diff --git a/src/libstd/ebml.rs b/src/libstd/ebml.rs
index 59e0a1e84fe8f..f782b6ff6a7bd 100644
--- a/src/libstd/ebml.rs
+++ b/src/libstd/ebml.rs
@@ -360,7 +360,7 @@ pub mod reader {
             f()
         }
 
-        fn read_struct<T>(&self, name: &str, f: fn() -> T) -> T {
+        fn read_struct<T>(&self, name: &str, _len: uint, f: fn() -> T) -> T {
             debug!("read_struct(name=%s)", name);
             f()
         }
@@ -631,7 +631,7 @@ pub mod writer {
         }
 
         fn emit_rec(&self, f: fn()) { f() }
-        fn emit_struct(&self, _name: &str, f: fn()) { f() }
+        fn emit_struct(&self, _name: &str, _len: uint, f: fn()) { f() }
         fn emit_field(&self, name: &str, _idx: uint, f: fn()) {
             self._emit_label(name);
             f()
diff --git a/src/libstd/json.rs b/src/libstd/json.rs
index 770d621d51fc7..1d46a54518589 100644
--- a/src/libstd/json.rs
+++ b/src/libstd/json.rs
@@ -148,7 +148,7 @@ pub impl Encoder: serialize::Encoder {
         f();
         self.wr.write_char('}');
     }
-    fn emit_struct(&self, _name: &str, f: fn()) {
+    fn emit_struct(&self, _name: &str, _len: uint, f: fn()) {
         self.wr.write_char('{');
         f();
         self.wr.write_char('}');
@@ -261,7 +261,7 @@ pub impl PrettyEncoder: serialize::Encoder {
         self.indent -= 2;
         self.wr.write_char('}');
     }
-    fn emit_struct(&self, _name: &str, f: fn()) {
+    fn emit_struct(&self, _name: &str, _len: uint, f: fn()) {
         self.emit_rec(f)
     }
     fn emit_field(&self, name: &str, idx: uint, f: fn()) {
@@ -861,7 +861,7 @@ pub impl Decoder: serialize::Decoder {
         move value
     }
 
-    fn read_struct<T>(&self, _name: &str, f: fn() -> T) -> T {
+    fn read_struct<T>(&self, _name: &str, _len: uint, f: fn() -> T) -> T {
         debug!("read_struct()");
         let value = f();
         self.pop();
diff --git a/src/libstd/prettyprint.rs b/src/libstd/prettyprint.rs
index ef26a8cb19035..5e477042b6be2 100644
--- a/src/libstd/prettyprint.rs
+++ b/src/libstd/prettyprint.rs
@@ -160,7 +160,7 @@ pub impl Encoder: serialize::Encoder {
         self.wr.write_str(~"}");
     }
 
-    fn emit_struct(&self, name: &str, f: fn()) {
+    fn emit_struct(&self, name: &str, _len: uint, f: fn()) {
         self.wr.write_str(fmt!("%s {", name));
         f();
         self.wr.write_str(~"}");
diff --git a/src/libstd/serialize.rs b/src/libstd/serialize.rs
index 9b1949256931c..a08d8defd5ce6 100644
--- a/src/libstd/serialize.rs
+++ b/src/libstd/serialize.rs
@@ -54,7 +54,7 @@ pub trait Encoder {
     fn emit_vec_elt(&self, idx: uint, f: fn());
 
     fn emit_rec(&self, f: fn());
-    fn emit_struct(&self, name: &str, f: fn());
+    fn emit_struct(&self, name: &str, _len: uint, f: fn());
     fn emit_field(&self, f_name: &str, f_idx: uint, f: fn());
 
     fn emit_tup(&self, len: uint, f: fn());
@@ -95,7 +95,7 @@ pub trait Decoder {
     fn read_vec_elt<T>(&self, idx: uint, f: fn() -> T) -> T;
 
     fn read_rec<T>(&self, f: fn() -> T) -> T;
-    fn read_struct<T>(&self, name: &str, f: fn() -> T) -> T;
+    fn read_struct<T>(&self, name: &str, _len: uint, f: fn() -> T) -> T;
     fn read_field<T>(&self, name: &str, idx: uint, f: fn() -> T) -> T;
 
     fn read_tup<T>(&self, sz: uint, f: fn() -> T) -> T;
diff --git a/src/libsyntax/ext/auto_encode.rs b/src/libsyntax/ext/auto_encode.rs
index 8eb37386364f4..680fa169afe79 100644
--- a/src/libsyntax/ext/auto_encode.rs
+++ b/src/libsyntax/ext/auto_encode.rs
@@ -25,7 +25,7 @@ would generate two implementations like:
 
     impl<S: Encoder> node_id: Encodable<S> {
         fn encode(s: &S) {
-            do s.emit_struct("Node") {
+            do s.emit_struct("Node", 1) {
                 s.emit_field("id", 0, || s.emit_uint(self))
             }
         }
@@ -33,7 +33,7 @@ would generate two implementations like:
 
     impl<D: Decoder> node_id: Decodable {
         static fn decode(d: &D) -> Node {
-            do d.read_struct("Node") {
+            do d.read_struct("Node", 1) {
                 Node {
                     id: d.read_field(~"x", 0, || decode(d))
                 }
@@ -686,6 +686,7 @@ fn mk_struct_ser_impl(
         ),
         ~[
             cx.lit_str(span, @cx.str_of(ident)),
+            cx.lit_uint(span, vec::len(fields)),
             cx.lambda_stmts(span, fields),
         ]
     );
@@ -712,6 +713,7 @@ fn mk_struct_deser_impl(
         ),
         ~[
             cx.lit_str(span, @cx.str_of(ident)),
+            cx.lit_uint(span, vec::len(fields)),
             cx.lambda_expr(
                 cx.expr(
                     span,