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/libstd/rand/reader.rs b/src/libstd/rand/reader.rs
index 48d7f2e7854..177b7380831 100644
--- a/src/libstd/rand/reader.rs
+++ b/src/libstd/rand/reader.rs
@@ -67,7 +67,7 @@ impl<R: Reader> Rng for ReaderRng<R> {
         if v.len() == 0 { return }
         match self.reader.read_at_least(v.len(), v) {
             Ok(_) => {}
-            Err(e) => panic!("ReaderRng.fill_bytes error: {}", e)
+            Err(e) => panic!("ReaderRng.fill_bytes error: {:?}", e)
         }
     }
 }