error.rs.html -- source
Description: Source to the Rust file `src/libstd/error.rs`.
Keywords: rust, rustlang, rust-lang

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    //! Traits for working with Errors.

    #![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 core::array;

    use crate::alloc::{AllocErr, LayoutErr, CannotReallocInPlace};
    use crate::any::TypeId;
    use crate::backtrace::Backtrace;
    use crate::borrow::Cow;
    use crate::cell;
    use crate::char;
    use crate::fmt::{self, Debug, Display};
    use crate::mem::transmute;
    use crate::num;
    use crate::str;
    use crate::string;

    /// `Error` is a trait representing the basic expectations for error values,
    /// i.e., values of type `E` in [`Result<T, E>`]. Errors must describe
    /// themselves through the [`Display`] and [`Debug`] traits, and may provide
    /// cause chain information:
    ///
    /// The [`source`] method is generally used when errors cross "abstraction
    /// boundaries". If one module must report an error that is caused by an error
    /// from a lower-level module, it can allow access to that error via the
    /// [`source`] method. This makes it possible for the high-level module to
    /// provide its own errors while also revealing some of the implementation for
    /// debugging via [`source`] chains.
    ///
    /// [`Result<T, E>`]: ../result/enum.Result.html
    /// [`Display`]: ../fmt/trait.Display.html
    /// [`Debug`]: ../fmt/trait.Debug.html
    /// [`source`]: trait.Error.html#method.source
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait Error: Debug + Display {
        /// **This method is soft-deprecated.**
        ///
        /// Although using it won’t cause compilation warning,
        /// new code should use [`Display`] instead
        /// and new `impl`s can omit it.
        ///
        /// To obtain error description as a string, use `to_string()`.
        ///
        /// [`Display`]: ../fmt/trait.Display.html
        ///
        /// # Examples
        ///
        /// ```
        /// match "xc".parse::<u32>() {
        ///     Err(e) => {
        ///         // Print `e` itself, not `e.description()`.
        ///         println!("Error: {}", e);
        ///     }
        ///     _ => println!("No error"),
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn description(&self) -> &str {
            "description() is deprecated; use Display"
        }

        /// 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<&dyn 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")]
        #[rustc_deprecated(since = "1.33.0", reason = "replaced by Error::source, which can support \
                                                       downcasting")]
        fn cause(&self) -> Option<&dyn Error> {
            self.source()
        }

        /// The lower-level source 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 source(&self) -> Option<&(dyn Error + 'static)> {
        ///         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.source().unwrap());
        ///         }
        ///         _ => println!("No error"),
        ///     }
        /// }
        /// ```
        #[stable(feature = "error_source", since = "1.30.0")]
        fn source(&self) -> Option<&(dyn Error + 'static)> { None }

        /// Gets the `TypeId` of `self`.
        #[doc(hidden)]
        #[unstable(feature = "error_type_id",
                   reason = "this is memory-unsafe to override in user code",
                   issue = "60784")]
        fn type_id(&self, _: private::Internal) -> TypeId where Self: 'static {
            TypeId::of::<Self>()
        }

        /// Returns a stack backtrace, if available, of where this error ocurred.
        ///
        /// This function allows inspecting the location, in code, of where an error
        /// happened. The returned `Backtrace` contains information about the stack
        /// trace of the OS thread of execution of where the error originated from.
        ///
        /// Note that not all errors contain a `Backtrace`. Also note that a
        /// `Backtrace` may actually be empty. For more information consult the
        /// `Backtrace` type itself.
        #[unstable(feature = "backtrace", issue = "53487")]
        fn backtrace(&self) -> Option<&Backtrace> {
            None
        }
    }

    mod private {
        // This is a hack to prevent `type_id` from being overridden by `Error`
        // implementations, since that can enable unsound downcasting.
        #[unstable(feature = "error_type_id", issue = "60784")]
        #[derive(Debug)]
        pub struct Internal;
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, E: Error + 'a> From<E> for Box<dyn Error + 'a> {
        /// Converts a type of [`Error`] into a box of dyn [`Error`].
        ///
        /// [`Error`]: ../error/trait.Error.html
        ///
        /// # Examples
        ///
        /// ```
        /// use std::error::Error;
        /// use std::fmt;
        /// use std::mem;
        ///
        /// #[derive(Debug)]
        /// struct AnError;
        ///
        /// impl fmt::Display for AnError {
        ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        ///         write!(f , "An error")
        ///     }
        /// }
        ///
        /// impl Error for AnError {
        ///     fn description(&self) -> &str {
        ///         "Description of an error"
        ///     }
        /// }
        ///
        /// let an_error = AnError;
        /// assert!(0 == mem::size_of_val(&an_error));
        /// let a_boxed_error = Box::<dyn Error>::from(an_error);
        /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
        /// ```
        fn from(err: E) -> Box<dyn Error + 'a> {
            Box::new(err)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<dyn Error + Send + Sync + 'a> {
        /// Converts a type of [`Error`] + [`trait@Send`] + [`trait@Sync`] into a box of
        /// dyn [`Error`] + [`trait@Send`] + [`trait@Sync`].
        ///
        /// [`Error`]: ../error/trait.Error.html
        ///
        /// # Examples
        ///
        /// ```
        /// use std::error::Error;
        /// use std::fmt;
        /// use std::mem;
        ///
        /// #[derive(Debug)]
        /// struct AnError;
        ///
        /// impl fmt::Display for AnError {
        ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        ///         write!(f , "An error")
        ///     }
        /// }
        ///
        /// impl Error for AnError {
        ///     fn description(&self) -> &str {
        ///         "Description of an error"
        ///     }
        /// }
        ///
        /// unsafe impl Send for AnError {}
        ///
        /// unsafe impl Sync for AnError {}
        ///
        /// let an_error = AnError;
        /// assert!(0 == mem::size_of_val(&an_error));
        /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(an_error);
        /// assert!(
        ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
        /// ```
        fn from(err: E) -> Box<dyn Error + Send + Sync + 'a> {
            Box::new(err)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl From<String> for Box<dyn Error + Send + Sync> {
        /// Converts a [`String`] into a box of dyn [`Error`] + [`trait@Send`] + [`trait@Sync`].
        ///
        /// [`Error`]: ../error/trait.Error.html
        ///
        /// # Examples
        ///
        /// ```
        /// use std::error::Error;
        /// use std::mem;
        ///
        /// let a_string_error = "a string error".to_string();
        /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_string_error);
        /// assert!(
        ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
        /// ```
        fn from(err: String) -> Box<dyn Error + Send + Sync> {
            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)
                }
            }

            // Purposefully skip printing "StringError(..)"
            impl Debug for StringError {
                fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                    Debug::fmt(&self.0, f)
                }
            }

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

    #[stable(feature = "string_box_error", since = "1.6.0")]
    impl From<String> for Box<dyn Error> {
        /// Converts a [`String`] into a box of dyn [`Error`].
        ///
        /// [`Error`]: ../error/trait.Error.html
        ///
        /// # Examples
        ///
        /// ```
        /// use std::error::Error;
        /// use std::mem;
        ///
        /// let a_string_error = "a string error".to_string();
        /// let a_boxed_error = Box::<dyn Error>::from(a_string_error);
        /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
        /// ```
        fn from(str_err: String) -> Box<dyn Error> {
            let err1: Box<dyn Error + Send + Sync> = From::from(str_err);
            let err2: Box<dyn Error> = err1;
            err2
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a> From<&str> for Box<dyn Error + Send + Sync + 'a> {
        /// Converts a [`str`] into a box of dyn [`Error`] + [`trait@Send`] + [`trait@Sync`].
        ///
        /// [`Error`]: ../error/trait.Error.html
        ///
        /// # Examples
        ///
        /// ```
        /// use std::error::Error;
        /// use std::mem;
        ///
        /// let a_str_error = "a str error";
        /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_str_error);
        /// assert!(
        ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
        /// ```
        fn from(err: &str) -> Box<dyn Error + Send + Sync + 'a> {
            From::from(String::from(err))
        }
    }

    #[stable(feature = "string_box_error", since = "1.6.0")]
    impl From<&str> for Box<dyn Error> {
        /// Converts a [`str`] into a box of dyn [`Error`].
        ///
        /// [`Error`]: ../error/trait.Error.html
        ///
        /// # Examples
        ///
        /// ```
        /// use std::error::Error;
        /// use std::mem;
        ///
        /// let a_str_error = "a str error";
        /// let a_boxed_error = Box::<dyn Error>::from(a_str_error);
        /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
        /// ```
        fn from(err: &str) -> Box<dyn Error> {
            From::from(String::from(err))
        }
    }

    #[stable(feature = "cow_box_error", since = "1.22.0")]
    impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + Send + Sync + 'a> {
        /// Converts a [`Cow`] into a box of dyn [`Error`] + [`trait@Send`] + [`trait@Sync`].
        ///
        /// [`Cow`]: ../borrow/enum.Cow.html
        /// [`Error`]: ../error/trait.Error.html
        ///
        /// # Examples
        ///
        /// ```
        /// use std::error::Error;
        /// use std::mem;
        /// use std::borrow::Cow;
        ///
        /// let a_cow_str_error = Cow::from("a str error");
        /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_cow_str_error);
        /// assert!(
        ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
        /// ```
        fn from(err: Cow<'b, str>) -> Box<dyn Error + Send + Sync + 'a> {
            From::from(String::from(err))
        }
    }

    #[stable(feature = "cow_box_error", since = "1.22.0")]
    impl<'a> From<Cow<'a, str>> for Box<dyn Error> {
        /// Converts a [`Cow`] into a box of dyn [`Error`].
        ///
        /// [`Cow`]: ../borrow/enum.Cow.html
        /// [`Error`]: ../error/trait.Error.html
        ///
        /// # Examples
        ///
        /// ```
        /// use std::error::Error;
        /// use std::mem;
        /// use std::borrow::Cow;
        ///
        /// let a_cow_str_error = Cow::from("a str error");
        /// let a_boxed_error = Box::<dyn Error>::from(a_cow_str_error);
        /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
        /// ```
        fn from(err: Cow<'a, str>) -> Box<dyn Error> {
            From::from(String::from(err))
        }
    }

    #[unstable(feature = "never_type", issue = "35121")]
    impl Error for ! {
        fn description(&self) -> &str { *self }
    }

    #[unstable(feature = "allocator_api",
               reason = "the precise API and guarantees it provides may be tweaked.",
               issue = "32838")]
    impl Error for AllocErr {
        fn description(&self) -> &str {
            "memory allocation failed"
        }
    }

    #[unstable(feature = "allocator_api",
               reason = "the precise API and guarantees it provides may be tweaked.",
               issue = "32838")]
    impl Error for LayoutErr {
        fn description(&self) -> &str {
            "invalid parameters to Layout::from_size_align"
        }
    }

    #[unstable(feature = "allocator_api",
               reason = "the precise API and guarantees it provides may be tweaked.",
               issue = "32838")]
    impl Error for CannotReallocInPlace {
        fn description(&self) -> &str {
            CannotReallocInPlace::description(self)
        }
    }

    #[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()
        }
    }

    #[stable(feature = "try_from", since = "1.34.0")]
    impl Error for num::TryFromIntError {
        fn description(&self) -> &str {
            self.__description()
        }
    }

    #[stable(feature = "try_from", since = "1.34.0")]
    impl Error for array::TryFromSliceError {
        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 = "1.8.0")]
    impl<T: Error> Error for Box<T> {
        fn description(&self) -> &str {
            Error::description(&**self)
        }

        #[allow(deprecated)]
        fn cause(&self) -> Option<&dyn Error> {
            Error::cause(&**self)
        }

        fn source(&self) -> Option<&(dyn Error + 'static)> {
            Error::source(&**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"
        }
    }

    #[stable(feature = "try_borrow", since = "1.13.0")]
    impl Error for cell::BorrowError {
        fn description(&self) -> &str {
            "already mutably borrowed"
        }
    }

    #[stable(feature = "try_borrow", since = "1.13.0")]
    impl Error for cell::BorrowMutError {
        fn description(&self) -> &str {
            "already borrowed"
        }
    }

    #[stable(feature = "try_from", since = "1.34.0")]
    impl Error for char::CharTryFromError {
        fn description(&self) -> &str {
            "converted integer out of range for `char`"
        }
    }

    #[stable(feature = "char_from_str", since = "1.20.0")]
    impl Error for char::ParseCharError {
        fn description(&self) -> &str {
            self.__description()
        }
    }

    // Copied from `any.rs`.
    impl dyn 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(private::Internal);

            // Compare both `TypeId`s 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 {
                    Some(&*(self as *const dyn Error 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 {
                    Some(&mut *(self as *mut dyn Error as *mut T))
                }
            } else {
                None
            }
        }
    }

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

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

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

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

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

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

    impl dyn Error {
        #[inline]
        #[stable(feature = "error_downcast", since = "1.3.0")]
        /// Attempts to downcast the box to a concrete type.
        pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<dyn Error>> {
            if self.is::<T>() {
                unsafe {
                    let raw: *mut dyn Error = Box::into_raw(self);
                    Ok(Box::from_raw(raw as *mut T))
                }
            } else {
                Err(self)
            }
        }

        /// Returns an iterator starting with the current error and continuing with
        /// recursively calling [`source`].
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(error_iter)]
        /// use std::error::Error;
        /// use std::fmt;
        ///
        /// #[derive(Debug)]
        /// struct A;
        ///
        /// #[derive(Debug)]
        /// struct B(Option<Box<dyn Error + 'static>>);
        ///
        /// 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 {}
        ///
        /// impl Error for B {
        ///     fn source(&self) -> Option<&(dyn Error + 'static)> {
        ///         self.0.as_ref().map(|e| e.as_ref())
        ///     }
        /// }
        ///
        /// let b = B(Some(Box::new(A)));
        ///
        /// // let err : Box<Error> = b.into(); // or
        /// let err = &b as &(dyn Error);
        ///
        /// let mut iter = err.iter_chain();
        ///
        /// assert_eq!("B".to_string(), iter.next().unwrap().to_string());
        /// assert_eq!("A".to_string(), iter.next().unwrap().to_string());
        /// assert!(iter.next().is_none());
        /// assert!(iter.next().is_none());
        /// ```
        ///
        /// [`source`]: trait.Error.html#method.source
        #[unstable(feature = "error_iter", issue = "58520")]
        #[inline]
        pub fn iter_chain(&self) -> ErrorIter<'_> {
            ErrorIter {
                current: Some(self),
            }
        }

        /// Returns an iterator starting with the [`source`] of this error
        /// and continuing with recursively calling [`source`].
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(error_iter)]
        /// use std::error::Error;
        /// use std::fmt;
        ///
        /// #[derive(Debug)]
        /// struct A;
        ///
        /// #[derive(Debug)]
        /// struct B(Option<Box<dyn Error + 'static>>);
        ///
        /// #[derive(Debug)]
        /// struct C(Option<Box<dyn Error + 'static>>);
        ///
        /// 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 fmt::Display for C {
        ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        ///         write!(f, "C")
        ///     }
        /// }
        ///
        /// impl Error for A {}
        ///
        /// impl Error for B {
        ///     fn source(&self) -> Option<&(dyn Error + 'static)> {
        ///         self.0.as_ref().map(|e| e.as_ref())
        ///     }
        /// }
        ///
        /// impl Error for C {
        ///     fn source(&self) -> Option<&(dyn Error + 'static)> {
        ///         self.0.as_ref().map(|e| e.as_ref())
        ///     }
        /// }
        ///
        /// let b = B(Some(Box::new(A)));
        /// let c = C(Some(Box::new(b)));
        ///
        /// // let err : Box<Error> = c.into(); // or
        /// let err = &c as &(dyn Error);
        ///
        /// let mut iter = err.iter_sources();
        ///
        /// assert_eq!("B".to_string(), iter.next().unwrap().to_string());
        /// assert_eq!("A".to_string(), iter.next().unwrap().to_string());
        /// assert!(iter.next().is_none());
        /// assert!(iter.next().is_none());
        /// ```
        ///
        /// [`source`]: trait.Error.html#method.source
        #[inline]
        #[unstable(feature = "error_iter", issue = "58520")]
        pub fn iter_sources(&self) -> ErrorIter<'_> {
            ErrorIter {
                current: self.source(),
            }
        }
    }

    /// An iterator over [`Error`]
    ///
    /// [`Error`]: trait.Error.html
    #[unstable(feature = "error_iter", issue = "58520")]
    #[derive(Copy, Clone, Debug)]
    pub struct ErrorIter<'a> {
        current: Option<&'a (dyn Error + 'static)>,
    }

    #[unstable(feature = "error_iter", issue = "58520")]
    impl<'a> Iterator for ErrorIter<'a> {
        type Item = &'a (dyn Error + 'static);

        fn next(&mut self) -> Option<Self::Item> {
            let current = self.current;
            self.current = self.current.and_then(Error::source);
            current
        }
    }

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

    impl dyn Error + Send + Sync {
        #[inline]
        #[stable(feature = "error_downcast", since = "1.3.0")]
        /// Attempts 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<dyn Error> = self;
            <dyn Error>::downcast(err).map_err(|s| unsafe {
                // Reapply the `Send + Sync` marker.
                transmute::<Box<dyn Error>, Box<dyn Error + Send + Sync>>(s)
            })
        }
    }

    #[cfg(test)]
    mod tests {
        use super::Error;
        use crate::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 a = &mut a as &mut (dyn 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<dyn Error> = Box::new(A);
            match a.downcast::<B>() {
                Ok(..) => panic!("expected error"),
                Err(e) => assert_eq!(*e.downcast::<A>().unwrap(), A),
            }
        }
    }

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