Use more consistent error messages for bad flattening

serde/src/private/de.rs

@@ -2095,7 +2095,7 @@ impl<'a, 'de, E> Deserializer<'de> for FlatMapDeserializer<'a, 'de, E>
     where
         V: Visitor<'de>,
     {
-        Err(Error::custom("can only flatten structs, maps and struct enum variants"))
+        Err(Error::custom("can only flatten structs and maps"))
     }
 
     fn deserialize_enum<V>(

serde/src/private/ser.rs

@@ -1043,9 +1043,7 @@ where
     M: SerializeMap + 'a
 {
     fn bad_type(self, what: Unsupported) -> M::Error {
-        ser::Error::custom(format_args!(
-            "cannot flatten serialize {} values", what
-        ))
+        ser::Error::custom(format_args!("can only flatten structs and maps (got {})", what))
     }
 }
 

test_suite/tests/test_annotations.rs

@@ -17,8 +17,8 @@ use self::serde::{Deserialize, Deserializer, Serialize, Serializer};
 use self::serde::de::{self, Unexpected};
 
 extern crate serde_test;
-use self::serde_test::{assert_de_tokens, assert_de_tokens_error, assert_ser_tokens, assert_tokens,
-                       Token};
+use self::serde_test::{assert_de_tokens, assert_de_tokens_error, assert_ser_tokens,
+                       assert_ser_tokens_error, assert_tokens, Token};
 
 trait MyDefault: Sized {
     fn my_default() -> Self;
@@ -1636,3 +1636,37 @@ fn test_complex_flatten() {
         ],
     );
 }
+
+#[test]
+fn test_flatten_unsupported_type() {
+    #[derive(Debug, PartialEq, Serialize, Deserialize)]
+    struct Outer {
+        outer: String,
+        #[serde(flatten)]
+        inner: String,
+    }
+
+    assert_ser_tokens_error(
+        &Outer {
+            outer: "foo".into(),
+            inner: "bar".into(),
+        },
+        &[
+            Token::Map { len: None },
+            Token::Str("outer"),
+            Token::Str("foo"),
+        ],
+        "can only flatten structs and maps (got a string)",
+    );
+    assert_de_tokens_error::<Outer>(
+        &[
+            Token::Map { len: None },
+            Token::Str("outer"),
+            Token::Str("foo"),
+            Token::Str("a"),
+            Token::Str("b"),
+            Token::MapEnd
+        ],
+        "can only flatten structs and maps",
+    );
+}