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]

src/libsyntax/attr.rs

@@ -28,6 +28,7 @@ use ptr::P;
 use std::cell::{RefCell, Cell};
 use std::collections::BitvSet;
 use std::collections::HashSet;
+use std::fmt;
 
 thread_local! { static USED_ATTRS: RefCell<BitvSet> = RefCell::new(BitvSet::new()) }
 
@@ -357,6 +358,12 @@ pub enum StabilityLevel {
     Locked
 }
 
+impl fmt::String for StabilityLevel {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Show::fmt(self, f)
+    }
+}
+
 pub fn find_stability_generic<'a,
                               AM: AttrMetaMethods,
                               I: Iterator<Item=&'a AM>>