std: Rename Show/String to Debug/Display

This commit is an implementation of [RFC 565][rfc] which is a stabilization of
the `std::fmt` module and the implementations of various formatting traits.
Specifically, the following changes were performed:

[rfc]: https://github.com/rust-lang/rfcs/blob/master/text/0565-show-string-guidelines.md

* The `Show` trait is now deprecated, it was renamed to `Debug`
* The `String` trait is now deprecated, it was renamed to `Display`
* Many `Debug` and `Display` implementations were audited in accordance with the
  RFC and audited implementations now have the `#[stable]` attribute
  * Integers and floats no longer print a suffix
  * Smart pointers no longer print details that they are a smart pointer
  * Paths with `Debug` are now quoted and escape characters
* The `unwrap` methods on `Result` now require `Display` instead of `Debug`
* The `Error` trait no longer has a `detail` method and now requires that
  `Display` must be implemented. With the loss of `String`, this has moved into
  libcore.
* `impl<E: Error> FromError<E> for Box<Error>` now exists
* `derive(Show)` has been renamed to `derive(Debug)`. This is not currently
  warned about due to warnings being emitted on stage1+

While backwards compatibility is attempted to be maintained with a blanket
implementation of `Display` for the old `String` trait (and the same for
`Show`/`Debug`) this is still a breaking change due to primitives no longer
implementing `String` as well as modifications such as `unwrap` and the `Error`
trait. Most code is fairly straightforward to update with a rename or tweaks of
method calls.

[breaking-change]
Closes #21436

10 files changed, 279 insertions, 88 deletions

diff --git a/src/libcore/any.rs b/src/libcore/any.rs
index 20ed2253861..6e9d2f349bf 100644
--- a/src/libcore/any.rs
+++ b/src/libcore/any.rs
@@ -34,11 +34,11 @@
 //! use runtime reflection instead.
 //!
 //! ```rust
-//! use std::fmt::Show;
+//! use std::fmt::Debug;
 //! use std::any::Any;
 //!
-//! // Logger function for any type that implements Show.
-//! fn log<T: Any+Show>(value: &T) {
+//! // Logger function for any type that implements Debug.
+//! fn log<T: Any + Debug>(value: &T) {
 //!     let value_any = value as &Any;
 //!
 //!     // try to convert our value to a String.  If successful, we want to
@@ -55,7 +55,7 @@
 //! }
 //!
 //! // This function wants to log its parameter out prior to doing work with it.
-//! fn do_work<T: Show+'static>(value: &T) {
+//! fn do_work<T: Debug + 'static>(value: &T) {
 //!     log(value);
 //!     // ...do some other work
 //! }
diff --git a/src/libcore/array.rs b/src/libcore/array.rs
index 0cc31bf70de..a83537e12f7 100644
--- a/src/libcore/array.rs
+++ b/src/libcore/array.rs
@@ -39,10 +39,10 @@ macro_rules! array_impls {
                 }
             }
 
-            #[unstable = "waiting for Show to stabilize"]
-            impl<T:fmt::Show> fmt::Show for [T; $N] {
+            #[stable]
+            impl<T: fmt::Debug> fmt::Debug for [T; $N] {
                 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-                    fmt::Show::fmt(&&self[], f)
+                    fmt::Debug::fmt(&&self[], f)
                 }
             }
 
diff --git a/src/libcore/borrow.rs b/src/libcore/borrow.rs
index 4363a0a4441..63614aaa463 100644
--- a/src/libcore/borrow.rs
+++ b/src/libcore/borrow.rs
@@ -133,7 +133,6 @@ impl<T> ToOwned<T> for T where T: Clone {
 ///     }
 /// }
 /// ```
-#[derive(Show)]
 pub enum Cow<'a, T, B: ?Sized + 'a> where B: ToOwned<T> {
     /// Borrowed data.
     Borrowed(&'a B),
@@ -239,14 +238,27 @@ impl<'a, T, B: ?Sized> PartialOrd for Cow<'a, T, B> where B: PartialOrd + ToOwne
 }
 
 #[stable]
-impl<'a, T, B: ?Sized> fmt::String for Cow<'a, T, B> where
-    B: fmt::String + ToOwned<T>,
-    T: fmt::String,
+impl<'a, T, B: ?Sized> fmt::Debug for Cow<'a, T, B> where
+    B: fmt::Debug + ToOwned<T>,
+    T: fmt::Debug,
 {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match *self {
-            Borrowed(ref b) => fmt::String::fmt(b, f),
-            Owned(ref o) => fmt::String::fmt(o, f),
+            Borrowed(ref b) => fmt::Debug::fmt(b, f),
+            Owned(ref o) => fmt::Debug::fmt(o, f),
+        }
+    }
+}
+
+#[stable]
+impl<'a, T, B: ?Sized> fmt::Display for Cow<'a, T, B> where
+    B: fmt::Display + ToOwned<T>,
+    T: fmt::Display,
+{
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            Borrowed(ref b) => fmt::Display::fmt(b, f),
+            Owned(ref o) => fmt::Display::fmt(o, f),
         }
     }
 }
diff --git a/src/libcore/error.rs b/src/libcore/error.rs
new file mode 100644
index 00000000000..9ff38028df9
--- /dev/null
+++ b/src/libcore/error.rs
@@ -0,0 +1,110 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Traits for working with Errors.
+//!
+//! # The `Error` trait
+//!
+//! `Error` is a trait representing the basic expectations for error values,
+//! i.e. values of type `E` in `Result<T, E>`. At a minimum, errors must provide
+//! a description, but they may optionally provide additional detail (via
+//! `Display`) and cause chain information:
+//!
+//! ```
+//! use std::fmt::Display;
+//!
+//! trait Error: Display {
+//!     fn description(&self) -> &str;
+//!
+//!     fn cause(&self) -> Option<&Error> { None }
+//! }
+//! ```
+//!
+//! The `cause` method is generally used when errors cross "abstraction
+//! boundaries", i.e.  when a one module must report an error that is "caused"
+//! by an error from a lower-level module. This setup makes it possible for the
+//! high-level module to provide its own errors that do not commit to any
+//! particular implementation, but also reveal some of its implementation for
+//! debugging via `cause` chains.
+//!
+//! # The `FromError` trait
+//!
+//! `FromError` is a simple trait that expresses conversions between different
+//! error types. To provide maximum flexibility, it does not require either of
+//! the types to actually implement the `Error` trait, although this will be the
+//! common case.
+//!
+//! The main use of this trait is in the `try!` macro, which uses it to
+//! automatically convert a given error to the error specified in a function's
+//! return type.
+//!
+//! For example,
+//!
+//! ```
+//! use std::error::FromError;
+//! use std::io::{File, IoError};
+//! use std::os::{MemoryMap, MapError};
+//! use std::path::Path;
+//!
+//! enum MyError {
+//!     Io(IoError),
+//!     Map(MapError)
+//! }
+//!
+//! impl FromError<IoError> for MyError {
+//!     fn from_error(err: IoError) -> MyError {
+//!         MyError::Io(err)
+//!     }
+//! }
+//!
+//! impl FromError<MapError> for MyError {
+//!     fn from_error(err: MapError) -> MyError {
+//!         MyError::Map(err)
+//!     }
+//! }
+//!
+//! #[allow(unused_variables)]
+//! fn open_and_map() -> Result<(), MyError> {
+//!     let f = try!(File::open(&Path::new("foo.txt")));
+//!     let m = try!(MemoryMap::new(0, &[]));
+//!     // do something interesting here...
+//!     Ok(())
+//! }
+//! ```
+
+#![stable]
+
+use prelude::*;
+use fmt::Display;
+
+/// Base functionality for all errors in Rust.
+#[unstable = "the exact API of this trait may change"]
+pub trait Error: Display {
+    /// A short description of the error; usually a static string.
+    fn description(&self) -> &str;
+
+    /// The lower-level cause of this error, if any.
+    fn cause(&self) -> Option<&Error> { None }
+}
+
+/// A trait for types that can be converted from a given error type `E`.
+#[stable]
+pub trait FromError<E> {
+    /// Perform the conversion.
+    fn from_error(err: E) -> Self;
+}
+
+// Any type is convertable from itself
+#[stable]
+impl<E> FromError<E> for E {
+    fn from_error(err: E) -> E {
+        err
+    }
+}
diff --git a/src/libcore/fmt/mod.rs b/src/libcore/fmt/mod.rs
index 535722f93bf..0e8d31a62ee 100644
--- a/src/libcore/fmt/mod.rs
+++ b/src/libcore/fmt/mod.rs
@@ -26,12 +26,15 @@ use ops::{Deref, FnOnce};
 use result;
 use slice::SliceExt;
 use slice;
-use str::{self, StrExt, Utf8Error};
+use str::{self, StrExt};
 
 pub use self::num::radix;
 pub use self::num::Radix;
 pub use self::num::RadixFmt;
 
+#[cfg(stage0)] pub use self::Debug as Show;
+#[cfg(stage0)] pub use self::Display as String;
+
 mod num;
 mod float;
 pub mod rt;
@@ -46,7 +49,7 @@ pub type Result = result::Result<(), Error>;
 /// occurred. Any extra information must be arranged to be transmitted through
 /// some other means.
 #[unstable = "core and I/O reconciliation may alter this definition"]
-#[derive(Copy)]
+#[derive(Copy, Show)]
 pub struct Error;
 
 /// A collection of methods that are required to format a message into a stream.
@@ -133,7 +136,7 @@ pub struct Argument<'a> {
 impl<'a> Argument<'a> {
     #[inline(never)]
     fn show_uint(x: &uint, f: &mut Formatter) -> Result {
-        Show::fmt(x, f)
+        Display::fmt(x, f)
     }
 
     fn new<'b, T>(x: &'b T, f: fn(&T, &mut Formatter) -> Result) -> Argument<'b> {
@@ -214,14 +217,15 @@ pub struct Arguments<'a> {
     args: &'a [Argument<'a>],
 }
 
-impl<'a> Show for Arguments<'a> {
+#[stable]
+impl<'a> Debug for Arguments<'a> {
     fn fmt(&self, fmt: &mut Formatter) -> Result {
-        String::fmt(self, fmt)
+        Display::fmt(self, fmt)
     }
 }
 
 #[stable]
-impl<'a> String for Arguments<'a> {
+impl<'a> Display for Arguments<'a> {
     fn fmt(&self, fmt: &mut Formatter) -> Result {
         write(fmt.buf, *self)
     }
@@ -229,20 +233,49 @@ impl<'a> String for Arguments<'a> {
 
 /// Format trait for the `:?` format. Useful for debugging, most all types
 /// should implement this.
-#[unstable = "I/O and core have yet to be reconciled"]
+#[deprecated = "renamed to Debug"]
+#[cfg(not(stage0))]
 pub trait Show {
     /// Formats the value using the given formatter.
     fn fmt(&self, &mut Formatter) -> Result;
 }
 
+/// Format trait for the `:?` format. Useful for debugging, most all types
+/// should implement this.
+#[unstable = "I/O and core have yet to be reconciled"]
+pub trait Debug {
+    /// Formats the value using the given formatter.
+    fn fmt(&self, &mut Formatter) -> Result;
+}
+
+#[cfg(not(stage0))]
+impl<T: Show + ?Sized> Debug for T {
+    #[allow(deprecated)]
+    fn fmt(&self, f: &mut Formatter) -> Result { Show::fmt(self, f) }
+}
+
+/// When a value can be semantically expressed as a String, this trait may be
+/// used. It corresponds to the default format, `{}`.
+#[deprecated = "renamed to Display"]
+#[cfg(not(stage0))]
+pub trait String {
+    /// Formats the value using the given formatter.
+    fn fmt(&self, &mut Formatter) -> Result;
+}
+
 /// When a value can be semantically expressed as a String, this trait may be
 /// used. It corresponds to the default format, `{}`.
 #[unstable = "I/O and core have yet to be reconciled"]
-pub trait String {
+pub trait Display {
     /// Formats the value using the given formatter.
     fn fmt(&self, &mut Formatter) -> Result;
 }
 
+#[cfg(not(stage0))]
+impl<T: String + ?Sized> Display for T {
+    #[allow(deprecated)]
+    fn fmt(&self, f: &mut Formatter) -> Result { String::fmt(self, f) }
+}
 
 /// Format trait for the `o` character
 #[unstable = "I/O and core have yet to be reconciled"]
@@ -583,9 +616,10 @@ impl<'a> Formatter<'a> {
     pub fn precision(&self) -> Option<uint> { self.precision }
 }
 
-impl Show for Error {
+#[stable]
+impl Display for Error {
     fn fmt(&self, f: &mut Formatter) -> Result {
-        String::fmt("an error occurred when formatting an argument", f)
+        Display::fmt("an error occurred when formatting an argument", f)
     }
 }
 
@@ -611,9 +645,11 @@ pub fn argumentuint<'a>(s: &'a uint) -> Argument<'a> {
 macro_rules! fmt_refs {
     ($($tr:ident),*) => {
         $(
+        #[stable]
         impl<'a, T: ?Sized + $tr> $tr for &'a T {
             fn fmt(&self, f: &mut Formatter) -> Result { $tr::fmt(&**self, f) }
         }
+        #[stable]
         impl<'a, T: ?Sized + $tr> $tr for &'a mut T {
             fn fmt(&self, f: &mut Formatter) -> Result { $tr::fmt(&**self, f) }
         }
@@ -621,22 +657,24 @@ macro_rules! fmt_refs {
     }
 }
 
-fmt_refs! { Show, String, Octal, Binary, LowerHex, UpperHex, LowerExp, UpperExp }
+fmt_refs! { Debug, Display, Octal, Binary, LowerHex, UpperHex, LowerExp, UpperExp }
 
-impl Show for bool {
+#[stable]
+impl Debug for bool {
     fn fmt(&self, f: &mut Formatter) -> Result {
-        String::fmt(self, f)
+        Display::fmt(self, f)
     }
 }
 
 #[stable]
-impl String for bool {
+impl Display for bool {
     fn fmt(&self, f: &mut Formatter) -> Result {
-        String::fmt(if *self { "true" } else { "false" }, f)
+        Display::fmt(if *self { "true" } else { "false" }, f)
     }
 }
 
-impl Show for str {
+#[stable]
+impl Debug for str {
     fn fmt(&self, f: &mut Formatter) -> Result {
         try!(write!(f, "\""));
         for c in self.chars().flat_map(|c| c.escape_default()) {
@@ -647,13 +685,14 @@ impl Show for str {
 }
 
 #[stable]
-impl String for str {
+impl Display for str {
     fn fmt(&self, f: &mut Formatter) -> Result {
         f.pad(self)
     }
 }
 
-impl Show for char {
+#[stable]
+impl Debug for char {
     fn fmt(&self, f: &mut Formatter) -> Result {
         use char::CharExt;
         try!(write!(f, "'"));
@@ -665,15 +704,16 @@ impl Show for char {
 }
 
 #[stable]
-impl String for char {
+impl Display for char {
     fn fmt(&self, f: &mut Formatter) -> Result {
         let mut utf8 = [0u8; 4];
         let amt = self.encode_utf8(&mut utf8).unwrap_or(0);
         let s: &str = unsafe { mem::transmute(&utf8[..amt]) };
-        String::fmt(s, f)
+        Display::fmt(s, f)
     }
 }
 
+#[stable]
 impl<T> Pointer for *const T {
     fn fmt(&self, f: &mut Formatter) -> Result {
         f.flags |= 1 << (rt::FlagAlternate as uint);
@@ -683,18 +723,21 @@ impl<T> Pointer for *const T {
     }
 }
 
+#[stable]
 impl<T> Pointer for *mut T {
     fn fmt(&self, f: &mut Formatter) -> Result {
         Pointer::fmt(&(*self as *const T), f)
     }
 }
 
+#[stable]
 impl<'a, T> Pointer for &'a T {
     fn fmt(&self, f: &mut Formatter) -> Result {
         Pointer::fmt(&(*self as *const T), f)
     }
 }
 
+#[stable]
 impl<'a, T> Pointer for &'a mut T {
     fn fmt(&self, f: &mut Formatter) -> Result {
         Pointer::fmt(&(&**self as *const T), f)
@@ -703,15 +746,15 @@ impl<'a, T> Pointer for &'a mut T {
 
 macro_rules! floating { ($ty:ident) => {
 
-    impl Show for $ty {
+    #[stable]
+    impl Debug for $ty {
         fn fmt(&self, fmt: &mut Formatter) -> Result {
-            try!(String::fmt(self, fmt));
-            fmt.write_str(stringify!($ty))
+            Display::fmt(self, fmt)
         }
     }
 
     #[stable]
-    impl String for $ty {
+    impl Display for $ty {
         fn fmt(&self, fmt: &mut Formatter) -> Result {
             use num::Float;
 
@@ -732,6 +775,7 @@ macro_rules! floating { ($ty:ident) => {
         }
     }
 
+    #[stable]
     impl LowerExp for $ty {
         fn fmt(&self, fmt: &mut Formatter) -> Result {
             use num::Float;
@@ -753,6 +797,7 @@ macro_rules! floating { ($ty:ident) => {
         }
     }
 
+    #[stable]
     impl UpperExp for $ty {
         fn fmt(&self, fmt: &mut Formatter) -> Result {
             use num::Float;
@@ -777,12 +822,14 @@ macro_rules! floating { ($ty:ident) => {
 floating! { f32 }
 floating! { f64 }
 
-// Implementation of Show for various core types
+// Implementation of Display/Debug for various core types
 
-impl<T> Show for *const T {
+#[stable]
+impl<T> Debug for *const T {
     fn fmt(&self, f: &mut Formatter) -> Result { Pointer::fmt(self, f) }
 }
-impl<T> Show for *mut T {
+#[stable]
+impl<T> Debug for *mut T {
     fn fmt(&self, f: &mut Formatter) -> Result { Pointer::fmt(self, f) }
 }
 
@@ -793,7 +840,8 @@ macro_rules! peel {
 macro_rules! tuple {
     () => ();
     ( $($name:ident,)+ ) => (
-        impl<$($name:Show),*> Show for ($($name,)*) {
+        #[stable]
+        impl<$($name:Debug),*> Debug for ($($name,)*) {
             #[allow(non_snake_case, unused_assignments)]
             fn fmt(&self, f: &mut Formatter) -> Result {
                 try!(write!(f, "("));
@@ -818,11 +866,13 @@ macro_rules! tuple {
 
 tuple! { T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, }
 
-impl<'a> Show for &'a (any::Any+'a) {
+#[stable]
+impl<'a> Debug for &'a (any::Any+'a) {
     fn fmt(&self, f: &mut Formatter) -> Result { f.pad("&Any") }
 }
 
-impl<T: Show> Show for [T] {
+#[stable]
+impl<T: Debug> Debug for [T] {
     fn fmt(&self, f: &mut Formatter) -> Result {
         if f.flags & (1 << (rt::FlagAlternate as uint)) == 0 {
             try!(write!(f, "["));
@@ -843,20 +893,22 @@ impl<T: Show> Show for [T] {
     }
 }
 
-impl Show for () {
+#[stable]
+impl Debug for () {
     fn fmt(&self, f: &mut Formatter) -> Result {
         f.pad("()")
     }
 }
 
-impl<T: Copy + Show> Show for Cell<T> {
+#[stable]
+impl<T: Copy + Debug> Debug for Cell<T> {
     fn fmt(&self, f: &mut Formatter) -> Result {
         write!(f, "Cell {{ value: {:?} }}", self.get())
     }
 }
 
-#[unstable]
-impl<T: Show> Show for RefCell<T> {
+#[stable]
+impl<T: Debug> Debug for RefCell<T> {
     fn fmt(&self, f: &mut Formatter) -> Result {
         match self.try_borrow() {
             Some(val) => write!(f, "RefCell {{ value: {:?} }}", val),
@@ -865,29 +917,17 @@ impl<T: Show> Show for RefCell<T> {
     }
 }
 
-impl<'b, T: Show> Show for Ref<'b, T> {
-    fn fmt(&self, f: &mut Formatter) -> Result {
-        Show::fmt(&**self, f)
-    }
-}
-
-impl<'b, T: Show> Show for RefMut<'b, T> {
+#[stable]
+impl<'b, T: Debug> Debug for Ref<'b, T> {
     fn fmt(&self, f: &mut Formatter) -> Result {
-        Show::fmt(&*(self.deref()), f)
+        Debug::fmt(&**self, f)
     }
 }
 
 #[stable]
-impl String for Utf8Error {
+impl<'b, T: Debug> Debug for RefMut<'b, T> {
     fn fmt(&self, f: &mut Formatter) -> Result {
-        match *self {
-            Utf8Error::InvalidByte(n) => {
-                write!(f, "invalid utf-8: invalid byte at index {}", n)
-            }
-            Utf8Error::TooShort => {
-                write!(f, "invalid utf-8: byte slice too short")
-            }
-        }
+        Debug::fmt(&*(self.deref()), f)
     }
 }
 
diff --git a/src/libcore/fmt/num.rs b/src/libcore/fmt/num.rs
index 1df6f845225..c456b3379e8 100644
--- a/src/libcore/fmt/num.rs
+++ b/src/libcore/fmt/num.rs
@@ -154,13 +154,14 @@ pub fn radix<T>(x: T, base: u8) -> RadixFmt<T, Radix> {
 
 macro_rules! radix_fmt {
     ($T:ty as $U:ty, $fmt:ident, $S:expr) => {
-        impl fmt::Show for RadixFmt<$T, Radix> {
+        #[stable]
+        impl fmt::Debug for RadixFmt<$T, Radix> {
             fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-                try!(fmt::String::fmt(self, f));
-                f.write_str($S)
+                fmt::Display::fmt(self, f)
             }
         }
-        impl fmt::String for RadixFmt<$T, Radix> {
+        #[stable]
+        impl fmt::Display for RadixFmt<$T, Radix> {
             fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                 match *self { RadixFmt(ref x, radix) => radix.$fmt(*x as $U, f) }
             }
@@ -169,6 +170,7 @@ macro_rules! radix_fmt {
 }
 macro_rules! int_base {
     ($Trait:ident for $T:ident as $U:ident -> $Radix:ident) => {
+        #[stable]
         impl fmt::$Trait for $T {
             fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                 $Radix.fmt_int(*self as $U, f)
@@ -179,10 +181,10 @@ macro_rules! int_base {
 
 macro_rules! show {
     ($T:ident with $S:expr) => {
-        impl fmt::Show for $T {
+        #[stable]
+        impl fmt::Debug for $T {
             fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-                try!(fmt::String::fmt(self, f));
-                f.write_str($S)
+                fmt::Display::fmt(self, f)
             }
         }
     }
@@ -192,7 +194,7 @@ macro_rules! integer {
         integer! { $Int, $Uint, stringify!($Int), stringify!($Uint) }
     };
     ($Int:ident, $Uint:ident, $SI:expr, $SU:expr) => {
-        int_base! { String   for $Int as $Int   -> Decimal }
+        int_base! { Display  for $Int as $Int   -> Decimal }
         int_base! { Binary   for $Int as $Uint  -> Binary }
         int_base! { Octal    for $Int as $Uint  -> Octal }
         int_base! { LowerHex for $Int as $Uint  -> LowerHex }
@@ -200,7 +202,7 @@ macro_rules! integer {
         radix_fmt! { $Int as $Int, fmt_int, $SI }
         show! { $Int with $SI }
 
-        int_base! { String   for $Uint as $Uint -> Decimal }
+        int_base! { Display  for $Uint as $Uint -> Decimal }
         int_base! { Binary   for $Uint as $Uint -> Binary }
         int_base! { Octal    for $Uint as $Uint -> Octal }
         int_base! { LowerHex for $Uint as $Uint -> LowerHex }
diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs
index 0b150d1ecf9..bbe5cfe1cbb 100644
--- a/src/libcore/lib.rs
+++ b/src/libcore/lib.rs
@@ -136,6 +136,7 @@ pub mod slice;
 pub mod str;
 pub mod hash;
 pub mod fmt;
+pub mod error;
 
 // note: does not need to be public
 mod tuple;
diff --git a/src/libcore/ops.rs b/src/libcore/ops.rs
index 7131253d5c4..853e4adb892 100644
--- a/src/libcore/ops.rs
+++ b/src/libcore/ops.rs
@@ -885,10 +885,10 @@ pub trait IndexMut<Index: ?Sized> {
 #[unstable = "API still in development"]
 pub struct FullRange;
 
-#[unstable = "API still in development"]
-impl fmt::Show for FullRange {
+#[stable]
+impl fmt::Debug for FullRange {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
-        fmt::Show::fmt("..", fmt)
+        fmt::Debug::fmt("..", fmt)
     }
 }
 
@@ -944,8 +944,8 @@ impl<Idx: Clone + Step> DoubleEndedIterator for Range<Idx> {
 #[unstable = "API still in development"]
 impl<Idx: Clone + Step> ExactSizeIterator for Range<Idx> {}
 
-#[unstable = "API still in development"]
-impl<Idx: fmt::Show> fmt::Show for Range<Idx> {
+#[stable]
+impl<Idx: fmt::Debug> fmt::Debug for Range<Idx> {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         write!(fmt, "{:?}..{:?}", self.start, self.end)
     }
@@ -973,8 +973,8 @@ impl<Idx: Clone + Step> Iterator for RangeFrom<Idx> {
     }
 }
 
-#[unstable = "API still in development"]
-impl<Idx: fmt::Show> fmt::Show for RangeFrom<Idx> {
+#[stable]
+impl<Idx: fmt::Debug> fmt::Debug for RangeFrom<Idx> {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         write!(fmt, "{:?}..", self.start)
     }
@@ -989,8 +989,8 @@ pub struct RangeTo<Idx> {
     pub end: Idx,
 }
 
-#[unstable = "API still in development"]
-impl<Idx: fmt::Show> fmt::Show for RangeTo<Idx> {
+#[stable]
+impl<Idx: fmt::Debug> fmt::Debug for RangeTo<Idx> {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         write!(fmt, "..{:?}", self.end)
     }
diff --git a/src/libcore/result.rs b/src/libcore/result.rs
index 1ab810f937d..c3d49e24978 100644
--- a/src/libcore/result.rs
+++ b/src/libcore/result.rs
@@ -229,7 +229,7 @@
 use self::Result::{Ok, Err};
 
 use clone::Clone;
-use fmt::Show;
+use fmt::Display;
 use iter::{Iterator, IteratorExt, DoubleEndedIterator, FromIterator, ExactSizeIterator};
 use ops::{FnMut, FnOnce};
 use option::Option::{self, None, Some};
@@ -714,7 +714,7 @@ impl<T, E> Result<T, E> {
 }
 
 #[stable]
-impl<T, E: Show> Result<T, E> {
+impl<T, E: Display> Result<T, E> {
     /// Unwraps a result, yielding the content of an `Ok`.
     ///
     /// # Panics
@@ -739,13 +739,13 @@ impl<T, E: Show> Result<T, E> {
         match self {
             Ok(t) => t,
             Err(e) =>
-                panic!("called `Result::unwrap()` on an `Err` value: {:?}", e)
+                panic!("called `Result::unwrap()` on an `Err` value: {}", e)
         }
     }
 }
 
 #[stable]
-impl<T: Show, E> Result<T, E> {
+impl<T: Display, E> Result<T, E> {
     /// Unwraps a result, yielding the content of an `Err`.
     ///
     /// # Panics
@@ -769,7 +769,7 @@ impl<T: Show, E> Result<T, E> {
     pub fn unwrap_err(self) -> E {
         match self {
             Ok(t) =>
-                panic!("called `Result::unwrap_err()` on an `Ok` value: {:?}", t),
+                panic!("called `Result::unwrap_err()` on an `Ok` value: {}", t),
             Err(e) => e
         }
     }
diff --git a/src/libcore/str/mod.rs b/src/libcore/str/mod.rs
index 6a542b2c458..f8bd48220ba 100644
--- a/src/libcore/str/mod.rs
+++ b/src/libcore/str/mod.rs
@@ -20,8 +20,10 @@ use self::Searcher::{Naive, TwoWay, TwoWayLong};
 
 use cmp::{self, Eq};
 use default::Default;
-use iter::range;
+use error::Error;
+use fmt;
 use iter::ExactSizeIterator;
+use iter::range;
 use iter::{Map, Iterator, IteratorExt, DoubleEndedIterator};
 use marker::Sized;
 use mem;
@@ -242,6 +244,30 @@ impl<'a> CharEq for &'a [char] {
     }
 }
 
+#[stable]
+impl Error for Utf8Error {
+    fn description(&self) -> &str {
+        match *self {
+            Utf8Error::TooShort => "invalid utf-8: not enough bytes",
+            Utf8Error::InvalidByte(..) => "invalid utf-8: corrupt contents",
+        }
+    }
+}
+
+#[stable]
+impl fmt::Display for Utf8Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            Utf8Error::InvalidByte(n) => {
+                write!(f, "invalid utf-8: invalid byte at index {}", n)
+            }
+            Utf8Error::TooShort => {
+                write!(f, "invalid utf-8: byte slice too short")
+            }
+        }
+    }
+}
+
 /*
 Section: Iterators
 */