Detect unknown struct variant fields deserializing from Value

src/value/de.rs

@@ -538,7 +538,7 @@ impl<'de> VariantAccess<'de> for VariantDeserializer {
         V: Visitor<'de>,
     {
         match self.value {
-            Some(Value::Object(v)) => visitor.visit_map(MapDeserializer::new(v)),
+            Some(Value::Object(v)) => visit_object(v, visitor),
             Some(other) => Err(serde::de::Error::invalid_type(
                 other.unexpected(),
                 &"struct variant",
@@ -1021,7 +1021,7 @@ impl<'de> VariantAccess<'de> for VariantRefDeserializer<'de> {
         V: Visitor<'de>,
     {
         match self.value {
-            Some(&Value::Object(ref v)) => visitor.visit_map(MapRefDeserializer::new(v)),
+            Some(&Value::Object(ref v)) => visit_object_ref(v, visitor),
             Some(other) => Err(serde::de::Error::invalid_type(
                 other.unexpected(),
                 &"struct variant",

tests/regression/issue795.rs

@@ -0,0 +1,57 @@
+use serde::de::{
+    Deserialize, Deserializer, EnumAccess, IgnoredAny, MapAccess, VariantAccess, Visitor,
+};
+use serde_json::json;
+use std::fmt;
+
+#[derive(Debug)]
+enum Enum {
+    Variant { x: u8 },
+}
+
+impl<'de> Deserialize<'de> for Enum {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        struct EnumVisitor;
+
+        impl<'de> Visitor<'de> for EnumVisitor {
+            type Value = Enum;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                formatter.write_str("enum Enum")
+            }
+
+            fn visit_enum<A>(self, data: A) -> Result<Self::Value, A::Error>
+            where
+                A: EnumAccess<'de>,
+            {
+                let (IgnoredAny, variant) = data.variant()?;
+                variant.struct_variant(&["x"], self)
+            }
+
+            fn visit_map<A>(self, mut data: A) -> Result<Self::Value, A::Error>
+            where
+                A: MapAccess<'de>,
+            {
+                let mut x = 0;
+                if let Some((IgnoredAny, value)) = data.next_entry()? {
+                    x = value;
+                }
+                Ok(Enum::Variant { x })
+            }
+        }
+
+        deserializer.deserialize_enum("Enum", &["Variant"], EnumVisitor)
+    }
+}
+
+#[test]
+fn test() {
+    let s = r#" {"Variant":{"x":0,"y":0}} "#;
+    assert!(serde_json::from_str::<Enum>(s).is_err());
+
+    let j = json!({"Variant":{"x":0,"y":0}});
+    assert!(serde_json::from_value::<Enum>(j).is_err());
+}