core: split into fmt::Show and fmt::String

fmt::Show is for debugging, and can and should be implemented for
all public types. This trait is used with `{:?}` syntax. There still
exists #[derive(Show)].

fmt::String is for types that faithfully be represented as a String.
Because of this, there is no way to derive fmt::String, all
implementations must be purposeful. It is used by the default format
syntax, `{}`.

This will break most instances of `{}`, since that now requires the type
to impl fmt::String. In most cases, replacing `{}` with `{:?}` is the
correct fix. Types that were being printed specifically for users should
receive a fmt::String implementation to fix this.

Part of #20013

[breaking-change]

3 files changed, 16 insertions, 4 deletions

diff --git a/src/liballoc/arc.rs b/src/liballoc/arc.rs
index 25f80ad11bd..7789620a64f 100644
--- a/src/liballoc/arc.rs
+++ b/src/liballoc/arc.rs
@@ -581,7 +581,7 @@ impl<T: Eq> Eq for Arc<T> {}
 
 impl<T: fmt::Show> fmt::Show for Arc<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        (**self).fmt(f)
+        write!(f, "Arc({:?})", (**self))
     }
 }
 
@@ -794,7 +794,7 @@ mod tests {
     #[test]
     fn show_arc() {
         let a = Arc::new(5u32);
-        assert!(format!("{}", a) == "5")
+        assert!(format!("{:?}", a) == "Arc(5u32)")
     }
 
     // Make sure deriving works with Arc<T>
diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index 6df8bb5f7aa..b4eb71df167 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -145,7 +145,13 @@ impl BoxAny for Box<Any> {
 
 impl<T: ?Sized + fmt::Show> fmt::Show for Box<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        (**self).fmt(f)
+        write!(f, "Box({:?})", &**self)
+    }
+}
+
+impl<Sized? T: fmt::String> fmt::String for Box<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::String::fmt(&**self, f)
     }
 }
 
diff --git a/src/liballoc/rc.rs b/src/liballoc/rc.rs
index 175bba4e71d..91566e89a16 100644
--- a/src/liballoc/rc.rs
+++ b/src/liballoc/rc.rs
@@ -607,7 +607,7 @@ impl<S: hash::Writer, T: Hash<S>> Hash<S> for Rc<T> {
 #[experimental = "Show is experimental."]
 impl<T: fmt::Show> fmt::Show for Rc<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        (**self).fmt(f)
+        write!(f, "Rc({:?})", **self)
     }
 }
 
@@ -962,4 +962,10 @@ mod tests {
         assert!(cow1_weak.upgrade().is_none());
     }
 
+    #[test]
+    fn test_show() {
+        let foo = Rc::new(75u);
+        assert!(format!("{:?}", foo) == "Rc(75u)")
+    }
+
 }