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, 2 insertions, 2 deletions

diff --git a/src/libstd/time/duration.rs b/src/libstd/time/duration.rs
index ac1f0c5d803..9bd6f78300e 100644
--- a/src/libstd/time/duration.rs
+++ b/src/libstd/time/duration.rs
@@ -48,7 +48,7 @@ macro_rules! try_opt {
 
 /// ISO 8601 time duration with nanosecond precision.
 /// This also allows for the negative duration; see individual methods for details.
-#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Show)]
 pub struct Duration {
     secs: i64,
     nanos: i32, // Always 0 <= nanos < NANOS_PER_SEC
@@ -337,7 +337,7 @@ impl Div<i32> for Duration {
     }
 }
 
-impl fmt::Show for Duration {
+impl fmt::String for Duration {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         // technically speaking, negative duration is not valid ISO 8601,
         // but we need to print it anyway.