Enhance auto_encode to take number of struct fields

emit_struct and read_struct takes an additional len:uint parameter which tells
us how many fields the struct we are working on has.

This is required to implement an Encoder for the msgpack [1] serialization
format. To serialize a struct with msgpack you have to use arrays and the size
of the array has to be know before each of the elements are written out. JSON
as an example doesn't have this problem as it uses '[' and ']' delimiters for
arrays.

[1]: www.msgpack.org

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()

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();

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(~"}");

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;

src/libsyntax/ext/auto_encode.rs

@@ -25,15 +25,15 @@ 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))
             }
         }
     }
 
     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,