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/libterm/terminfo/parser/compiled.rs b/src/libterm/terminfo/parser/compiled.rs
index 5f0111c7d7a..3e70aa886e0 100644
--- a/src/libterm/terminfo/parser/compiled.rs
+++ b/src/libterm/terminfo/parser/compiled.rs
@@ -163,7 +163,7 @@ pub fn parse(file: &mut io::Reader, longnames: bool)
     macro_rules! try { ($e:expr) => (
         match $e {
             Ok(e) => e,
-            Err(e) => return Err(format!("{}", e))
+            Err(e) => return Err(format!("{:?}", e))
         }
     ) }