote_arm!(cx,
                 __Field::$name => {
+                    if $name.is_some() {
+                        return Err(<__V::Error as _serde::de::Error>::duplicate_field($name_str));
+                    }
                     $name = Some($visit);
                 }
             )

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 {