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Diffstat (limited to 'library/std/src/error.rs')
| -rw-r--r-- | library/std/src/error.rs | 802 |
1 files changed, 802 insertions, 0 deletions
diff --git a/library/std/src/error.rs b/library/std/src/error.rs new file mode 100644 index 00000000000..3b4cb859dd4 --- /dev/null +++ b/library/std/src/error.rs @@ -0,0 +1,802 @@ +//! 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 core::convert::Infallible; + +use crate::alloc::{AllocErr, 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); + /// 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 = "1.42.0", 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)) + /// ``` + #[inline] + 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)) + /// ``` + #[inline] + 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 {} + +#[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 Infallible { + 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() + } +} + +#[unstable(feature = "try_reserve", reason = "new API", issue = "48043")] +impl Error for alloc::collections::TryReserveError {} + +// 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), + } + } +} |