src/libstd/error.rs - third_party/rust - Git at Google

 // 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.

 #![stable(feature = "rust1", since = "1.0.0")]

 // A note about crates and the facade:
 //
 // Originally, the `Error` trait was defined in libcore, and the impls
 // were scattered about. However, coherence objected to this
 // arrangement, because to create the blanket impls for `Box` required
 // knowing that `&str: !Error`, and we have no means to deal with that
 // sort of conflict just now. Therefore, for the time being, we have
 // moved the `Error` trait into libstd. As we evolve a sol'n to the
 // coherence challenge (e.g., specialization, neg impls, etc) we can
 // reconsider what crate these items belong in.

 use any::TypeId;
 use boxed::Box;
 use char;
 use fmt::{self, Debug, Display};
 use marker::{Send, Sync, Reflect};
 use mem::transmute;
 use num;
 use raw::TraitObject;
 use str;
 use string::{self, String};

 /// Base functionality for all errors in Rust.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait Error: Debug + Display + Reflect {
     /// A short description of the error.
     ///
     /// The description should not contain newlines or sentence-ending
     /// punctuation, to facilitate embedding in larger user-facing
     /// strings.
     ///
     /// # Examples
     ///
     /// ```
     /// use std::error::Error;
     ///
     /// match "xc".parse::<u32>() {
     ///     Err(e) => {
     ///         println!("Error: {}", e.description());
     ///     }
     ///     _ => println!("No error"),
     /// }
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     fn description(&self) -> &str;

     /// The lower-level cause of this error, if any.
     ///
     /// # Examples
     ///
     /// ```
     /// use std::error::Error;
     /// use std::fmt;
     ///
     /// #[derive(Debug)]
     /// struct SuperError {
     ///     side: SuperErrorSideKick,
     /// }
     ///
     /// impl fmt::Display for SuperError {
     ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
     ///         write!(f, "SuperError is here!")
     ///     }
     /// }
     ///
     /// impl Error for SuperError {
     ///     fn description(&self) -> &str {
     ///         "I'm the superhero of errors!"
     ///     }
     ///
     ///     fn cause(&self) -> Option<&Error> {
     ///         Some(&self.side)
     ///     }
     /// }
     ///
     /// #[derive(Debug)]
     /// struct SuperErrorSideKick;
     ///
     /// impl fmt::Display for SuperErrorSideKick {
     ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
     ///         write!(f, "SuperErrorSideKick is here!")
     ///     }
     /// }
     ///
     /// impl Error for SuperErrorSideKick {
     ///     fn description(&self) -> &str {
     ///         "I'm SuperError side kick!"
     ///     }
     /// }
     ///
     /// fn get_super_error() -> Result<(), SuperError> {
     ///     Err(SuperError { side: SuperErrorSideKick })
     /// }
     ///
     /// fn main() {
     ///     match get_super_error() {
     ///         Err(e) => {
     ///             println!("Error: {}", e.description());
     ///             println!("Caused by: {}", e.cause().unwrap());
     ///         }
     ///         _ => println!("No error"),
     ///     }
     /// }
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     fn cause(&self) -> Option<&Error> { None }

     /// Get the `TypeId` of `self`
     #[doc(hidden)]
     #[unstable(feature = "error_type_id",
                reason = "unclear whether to commit to this public implementation detail",
                issue = "27745")]
     fn type_id(&self) -> TypeId where Self: 'static {
         TypeId::of::<Self>()
     }
 }

 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, E: Error + 'a> From<E> for Box<Error + 'a> {
     fn from(err: E) -> Box<Error + 'a> {
         Box::new(err)
     }
 }

 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<Error + Send + Sync + 'a> {
     fn from(err: E) -> Box<Error + Send + Sync + 'a> {
         Box::new(err)
     }
 }

 #[stable(feature = "rust1", since = "1.0.0")]
 impl From<String> for Box<Error + Send + Sync> {
     fn from(err: String) -> Box<Error + Send + Sync> {
         #[derive(Debug)]
         struct StringError(String);

         impl Error for StringError {
             fn description(&self) -> &str { &self.0 }
         }

         impl Display for StringError {
             fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                 Display::fmt(&self.0, f)
             }
         }

         Box::new(StringError(err))
     }
 }

 #[stable(feature = "string_box_error", since = "1.7.0")]
 impl From<String> for Box<Error> {
     fn from(str_err: String) -> Box<Error> {
         let err1: Box<Error + Send + Sync> = From::from(str_err);
         let err2: Box<Error> = err1;
         err2
     }
 }

 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, 'b> From<&'b str> for Box<Error + Send + Sync + 'a> {
     fn from(err: &'b str) -> Box<Error + Send + Sync + 'a> {
         From::from(String::from(err))
     }
 }

 #[stable(feature = "string_box_error", since = "1.7.0")]
 impl<'a> From<&'a str> for Box<Error> {
     fn from(err: &'a str) -> Box<Error> {
         From::from(String::from(err))
     }
 }

 #[stable(feature = "rust1", since = "1.0.0")]
 impl Error for str::ParseBoolError {
     fn description(&self) -> &str { "failed to parse bool" }
 }

 #[stable(feature = "rust1", since = "1.0.0")]
 impl Error for str::Utf8Error {
     fn description(&self) -> &str {
         "invalid utf-8: corrupt contents"
     }
 }

 #[stable(feature = "rust1", since = "1.0.0")]
 impl Error for num::ParseIntError {
     fn description(&self) -> &str {
         self.__description()
     }
 }

 #[unstable(feature = "try_from", issue = "33417")]
 impl Error for num::TryFromIntError {
     fn description(&self) -> &str {
         self.__description()
     }
 }

 #[stable(feature = "rust1", since = "1.0.0")]
 impl Error for num::ParseFloatError {
     fn description(&self) -> &str {
         self.__description()
     }
 }

 #[stable(feature = "rust1", since = "1.0.0")]
 impl Error for string::FromUtf8Error {
     fn description(&self) -> &str {
         "invalid utf-8"
     }
 }

 #[stable(feature = "rust1", since = "1.0.0")]
 impl Error for string::FromUtf16Error {
     fn description(&self) -> &str {
         "invalid utf-16"
     }
 }

 #[stable(feature = "str_parse_error2", since = "1.8.0")]
 impl Error for string::ParseError {
     fn description(&self) -> &str {
         match *self {}
     }
 }

 #[stable(feature = "decode_utf16", since = "1.9.0")]
 impl Error for char::DecodeUtf16Error {
     fn description(&self) -> &str {
         "unpaired surrogate found"
     }
 }

 #[stable(feature = "box_error", since = "1.7.0")]
 impl<T: Error> Error for Box<T> {
     fn description(&self) -> &str {
         Error::description(&**self)
     }

     fn cause(&self) -> Option<&Error> {
         Error::cause(&**self)
     }
 }

 #[stable(feature = "fmt_error", since = "1.11.0")]
 impl Error for fmt::Error {
     fn description(&self) -> &str {
         "an error occurred when formatting an argument"
     }
 }

 // copied from any.rs
 impl Error + 'static {
     /// Returns true if the boxed type is the same as `T`
     #[stable(feature = "error_downcast", since = "1.3.0")]
     #[inline]
     pub fn is<T: Error + 'static>(&self) -> bool {
         // Get TypeId of the type this function is instantiated with
         let t = TypeId::of::<T>();

         // Get TypeId of the type in the trait object
         let boxed = self.type_id();

         // Compare both TypeIds on equality
         t == boxed
     }

     /// Returns some reference to the boxed value if it is of type `T`, or
     /// `None` if it isn't.
     #[stable(feature = "error_downcast", since = "1.3.0")]
     #[inline]
     pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
         if self.is::<T>() {
             unsafe {
                 // Get the raw representation of the trait object
                 let to: TraitObject = transmute(self);

                 // Extract the data pointer
                 Some(&*(to.data as *const T))
             }
         } else {
             None
         }
     }

     /// Returns some mutable reference to the boxed value if it is of type `T`, or
     /// `None` if it isn't.
     #[stable(feature = "error_downcast", since = "1.3.0")]
     #[inline]
     pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
         if self.is::<T>() {
             unsafe {
                 // Get the raw representation of the trait object
                 let to: TraitObject = transmute(self);

                 // Extract the data pointer
                 Some(&mut *(to.data as *const T as *mut T))
             }
         } else {
             None
         }
     }
 }

 impl Error + 'static + Send {
     /// Forwards to the method defined on the type `Any`.
     #[stable(feature = "error_downcast", since = "1.3.0")]
     #[inline]
     pub fn is<T: Error + 'static>(&self) -> bool {
         <Error + 'static>::is::<T>(self)
     }

     /// Forwards to the method defined on the type `Any`.
     #[stable(feature = "error_downcast", since = "1.3.0")]
     #[inline]
     pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
         <Error + 'static>::downcast_ref::<T>(self)
     }

     /// Forwards to the method defined on the type `Any`.
     #[stable(feature = "error_downcast", since = "1.3.0")]
     #[inline]
     pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
         <Error + 'static>::downcast_mut::<T>(self)
     }
 }

 impl Error + 'static + Send + Sync {
     /// Forwards to the method defined on the type `Any`.
     #[stable(feature = "error_downcast", since = "1.3.0")]
     #[inline]
     pub fn is<T: Error + 'static>(&self) -> bool {
         <Error + 'static>::is::<T>(self)
     }

     /// Forwards to the method defined on the type `Any`.
     #[stable(feature = "error_downcast", since = "1.3.0")]
     #[inline]
     pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
         <Error + 'static>::downcast_ref::<T>(self)
     }

     /// Forwards to the method defined on the type `Any`.
     #[stable(feature = "error_downcast", since = "1.3.0")]
     #[inline]
     pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
         <Error + 'static>::downcast_mut::<T>(self)
     }
 }

 impl Error {
     #[inline]
     #[stable(feature = "error_downcast", since = "1.3.0")]
     /// Attempt to downcast the box to a concrete type.
     pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<Error>> {
         if self.is::<T>() {
             unsafe {
                 // Get the raw representation of the trait object
                 let raw = Box::into_raw(self);
                 let to: TraitObject =
                     transmute::<*mut Error, TraitObject>(raw);

                 // Extract the data pointer
                 Ok(Box::from_raw(to.data as *mut T))
             }
         } else {
             Err(self)
         }
     }
 }

 impl Error + Send {
     #[inline]
     #[stable(feature = "error_downcast", since = "1.3.0")]
     /// Attempt to downcast the box to a concrete type.
     pub fn downcast<T: Error + 'static>(self: Box<Self>)
                                         -> Result<Box<T>, Box<Error + Send>> {
         let err: Box<Error> = self;
         <Error>::downcast(err).map_err(|s| unsafe {
             // reapply the Send marker
             transmute::<Box<Error>, Box<Error + Send>>(s)
         })
     }
 }

 impl Error + Send + Sync {
     #[inline]
     #[stable(feature = "error_downcast", since = "1.3.0")]
     /// Attempt to downcast the box to a concrete type.
     pub fn downcast<T: Error + 'static>(self: Box<Self>)
                                         -> Result<Box<T>, Box<Self>> {
         let err: Box<Error> = self;
         <Error>::downcast(err).map_err(|s| unsafe {
             // reapply the Send+Sync marker
             transmute::<Box<Error>, Box<Error + Send + Sync>>(s)
         })
     }
 }

 #[cfg(test)]
 mod tests {
     use prelude::v1::*;
     use super::Error;
     use fmt;

     #[derive(Debug, PartialEq)]
     struct A;
     #[derive(Debug, PartialEq)]
     struct B;

     impl fmt::Display for A {
         fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
             write!(f, "A")
         }
     }
     impl fmt::Display for B {
         fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
             write!(f, "B")
         }
     }

     impl Error for A {
         fn description(&self) -> &str { "A-desc" }
     }
     impl Error for B {
         fn description(&self) -> &str { "A-desc" }
     }

     #[test]
     fn downcasting() {
         let mut a = A;
         let mut a = &mut a as &mut (Error + 'static);
         assert_eq!(a.downcast_ref::<A>(), Some(&A));
         assert_eq!(a.downcast_ref::<B>(), None);
         assert_eq!(a.downcast_mut::<A>(), Some(&mut A));
         assert_eq!(a.downcast_mut::<B>(), None);

         let a: Box<Error> = Box::new(A);
         match a.downcast::<B>() {
             Ok(..) => panic!("expected error"),
             Err(e) => assert_eq!(*e.downcast::<A>().unwrap(), A),
         }
     }
 }
	// 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.

	#![stable(feature = "rust1", since = "1.0.0")]

	// A note about crates and the facade:
	//
	// Originally, the `Error` trait was defined in libcore, and the impls
	// were scattered about. However, coherence objected to this
	// arrangement, because to create the blanket impls for `Box` required
	// knowing that `&str: !Error`, and we have no means to deal with that
	// sort of conflict just now. Therefore, for the time being, we have
	// moved the `Error` trait into libstd. As we evolve a sol'n to the
	// coherence challenge (e.g., specialization, neg impls, etc) we can
	// reconsider what crate these items belong in.

	use any::TypeId;
	use boxed::Box;
	use char;
	use fmt::{self, Debug, Display};
	use marker::{Send, Sync, Reflect};
	use mem::transmute;
	use num;
	use raw::TraitObject;
	use str;
	use string::{self, String};

	/// Base functionality for all errors in Rust.
	#[stable(feature = "rust1", since = "1.0.0")]
	pub trait Error: Debug + Display + Reflect {
	/// A short description of the error.
	///
	/// The description should not contain newlines or sentence-ending
	/// punctuation, to facilitate embedding in larger user-facing
	/// strings.
	///
	/// # Examples
	///
	/// ```
	/// use std::error::Error;
	///
	/// match "xc".parse::<u32>() {
	/// Err(e) => {
	/// println!("Error: {}", e.description());
	/// }
	/// _ => println!("No error"),
	/// }
	/// ```
	#[stable(feature = "rust1", since = "1.0.0")]
	fn description(&self) -> &str;

	/// The lower-level cause of this error, if any.
	///
	/// # Examples
	///
	/// ```
	/// use std::error::Error;
	/// use std::fmt;
	///
	/// #[derive(Debug)]
	/// struct SuperError {
	/// side: SuperErrorSideKick,
	/// }
	///
	/// impl fmt::Display for SuperError {
	/// fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
	/// write!(f, "SuperError is here!")
	/// }
	/// }
	///
	/// impl Error for SuperError {
	/// fn description(&self) -> &str {
	/// "I'm the superhero of errors!"
	/// }
	///
	/// fn cause(&self) -> Option<&Error> {
	/// Some(&self.side)
	/// }
	/// }
	///
	/// #[derive(Debug)]
	/// struct SuperErrorSideKick;
	///
	/// impl fmt::Display for SuperErrorSideKick {
	/// fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
	/// write!(f, "SuperErrorSideKick is here!")
	/// }
	/// }
	///
	/// impl Error for SuperErrorSideKick {
	/// fn description(&self) -> &str {
	/// "I'm SuperError side kick!"
	/// }
	/// }
	///
	/// fn get_super_error() -> Result<(), SuperError> {
	/// Err(SuperError { side: SuperErrorSideKick })
	/// }
	///
	/// fn main() {
	/// match get_super_error() {
	/// Err(e) => {
	/// println!("Error: {}", e.description());
	/// println!("Caused by: {}", e.cause().unwrap());
	/// }
	/// _ => println!("No error"),
	/// }
	/// }
	/// ```
	#[stable(feature = "rust1", since = "1.0.0")]
	fn cause(&self) -> Option<&Error> { None }

	/// Get the `TypeId` of `self`
	#[doc(hidden)]
	#[unstable(feature = "error_type_id",
	reason = "unclear whether to commit to this public implementation detail",
	issue = "27745")]
	fn type_id(&self) -> TypeId where Self: 'static {
	TypeId::of::<Self>()
	}
	}

	#[stable(feature = "rust1", since = "1.0.0")]
	impl<'a, E: Error + 'a> From<E> for Box<Error + 'a> {
	fn from(err: E) -> Box<Error + 'a> {
	Box::new(err)
	}
	}

	#[stable(feature = "rust1", since = "1.0.0")]
	impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<Error + Send + Sync + 'a> {
	fn from(err: E) -> Box<Error + Send + Sync + 'a> {
	Box::new(err)
	}
	}

	#[stable(feature = "rust1", since = "1.0.0")]
	impl From<String> for Box<Error + Send + Sync> {
	fn from(err: String) -> Box<Error + Send + Sync> {
	#[derive(Debug)]
	struct StringError(String);

	impl Error for StringError {
	fn description(&self) -> &str { &self.0 }
	}

	impl Display for StringError {
	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
	Display::fmt(&self.0, f)
	}
	}

	Box::new(StringError(err))
	}
	}

	#[stable(feature = "string_box_error", since = "1.7.0")]
	impl From<String> for Box<Error> {
	fn from(str_err: String) -> Box<Error> {
	let err1: Box<Error + Send + Sync> = From::from(str_err);
	let err2: Box<Error> = err1;
	err2
	}
	}

	#[stable(feature = "rust1", since = "1.0.0")]
	impl<'a, 'b> From<&'b str> for Box<Error + Send + Sync + 'a> {
	fn from(err: &'b str) -> Box<Error + Send + Sync + 'a> {
	From::from(String::from(err))
	}
	}

	#[stable(feature = "string_box_error", since = "1.7.0")]
	impl<'a> From<&'a str> for Box<Error> {
	fn from(err: &'a str) -> Box<Error> {
	From::from(String::from(err))
	}
	}

	#[stable(feature = "rust1", since = "1.0.0")]
	impl Error for str::ParseBoolError {
	fn description(&self) -> &str { "failed to parse bool" }
	}

	#[stable(feature = "rust1", since = "1.0.0")]
	impl Error for str::Utf8Error {
	fn description(&self) -> &str {
	"invalid utf-8: corrupt contents"
	}
	}

	#[stable(feature = "rust1", since = "1.0.0")]
	impl Error for num::ParseIntError {
	fn description(&self) -> &str {
	self.__description()
	}
	}

	#[unstable(feature = "try_from", issue = "33417")]
	impl Error for num::TryFromIntError {
	fn description(&self) -> &str {
	self.__description()
	}
	}

	#[stable(feature = "rust1", since = "1.0.0")]
	impl Error for num::ParseFloatError {
	fn description(&self) -> &str {
	self.__description()
	}
	}

	#[stable(feature = "rust1", since = "1.0.0")]
	impl Error for string::FromUtf8Error {
	fn description(&self) -> &str {
	"invalid utf-8"
	}
	}

	#[stable(feature = "rust1", since = "1.0.0")]
	impl Error for string::FromUtf16Error {
	fn description(&self) -> &str {
	"invalid utf-16"
	}
	}

	#[stable(feature = "str_parse_error2", since = "1.8.0")]
	impl Error for string::ParseError {
	fn description(&self) -> &str {
	match *self {}
	}
	}

	#[stable(feature = "decode_utf16", since = "1.9.0")]
	impl Error for char::DecodeUtf16Error {
	fn description(&self) -> &str {
	"unpaired surrogate found"
	}
	}

	#[stable(feature = "box_error", since = "1.7.0")]
	impl<T: Error> Error for Box<T> {
	fn description(&self) -> &str {
	Error::description(&**self)
	}

	fn cause(&self) -> Option<&Error> {
	Error::cause(&**self)
	}
	}

	#[stable(feature = "fmt_error", since = "1.11.0")]
	impl Error for fmt::Error {
	fn description(&self) -> &str {
	"an error occurred when formatting an argument"
	}
	}

	// copied from any.rs
	impl Error + 'static {
	/// Returns true if the boxed type is the same as `T`
	#[stable(feature = "error_downcast", since = "1.3.0")]
	#[inline]
	pub fn is<T: Error + 'static>(&self) -> bool {
	// Get TypeId of the type this function is instantiated with
	let t = TypeId::of::<T>();

	// Get TypeId of the type in the trait object
	let boxed = self.type_id();

	// Compare both TypeIds on equality
	t == boxed
	}

	/// Returns some reference to the boxed value if it is of type `T`, or
	/// `None` if it isn't.
	#[stable(feature = "error_downcast", since = "1.3.0")]
	#[inline]
	pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
	if self.is::<T>() {
	unsafe {
	// Get the raw representation of the trait object
	let to: TraitObject = transmute(self);

	// Extract the data pointer
	Some(&(to.data as const T))
	}
	} else {
	None
	}
	}

	/// Returns some mutable reference to the boxed value if it is of type `T`, or
	/// `None` if it isn't.
	#[stable(feature = "error_downcast", since = "1.3.0")]
	#[inline]
	pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
	if self.is::<T>() {
	unsafe {
	// Get the raw representation of the trait object
	let to: TraitObject = transmute(self);

	// Extract the data pointer
	Some(&mut (to.data as const T as *mut T))
	}
	} else {
	None
	}
	}
	}

	impl Error + 'static + Send {
	/// Forwards to the method defined on the type `Any`.
	#[stable(feature = "error_downcast", since = "1.3.0")]
	#[inline]
	pub fn is<T: Error + 'static>(&self) -> bool {
	<Error + 'static>::is::<T>(self)
	}

	/// Forwards to the method defined on the type `Any`.
	#[stable(feature = "error_downcast", since = "1.3.0")]
	#[inline]
	pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
	<Error + 'static>::downcast_ref::<T>(self)
	}

	/// Forwards to the method defined on the type `Any`.
	#[stable(feature = "error_downcast", since = "1.3.0")]
	#[inline]
	pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
	<Error + 'static>::downcast_mut::<T>(self)
	}
	}

	impl Error + 'static + Send + Sync {
	/// Forwards to the method defined on the type `Any`.
	#[stable(feature = "error_downcast", since = "1.3.0")]
	#[inline]
	pub fn is<T: Error + 'static>(&self) -> bool {
	<Error + 'static>::is::<T>(self)
	}

	/// Forwards to the method defined on the type `Any`.
	#[stable(feature = "error_downcast", since = "1.3.0")]
	#[inline]
	pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
	<Error + 'static>::downcast_ref::<T>(self)
	}

	/// Forwards to the method defined on the type `Any`.
	#[stable(feature = "error_downcast", since = "1.3.0")]
	#[inline]
	pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
	<Error + 'static>::downcast_mut::<T>(self)
	}
	}

	impl Error {
	#[inline]
	#[stable(feature = "error_downcast", since = "1.3.0")]
	/// Attempt to downcast the box to a concrete type.
	pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<Error>> {
	if self.is::<T>() {
	unsafe {
	// Get the raw representation of the trait object
	let raw = Box::into_raw(self);
	let to: TraitObject =
	transmute::<*mut Error, TraitObject>(raw);

	// Extract the data pointer
	Ok(Box::from_raw(to.data as *mut T))
	}
	} else {
	Err(self)
	}
	}
	}

	impl Error + Send {
	#[inline]
	#[stable(feature = "error_downcast", since = "1.3.0")]
	/// Attempt to downcast the box to a concrete type.
	pub fn downcast<T: Error + 'static>(self: Box<Self>)
	-> Result<Box<T>, Box<Error + Send>> {
	let err: Box<Error> = self;
	<Error>::downcast(err).map_err(\|s\| unsafe {
	// reapply the Send marker
	transmute::<Box<Error>, Box<Error + Send>>(s)
	})
	}
	}

	impl Error + Send + Sync {
	#[inline]
	#[stable(feature = "error_downcast", since = "1.3.0")]
	/// Attempt to downcast the box to a concrete type.
	pub fn downcast<T: Error + 'static>(self: Box<Self>)
	-> Result<Box<T>, Box<Self>> {
	let err: Box<Error> = self;
	<Error>::downcast(err).map_err(\|s\| unsafe {
	// reapply the Send+Sync marker
	transmute::<Box<Error>, Box<Error + Send + Sync>>(s)
	})
	}
	}

	#[cfg(test)]
	mod tests {
	use prelude::v1::*;
	use super::Error;
	use fmt;

	#[derive(Debug, PartialEq)]
	struct A;
	#[derive(Debug, PartialEq)]
	struct B;

	impl fmt::Display for A {
	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
	write!(f, "A")
	}
	}
	impl fmt::Display for B {
	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
	write!(f, "B")
	}
	}

	impl Error for A {
	fn description(&self) -> &str { "A-desc" }
	}
	impl Error for B {
	fn description(&self) -> &str { "A-desc" }
	}

	#[test]
	fn downcasting() {
	let mut a = A;
	let mut a = &mut a as &mut (Error + 'static);
	assert_eq!(a.downcast_ref::<A>(), Some(&A));
	assert_eq!(a.downcast_ref::<B>(), None);
	assert_eq!(a.downcast_mut::<A>(), Some(&mut A));
	assert_eq!(a.downcast_mut::<B>(), None);

	let a: Box<Error> = Box::new(A);
	match a.downcast::<B>() {
	Ok(..) => panic!("expected error"),
	Err(e) => assert_eq!(*e.downcast::<A>().unwrap(), A),
	}
	}
	}