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, 1 insertions, 1 deletions

diff --git a/src/libcore/macros.rs b/src/libcore/macros.rs
index a579f9db416..99e49cc21ed 100644
--- a/src/libcore/macros.rs
+++ b/src/libcore/macros.rs
@@ -83,7 +83,7 @@ macro_rules! assert_eq {
                 if !((*left_val == *right_val) &&
                      (*right_val == *left_val)) {
                     panic!("assertion failed: `(left == right) && (right == left)` \
-                           (left: `{}`, right: `{}`)", *left_val, *right_val)
+                           (left: `{:?}`, right: `{:?}`)", *left_val, *right_val)
                 }
             }
         }