add a "ignore_field" handler when a deserializable is skipping a field

serde_macros/src/lib.rs

@@ -389,6 +389,7 @@ fn deserialize_struct_from_map(
                     $name = Some(
                         try!(::serde::de::Deserializable::deserialize($deserializer))
                     );
+                    continue;
                 })
         })
         .collect();
@@ -465,13 +466,11 @@ fn deserialize_struct_from_map(
 
                 match key {
                     $key_arms
-                    _ => {
-                        // Ignore unknown fields.
-                        let _: ::serde::de::IgnoreTokens =
-                            try!(Deserializable::deserialize($deserializer));
-                    }
+                    _ => { }
                 }
             }
+
+            try!($deserializer.ignore_field(token))
         }
 
         match $fields_tuple {

src/de.rs

@@ -193,6 +193,12 @@ pub trait Deserializer<E>: Iterator<Result<Token, E>> {
     /// named `field`.
     fn missing_field_error(&mut self, field: &'static str) -> E;
 
+    /// Called when a deserializable has decided to not consume this token.
+    fn ignore_field(&mut self, _token: Token) -> Result<(), E> {
+        let _: IgnoreTokens = try!(Deserializable::deserialize(self));
+        Ok(())
+    }
+
     #[inline]
     fn expect_token(&mut self) -> Result<Token, E> {
         match self.next() {