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]

1 files changed, 7 insertions, 1 deletions

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)
     }
 }