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/io/stdio.rs b/src/libstd/io/stdio.rs
index f571bed3ba2..d7bc572106e 100644
--- a/src/libstd/io/stdio.rs
+++ b/src/libstd/io/stdio.rs
@@ -349,7 +349,7 @@ fn with_task_stdout<F>(f: F) where F: FnOnce(&mut Writer) -> IoResult<()> {
     });
     match result {
         Ok(()) => {}
-        Err(e) => panic!("failed printing to stdout: {}", e),
+        Err(e) => panic!("failed printing to stdout: {:?}", e),
     }
 }