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

 //! 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, CannotReallocInPlace, LayoutErr};
 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 {
     /// 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 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 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 occurred.
     ///
     /// 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
     }

     /// ```
     /// if let Err(e) = "xc".parse::<u32>() {
     ///     // Print `e` itself, no need for description().
     ///     eprintln!("Error: {}", e);
     /// }
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_deprecated(since = "1.41.0", reason = "use the Display impl or to_string()")]
     fn description(&self) -> &str {
         "description() is deprecated; use Display"
     }

     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_deprecated(
         since = "1.33.0",
         reason = "replaced by Error::source, which can support downcasting"
     )]
     #[allow(missing_docs)]
     fn cause(&self) -> Option<&dyn Error> {
         self.source()
     }
 }

 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 {}
     ///
     /// 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`] + [`Send`] + [`Sync`] into a box of
     /// dyn [`Error`] + [`Send`] + [`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 {}
     ///
     /// 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`] + [`Send`] + [`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 {
             #[allow(deprecated)]
             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`] + [`Send`] + [`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`] + [`Send`] + [`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 ! {}

 #[unstable(
     feature = "allocator_api",
     reason = "the precise API and guarantees it provides may be tweaked.",
     issue = "32838"
 )]
 impl Error for AllocErr {}

 #[unstable(
     feature = "allocator_api",
     reason = "the precise API and guarantees it provides may be tweaked.",
     issue = "32838"
 )]
 impl Error for LayoutErr {}

 #[unstable(
     feature = "allocator_api",
     reason = "the precise API and guarantees it provides may be tweaked.",
     issue = "32838"
 )]
 impl Error for CannotReallocInPlace {}

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

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

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

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

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

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

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

 #[stable(feature = "rust1", since = "1.0.0")]
 impl Error for string::FromUtf16Error {
     #[allow(deprecated)]
     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 {
     #[allow(deprecated)]
     fn description(&self) -> &str {
         "unpaired surrogate found"
     }
 }

 #[stable(feature = "box_error", since = "1.8.0")]
 impl<T: Error> Error for Box<T> {
     #[allow(deprecated, deprecated_in_future)]
     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 {
     #[allow(deprecated)]
     fn description(&self) -> &str {
         "an error occurred when formatting an argument"
     }
 }

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

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

 #[stable(feature = "try_from", since = "1.34.0")]
 impl Error for char::CharTryFromError {
     #[allow(deprecated)]
     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 {
     #[allow(deprecated)]
     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`].
     ///
     /// If you want to omit the current error and only use its sources,
     /// use `skip(1)`.
     ///
     /// # 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.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 chain(&self) -> Chain<'_> {
         Chain { current: Some(self) }
     }
 }

 /// An iterator over an [`Error`] and its sources.
 ///
 /// If you want to omit the initial error and only process
 /// its sources, use `skip(1)`.
 ///
 /// [`Error`]: trait.Error.html
 #[unstable(feature = "error_iter", issue = "58520")]
 #[derive(Clone, Debug)]
 pub struct Chain<'a> {
     current: Option<&'a (dyn Error + 'static)>,
 }

 #[unstable(feature = "error_iter", issue = "58520")]
 impl<'a> Iterator for Chain<'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 {}
     impl Error for B {}

     #[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),
         }
     }
 }