std: change Decoder::read_option to return a generic type

This allows read_option to be used with a custom option type instead
of just core::Option.

src/libstd/ebml.rs

@@ -411,13 +411,13 @@ fn read_tup_elt<T>(&self, idx: uint, f: &fn() -> T) -> T {
         }
 
         #[cfg(stage0)]
-        fn read_option<T>(&self, f: &fn() -> T) -> Option<T> {
+        fn read_option<T>(&self, f: &fn(bool) -> T) -> T {
             debug!("read_option()");
             do self.read_enum("Option") || {
                 do self.read_enum_variant |idx| {
                     match idx {
-                        0 => None,
-                        1 => Some(f()),
+                        0 => f(false),
+                        1 => f(true),
                         _ => fail!(),
                     }
                 }
@@ -427,13 +427,13 @@ fn read_option<T>(&self, f: &fn() -> T) -> Option<T> {
         #[cfg(stage1)]
         #[cfg(stage2)]
         #[cfg(stage3)]
-        fn read_option<T>(&self, f: &fn() -> T) -> Option<T> {
+        fn read_option<T>(&self, f: &fn(bool) -> T) -> T {
             debug!("read_option()");
             do self.read_enum("Option") || {
                 do self.read_enum_variant(["None", "Some"]) |idx| {
                     match idx {
-                        0 => None,
-                        1 => Some(f()),
+                        0 => f(false),
+                        1 => f(true),
                         _ => fail!(),
                     }
                 }

src/libstd/json.rs

@@ -980,10 +980,10 @@ fn read_tup_elt<T>(&self, idx: uint, f: &fn() -> T) -> T {
         }
     }
 
-    fn read_option<T>(&self, f: &fn() -> T) -> Option<T> {
+    fn read_option<T>(&self, f: &fn(bool) -> T) -> T {
         match *self.peek() {
-            Null => { self.pop(); None }
-            _ => Some(f()),
+            Null => { self.pop(); f(false) }
+            _ => f(true),
         }
     }
 }

src/libstd/serialize.rs

@@ -118,7 +118,7 @@ pub trait Decoder {
     fn read_tup_elt<T>(&self, idx: uint, f: &fn() -> T) -> T;
 
     // Specialized types:
-    fn read_option<T>(&self, f: &fn() -> T) -> Option<T>;
+    fn read_option<T>(&self, f: &fn(bool) -> T) -> T;
 }
 
 pub trait Encodable<S:Encoder> {
@@ -395,7 +395,13 @@ fn encode(&self, s: &S) {
 
 impl<D:Decoder,T:Decodable<D>> Decodable<D> for Option<T> {
     fn decode(d: &D) -> Option<T> {
-        d.read_option(|| Decodable::decode(d))
+        do d.read_option |b| {
+            if b {
+                Some(Decodable::decode(d))
+            } else {
+                None
+            }
+        }
     }
 }