Allow multiple flattened maps to see the same fields

Before this change, flattening anything after a flattened map was
nonsensical because the later flattened field would always observe no
input fields.

    #[derive(Deserialize)]
    struct S {
        #[serde(flatten)]
        map: Map<K, V>,
        #[serde(flatten)]
        other: Other, // always empty
    }

This change makes a flattened map not consume any of the input fields,
leaving them available to later flattened fields in the same struct. The
new behavior is useful when two flattened fields that both use
deserialize_map care about disjoint subsets of the fields in the input.

    #[derive(Deserialize)]
    struct S {
        // Looks at fields with a "player1_" prefix.
        #[serde(flatten, with = "prefix_player1")]
        player1: Player,
        // Looks at fields with a "player2_" prefix.
        #[serde(flatten, with = "prefix_player2")]
        player2: Player,
    }

test_suite/tests/test_annotations.rs

@@ -14,7 +14,7 @@ extern crate serde_derive;
 extern crate serde;
 use self::serde::de::{self, Unexpected};
 use self::serde::{Deserialize, Deserializer, Serialize, Serializer};
-use std::collections::HashMap;
+use std::collections::{BTreeMap, HashMap};
 use std::marker::PhantomData;
 
 extern crate serde_test;
@@ -1683,6 +1683,49 @@ fn test_complex_flatten() {
     );
 }
 
+#[test]
+fn test_flatten_map_twice() {
+    #[derive(Debug, PartialEq, Deserialize)]
+    struct Outer {
+        #[serde(flatten)]
+        first: BTreeMap<String, String>,
+        #[serde(flatten)]
+        between: Inner,
+        #[serde(flatten)]
+        second: BTreeMap<String, String>,
+    }
+
+    #[derive(Debug, PartialEq, Deserialize)]
+    struct Inner {
+        y: String,
+    }
+
+    assert_de_tokens(
+        &Outer {
+            first: {
+                let mut first = BTreeMap::new();
+                first.insert("x".to_owned(), "X".to_owned());
+                first.insert("y".to_owned(), "Y".to_owned());
+                first
+            },
+            between: Inner { y: "Y".to_owned() },
+            second: {
+                let mut second = BTreeMap::new();
+                second.insert("x".to_owned(), "X".to_owned());
+                second
+            },
+        },
+        &[
+            Token::Map { len: None },
+            Token::Str("x"),
+            Token::Str("X"),
+            Token::Str("y"),
+            Token::Str("Y"),
+            Token::MapEnd,
+        ],
+    );
+}
+
 #[test]
 fn test_flatten_unsupported_type() {
     #[derive(Debug, PartialEq, Serialize, Deserialize)]