use crate::backtrace::Backtrace; use crate::chain::Chain; #[cfg(any(feature = "std", anyhow_no_ptr_addr_of))] use crate::ptr::Mut; use crate::ptr::{Own, Ref}; use crate::{Error, StdError}; use alloc::boxed::Box; #[cfg(backtrace)] use core::any::Demand; use core::any::TypeId; use core::fmt::{self, Debug, Display}; use core::mem::ManuallyDrop; #[cfg(not(anyhow_no_ptr_addr_of))] use core::ptr; use core::ptr::NonNull; #[cfg(feature = "std")] use core::ops::{Deref, DerefMut}; impl Error { /// Create a new error object from any error type. /// /// The error type must be threadsafe and `'static`, so that the `Error` /// will be as well. /// /// If the error type does not provide a backtrace, a backtrace will be /// created here to ensure that a backtrace exists. #[cfg(feature = "std")] #[cfg_attr(doc_cfg, doc(cfg(feature = "std")))] #[cold] #[must_use] pub fn new(error: E) -> Self where E: StdError + Send + Sync + 'static, { let backtrace = backtrace_if_absent!(&error); Error::from_std(error, backtrace) } /// Create a new error object from a printable error message. /// /// If the argument implements std::error::Error, prefer `Error::new` /// instead which preserves the underlying error's cause chain and /// backtrace. If the argument may or may not implement std::error::Error /// now or in the future, use `anyhow!(err)` which handles either way /// correctly. /// /// `Error::msg("...")` is equivalent to `anyhow!("...")` but occasionally /// convenient in places where a function is preferable over a macro, such /// as iterator or stream combinators: /// /// ``` /// # mod ffi { /// # pub struct Input; /// # pub struct Output; /// # pub async fn do_some_work(_: Input) -> Result { /// # unimplemented!() /// # } /// # } /// # /// # use ffi::{Input, Output}; /// # /// use anyhow::{Error, Result}; /// use futures::stream::{Stream, StreamExt, TryStreamExt}; /// /// async fn demo(stream: S) -> Result> /// where /// S: Stream, /// { /// stream /// .then(ffi::do_some_work) // returns Result /// .map_err(Error::msg) /// .try_collect() /// .await /// } /// ``` #[cold] #[must_use] pub fn msg(message: M) -> Self where M: Display + Debug + Send + Sync + 'static, { Error::from_adhoc(message, backtrace!()) } #[cfg(feature = "std")] #[cold] pub(crate) fn from_std(error: E, backtrace: Option) -> Self where E: StdError + Send + Sync + 'static, { let vtable = &ErrorVTable { object_drop: object_drop::, object_ref: object_ref::, #[cfg(anyhow_no_ptr_addr_of)] object_mut: object_mut::, object_boxed: object_boxed::, object_downcast: object_downcast::, #[cfg(anyhow_no_ptr_addr_of)] object_downcast_mut: object_downcast_mut::, object_drop_rest: object_drop_front::, #[cfg(all(not(backtrace), feature = "backtrace"))] object_backtrace: no_backtrace, }; // Safety: passing vtable that operates on the right type E. unsafe { Error::construct(error, vtable, backtrace) } } #[cold] pub(crate) fn from_adhoc(message: M, backtrace: Option) -> Self where M: Display + Debug + Send + Sync + 'static, { use crate::wrapper::MessageError; let error: MessageError = MessageError(message); let vtable = &ErrorVTable { object_drop: object_drop::>, object_ref: object_ref::>, #[cfg(all(feature = "std", anyhow_no_ptr_addr_of))] object_mut: object_mut::>, object_boxed: object_boxed::>, object_downcast: object_downcast::, #[cfg(anyhow_no_ptr_addr_of)] object_downcast_mut: object_downcast_mut::, object_drop_rest: object_drop_front::, #[cfg(all(not(backtrace), feature = "backtrace"))] object_backtrace: no_backtrace, }; // Safety: MessageError is repr(transparent) so it is okay for the // vtable to allow casting the MessageError to M. unsafe { Error::construct(error, vtable, backtrace) } } #[cold] pub(crate) fn from_display(message: M, backtrace: Option) -> Self where M: Display + Send + Sync + 'static, { use crate::wrapper::DisplayError; let error: DisplayError = DisplayError(message); let vtable = &ErrorVTable { object_drop: object_drop::>, object_ref: object_ref::>, #[cfg(all(feature = "std", anyhow_no_ptr_addr_of))] object_mut: object_mut::>, object_boxed: object_boxed::>, object_downcast: object_downcast::, #[cfg(anyhow_no_ptr_addr_of)] object_downcast_mut: object_downcast_mut::, object_drop_rest: object_drop_front::, #[cfg(all(not(backtrace), feature = "backtrace"))] object_backtrace: no_backtrace, }; // Safety: DisplayError is repr(transparent) so it is okay for the // vtable to allow casting the DisplayError to M. unsafe { Error::construct(error, vtable, backtrace) } } #[cfg(feature = "std")] #[cold] pub(crate) fn from_context(context: C, error: E, backtrace: Option) -> Self where C: Display + Send + Sync + 'static, E: StdError + Send + Sync + 'static, { let error: ContextError = ContextError { context, error }; let vtable = &ErrorVTable { object_drop: object_drop::>, object_ref: object_ref::>, #[cfg(anyhow_no_ptr_addr_of)] object_mut: object_mut::>, object_boxed: object_boxed::>, object_downcast: context_downcast::, #[cfg(anyhow_no_ptr_addr_of)] object_downcast_mut: context_downcast_mut::, object_drop_rest: context_drop_rest::, #[cfg(all(not(backtrace), feature = "backtrace"))] object_backtrace: no_backtrace, }; // Safety: passing vtable that operates on the right type. unsafe { Error::construct(error, vtable, backtrace) } } #[cfg(feature = "std")] #[cold] pub(crate) fn from_boxed( error: Box, backtrace: Option, ) -> Self { use crate::wrapper::BoxedError; let error = BoxedError(error); let vtable = &ErrorVTable { object_drop: object_drop::, object_ref: object_ref::, #[cfg(anyhow_no_ptr_addr_of)] object_mut: object_mut::, object_boxed: object_boxed::, object_downcast: object_downcast::>, #[cfg(anyhow_no_ptr_addr_of)] object_downcast_mut: object_downcast_mut::>, object_drop_rest: object_drop_front::>, #[cfg(all(not(backtrace), feature = "backtrace"))] object_backtrace: no_backtrace, }; // Safety: BoxedError is repr(transparent) so it is okay for the vtable // to allow casting to Box. unsafe { Error::construct(error, vtable, backtrace) } } // Takes backtrace as argument rather than capturing it here so that the // user sees one fewer layer of wrapping noise in the backtrace. // // Unsafe because the given vtable must have sensible behavior on the error // value of type E. #[cold] unsafe fn construct( error: E, vtable: &'static ErrorVTable, backtrace: Option, ) -> Self where E: StdError + Send + Sync + 'static, { let inner: Box> = Box::new(ErrorImpl { vtable, backtrace, _object: error, }); // Erase the concrete type of E from the compile-time type system. This // is equivalent to the safe unsize coercion from Box> to // Box> except that the // result is a thin pointer. The necessary behavior for manipulating the // underlying ErrorImpl is preserved in the vtable provided by the // caller rather than a builtin fat pointer vtable. let inner = Own::new(inner).cast::(); Error { inner } } /// Wrap the error value with additional context. /// /// For attaching context to a `Result` as it is propagated, the /// [`Context`][crate::Context] extension trait may be more convenient than /// this function. /// /// The primary reason to use `error.context(...)` instead of /// `result.context(...)` via the `Context` trait would be if the context /// needs to depend on some data held by the underlying error: /// /// ``` /// # use std::fmt::{self, Debug, Display}; /// # /// # type T = (); /// # /// # impl std::error::Error for ParseError {} /// # impl Debug for ParseError { /// # fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result { /// # unimplemented!() /// # } /// # } /// # impl Display for ParseError { /// # fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result { /// # unimplemented!() /// # } /// # } /// # /// use anyhow::Result; /// use std::fs::File; /// use std::path::Path; /// /// struct ParseError { /// line: usize, /// column: usize, /// } /// /// fn parse_impl(file: File) -> Result { /// # const IGNORE: &str = stringify! { /// ... /// # }; /// # unimplemented!() /// } /// /// pub fn parse(path: impl AsRef) -> Result { /// let file = File::open(&path)?; /// parse_impl(file).map_err(|error| { /// let context = format!( /// "only the first {} lines of {} are valid", /// error.line, path.as_ref().display(), /// ); /// anyhow::Error::new(error).context(context) /// }) /// } /// ``` #[cold] #[must_use] pub fn context(self, context: C) -> Self where C: Display + Send + Sync + 'static, { let error: ContextError = ContextError { context, error: self, }; let vtable = &ErrorVTable { object_drop: object_drop::>, object_ref: object_ref::>, #[cfg(all(feature = "std", anyhow_no_ptr_addr_of))] object_mut: object_mut::>, object_boxed: object_boxed::>, object_downcast: context_chain_downcast::, #[cfg(anyhow_no_ptr_addr_of)] object_downcast_mut: context_chain_downcast_mut::, object_drop_rest: context_chain_drop_rest::, #[cfg(all(not(backtrace), feature = "backtrace"))] object_backtrace: context_backtrace::, }; // As the cause is anyhow::Error, we already have a backtrace for it. let backtrace = None; // Safety: passing vtable that operates on the right type. unsafe { Error::construct(error, vtable, backtrace) } } /// Get the backtrace for this Error. /// /// In order for the backtrace to be meaningful, one of the two environment /// variables `RUST_LIB_BACKTRACE=1` or `RUST_BACKTRACE=1` must be defined /// and `RUST_LIB_BACKTRACE` must not be `0`. Backtraces are somewhat /// expensive to capture in Rust, so we don't necessarily want to be /// capturing them all over the place all the time. /// /// - If you want panics and errors to both have backtraces, set /// `RUST_BACKTRACE=1`; /// - If you want only errors to have backtraces, set /// `RUST_LIB_BACKTRACE=1`; /// - If you want only panics to have backtraces, set `RUST_BACKTRACE=1` and /// `RUST_LIB_BACKTRACE=0`. /// /// # Stability /// /// Standard library backtraces are only available on the nightly channel. /// Tracking issue: [rust-lang/rust#53487][tracking]. /// /// On stable compilers, this function is only available if the crate's /// "backtrace" feature is enabled, and will use the `backtrace` crate as /// the underlying backtrace implementation. /// /// ```toml /// [dependencies] /// anyhow = { version = "1.0", features = ["backtrace"] } /// ``` /// /// [tracking]: https://github.com/rust-lang/rust/issues/53487 #[cfg(any(backtrace, feature = "backtrace"))] #[cfg_attr(doc_cfg, doc(cfg(any(nightly, feature = "backtrace"))))] pub fn backtrace(&self) -> &impl_backtrace!() { unsafe { ErrorImpl::backtrace(self.inner.by_ref()) } } /// An iterator of the chain of source errors contained by this Error. /// /// This iterator will visit every error in the cause chain of this error /// object, beginning with the error that this error object was created /// from. /// /// # Example /// /// ``` /// use anyhow::Error; /// use std::io; /// /// pub fn underlying_io_error_kind(error: &Error) -> Option { /// for cause in error.chain() { /// if let Some(io_error) = cause.downcast_ref::() { /// return Some(io_error.kind()); /// } /// } /// None /// } /// ``` #[cfg(feature = "std")] #[cfg_attr(doc_cfg, doc(cfg(feature = "std")))] #[cold] pub fn chain(&self) -> Chain { unsafe { ErrorImpl::chain(self.inner.by_ref()) } } /// The lowest level cause of this error — this error's cause's /// cause's cause etc. /// /// The root cause is the last error in the iterator produced by /// [`chain()`][Error::chain]. #[cfg(feature = "std")] #[cfg_attr(doc_cfg, doc(cfg(feature = "std")))] pub fn root_cause(&self) -> &(dyn StdError + 'static) { self.chain().last().unwrap() } /// Returns true if `E` is the type held by this error object. /// /// For errors with context, this method returns true if `E` matches the /// type of the context `C` **or** the type of the error on which the /// context has been attached. For details about the interaction between /// context and downcasting, [see here]. /// /// [see here]: trait.Context.html#effect-on-downcasting pub fn is(&self) -> bool where E: Display + Debug + Send + Sync + 'static, { self.downcast_ref::().is_some() } /// Attempt to downcast the error object to a concrete type. pub fn downcast(mut self) -> Result where E: Display + Debug + Send + Sync + 'static, { let target = TypeId::of::(); let inner = self.inner.by_mut(); unsafe { // Use vtable to find NonNull<()> which points to a value of type E // somewhere inside the data structure. #[cfg(not(anyhow_no_ptr_addr_of))] let addr = match (vtable(inner.ptr).object_downcast)(inner.by_ref(), target) { Some(addr) => addr.by_mut().extend(), None => return Err(self), }; #[cfg(anyhow_no_ptr_addr_of)] let addr = match (vtable(inner.ptr).object_downcast_mut)(inner, target) { Some(addr) => addr.extend(), None => return Err(self), }; // Prepare to read E out of the data structure. We'll drop the rest // of the data structure separately so that E is not dropped. let outer = ManuallyDrop::new(self); // Read E from where the vtable found it. let error = addr.cast::().read(); // Drop rest of the data structure outside of E. (vtable(outer.inner.ptr).object_drop_rest)(outer.inner, target); Ok(error) } } /// Downcast this error object by reference. /// /// # Example /// /// ``` /// # use anyhow::anyhow; /// # use std::fmt::{self, Display}; /// # use std::task::Poll; /// # /// # #[derive(Debug)] /// # enum DataStoreError { /// # Censored(()), /// # } /// # /// # impl Display for DataStoreError { /// # fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result { /// # unimplemented!() /// # } /// # } /// # /// # impl std::error::Error for DataStoreError {} /// # /// # const REDACTED_CONTENT: () = (); /// # /// # let error = anyhow!("..."); /// # let root_cause = &error; /// # /// # let ret = /// // If the error was caused by redaction, then return a tombstone instead /// // of the content. /// match root_cause.downcast_ref::() { /// Some(DataStoreError::Censored(_)) => Ok(Poll::Ready(REDACTED_CONTENT)), /// None => Err(error), /// } /// # ; /// ``` pub fn downcast_ref(&self) -> Option<&E> where E: Display + Debug + Send + Sync + 'static, { let target = TypeId::of::(); unsafe { // Use vtable to find NonNull<()> which points to a value of type E // somewhere inside the data structure. let addr = (vtable(self.inner.ptr).object_downcast)(self.inner.by_ref(), target)?; Some(addr.cast::().deref()) } } /// Downcast this error object by mutable reference. pub fn downcast_mut(&mut self) -> Option<&mut E> where E: Display + Debug + Send + Sync + 'static, { let target = TypeId::of::(); unsafe { // Use vtable to find NonNull<()> which points to a value of type E // somewhere inside the data structure. #[cfg(not(anyhow_no_ptr_addr_of))] let addr = (vtable(self.inner.ptr).object_downcast)(self.inner.by_ref(), target)?.by_mut(); #[cfg(anyhow_no_ptr_addr_of)] let addr = (vtable(self.inner.ptr).object_downcast_mut)(self.inner.by_mut(), target)?; Some(addr.cast::().deref_mut()) } } } #[cfg(backtrace)] impl std::any::Provider for Error { // Called by thiserror when you have `#[source] anyhow::Error`. This provide // implementation includes the anyhow::Error's Backtrace if any, unlike // deref'ing to dyn Error where the provide implementation would include // only the original error's Backtrace from before it got wrapped into an // anyhow::Error. fn provide<'a>(&'a self, demand: &mut Demand<'a>) { unsafe { ErrorImpl::provide(self.inner.by_ref(), demand) } } } #[cfg(feature = "std")] #[cfg_attr(doc_cfg, doc(cfg(feature = "std")))] impl From for Error where E: StdError + Send + Sync + 'static, { #[cold] fn from(error: E) -> Self { let backtrace = backtrace_if_absent!(&error); Error::from_std(error, backtrace) } } #[cfg(feature = "std")] #[cfg_attr(doc_cfg, doc(cfg(feature = "std")))] impl Deref for Error { type Target = dyn StdError + Send + Sync + 'static; fn deref(&self) -> &Self::Target { unsafe { ErrorImpl::error(self.inner.by_ref()) } } } #[cfg(feature = "std")] #[cfg_attr(doc_cfg, doc(cfg(feature = "std")))] impl DerefMut for Error { fn deref_mut(&mut self) -> &mut Self::Target { unsafe { ErrorImpl::error_mut(self.inner.by_mut()) } } } impl Display for Error { fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result { unsafe { ErrorImpl::display(self.inner.by_ref(), formatter) } } } impl Debug for Error { fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result { unsafe { ErrorImpl::debug(self.inner.by_ref(), formatter) } } } impl Drop for Error { fn drop(&mut self) { unsafe { // Invoke the vtable's drop behavior. (vtable(self.inner.ptr).object_drop)(self.inner); } } } struct ErrorVTable { object_drop: unsafe fn(Own), object_ref: unsafe fn(Ref) -> Ref, #[cfg(all(feature = "std", anyhow_no_ptr_addr_of))] object_mut: unsafe fn(Mut) -> &mut (dyn StdError + Send + Sync + 'static), object_boxed: unsafe fn(Own) -> Box, object_downcast: unsafe fn(Ref, TypeId) -> Option>, #[cfg(anyhow_no_ptr_addr_of)] object_downcast_mut: unsafe fn(Mut, TypeId) -> Option>, object_drop_rest: unsafe fn(Own, TypeId), #[cfg(all(not(backtrace), feature = "backtrace"))] object_backtrace: unsafe fn(Ref) -> Option<&Backtrace>, } // Safety: requires layout of *e to match ErrorImpl. unsafe fn object_drop(e: Own) { // Cast back to ErrorImpl so that the allocator receives the correct // Layout to deallocate the Box's memory. let unerased = e.cast::>().boxed(); drop(unerased); } // Safety: requires layout of *e to match ErrorImpl. unsafe fn object_drop_front(e: Own, target: TypeId) { // Drop the fields of ErrorImpl other than E as well as the Box allocation, // without dropping E itself. This is used by downcast after doing a // ptr::read to take ownership of the E. let _ = target; let unerased = e.cast::>>().boxed(); drop(unerased); } // Safety: requires layout of *e to match ErrorImpl. unsafe fn object_ref(e: Ref) -> Ref where E: StdError + Send + Sync + 'static, { // Attach E's native StdError vtable onto a pointer to self._object. let unerased = e.cast::>(); #[cfg(not(anyhow_no_ptr_addr_of))] return Ref::from_raw(NonNull::new_unchecked( ptr::addr_of!((*unerased.as_ptr())._object) as *mut E, )); #[cfg(anyhow_no_ptr_addr_of)] return Ref::new(&unerased.deref()._object); } // Safety: requires layout of *e to match ErrorImpl, and for `e` to be derived // from a `&mut` #[cfg(all(feature = "std", anyhow_no_ptr_addr_of))] unsafe fn object_mut(e: Mut) -> &mut (dyn StdError + Send + Sync + 'static) where E: StdError + Send + Sync + 'static, { // Attach E's native StdError vtable onto a pointer to self._object. &mut e.cast::>().deref_mut()._object } // Safety: requires layout of *e to match ErrorImpl. unsafe fn object_boxed(e: Own) -> Box where E: StdError + Send + Sync + 'static, { // Attach ErrorImpl's native StdError vtable. The StdError impl is below. e.cast::>().boxed() } // Safety: requires layout of *e to match ErrorImpl. unsafe fn object_downcast(e: Ref, target: TypeId) -> Option> where E: 'static, { if TypeId::of::() == target { // Caller is looking for an E pointer and e is ErrorImpl, take a // pointer to its E field. let unerased = e.cast::>(); #[cfg(not(anyhow_no_ptr_addr_of))] return Some( Ref::from_raw(NonNull::new_unchecked( ptr::addr_of!((*unerased.as_ptr())._object) as *mut E, )) .cast::<()>(), ); #[cfg(anyhow_no_ptr_addr_of)] return Some(Ref::new(&unerased.deref()._object).cast::<()>()); } else { None } } // Safety: requires layout of *e to match ErrorImpl. #[cfg(anyhow_no_ptr_addr_of)] unsafe fn object_downcast_mut(e: Mut, target: TypeId) -> Option> where E: 'static, { if TypeId::of::() == target { // Caller is looking for an E pointer and e is ErrorImpl, take a // pointer to its E field. let unerased = e.cast::>().deref_mut(); Some(Mut::new(&mut unerased._object).cast::<()>()) } else { None } } #[cfg(all(not(backtrace), feature = "backtrace"))] fn no_backtrace(e: Ref) -> Option<&Backtrace> { let _ = e; None } // Safety: requires layout of *e to match ErrorImpl>. #[cfg(feature = "std")] unsafe fn context_downcast(e: Ref, target: TypeId) -> Option> where C: 'static, E: 'static, { if TypeId::of::() == target { let unerased = e.cast::>>().deref(); Some(Ref::new(&unerased._object.context).cast::<()>()) } else if TypeId::of::() == target { let unerased = e.cast::>>().deref(); Some(Ref::new(&unerased._object.error).cast::<()>()) } else { None } } // Safety: requires layout of *e to match ErrorImpl>. #[cfg(all(feature = "std", anyhow_no_ptr_addr_of))] unsafe fn context_downcast_mut(e: Mut, target: TypeId) -> Option> where C: 'static, E: 'static, { if TypeId::of::() == target { let unerased = e.cast::>>().deref_mut(); Some(Mut::new(&mut unerased._object.context).cast::<()>()) } else if TypeId::of::() == target { let unerased = e.cast::>>().deref_mut(); Some(Mut::new(&mut unerased._object.error).cast::<()>()) } else { None } } // Safety: requires layout of *e to match ErrorImpl>. #[cfg(feature = "std")] unsafe fn context_drop_rest(e: Own, target: TypeId) where C: 'static, E: 'static, { // Called after downcasting by value to either the C or the E and doing a // ptr::read to take ownership of that value. if TypeId::of::() == target { let unerased = e .cast::, E>>>() .boxed(); drop(unerased); } else { let unerased = e .cast::>>>() .boxed(); drop(unerased); } } // Safety: requires layout of *e to match ErrorImpl>. unsafe fn context_chain_downcast(e: Ref, target: TypeId) -> Option> where C: 'static, { let unerased = e.cast::>>().deref(); if TypeId::of::() == target { Some(Ref::new(&unerased._object.context).cast::<()>()) } else { // Recurse down the context chain per the inner error's vtable. let source = &unerased._object.error; (vtable(source.inner.ptr).object_downcast)(source.inner.by_ref(), target) } } // Safety: requires layout of *e to match ErrorImpl>. #[cfg(anyhow_no_ptr_addr_of)] unsafe fn context_chain_downcast_mut(e: Mut, target: TypeId) -> Option> where C: 'static, { let unerased = e.cast::>>().deref_mut(); if TypeId::of::() == target { Some(Mut::new(&mut unerased._object.context).cast::<()>()) } else { // Recurse down the context chain per the inner error's vtable. let source = &mut unerased._object.error; (vtable(source.inner.ptr).object_downcast_mut)(source.inner.by_mut(), target) } } // Safety: requires layout of *e to match ErrorImpl>. unsafe fn context_chain_drop_rest(e: Own, target: TypeId) where C: 'static, { // Called after downcasting by value to either the C or one of the causes // and doing a ptr::read to take ownership of that value. if TypeId::of::() == target { let unerased = e .cast::, Error>>>() .boxed(); // Drop the entire rest of the data structure rooted in the next Error. drop(unerased); } else { let unerased = e .cast::>>>() .boxed(); // Read the Own from the next error. let inner = unerased._object.error.inner; drop(unerased); let vtable = vtable(inner.ptr); // Recursively drop the next error using the same target typeid. (vtable.object_drop_rest)(inner, target); } } // Safety: requires layout of *e to match ErrorImpl>. #[cfg(all(not(backtrace), feature = "backtrace"))] #[allow(clippy::unnecessary_wraps)] unsafe fn context_backtrace(e: Ref) -> Option<&Backtrace> where C: 'static, { let unerased = e.cast::>>().deref(); let backtrace = ErrorImpl::backtrace(unerased._object.error.inner.by_ref()); Some(backtrace) } // NOTE: If working with `ErrorImpl<()>`, references should be avoided in favor // of raw pointers and `NonNull`. // repr C to ensure that E remains in the final position. #[repr(C)] pub(crate) struct ErrorImpl { vtable: &'static ErrorVTable, backtrace: Option, // NOTE: Don't use directly. Use only through vtable. Erased type may have // different alignment. _object: E, } // Reads the vtable out of `p`. This is the same as `p.as_ref().vtable`, but // avoids converting `p` into a reference. unsafe fn vtable(p: NonNull) -> &'static ErrorVTable { // NOTE: This assumes that `ErrorVTable` is the first field of ErrorImpl. *(p.as_ptr() as *const &'static ErrorVTable) } // repr C to ensure that ContextError has the same layout as // ContextError, E> and ContextError>. #[repr(C)] pub(crate) struct ContextError { pub context: C, pub error: E, } impl ErrorImpl { fn erase(&self) -> Ref { // Erase the concrete type of E but preserve the vtable in self.vtable // for manipulating the resulting thin pointer. This is analogous to an // unsize coercion. Ref::new(self).cast::() } } impl ErrorImpl { pub(crate) unsafe fn error(this: Ref) -> &(dyn StdError + Send + Sync + 'static) { // Use vtable to attach E's native StdError vtable for the right // original type E. (vtable(this.ptr).object_ref)(this).deref() } #[cfg(feature = "std")] pub(crate) unsafe fn error_mut(this: Mut) -> &mut (dyn StdError + Send + Sync + 'static) { // Use vtable to attach E's native StdError vtable for the right // original type E. #[cfg(not(anyhow_no_ptr_addr_of))] return (vtable(this.ptr).object_ref)(this.by_ref()) .by_mut() .deref_mut(); #[cfg(anyhow_no_ptr_addr_of)] return (vtable(this.ptr).object_mut)(this); } #[cfg(any(backtrace, feature = "backtrace"))] pub(crate) unsafe fn backtrace(this: Ref) -> &Backtrace { // This unwrap can only panic if the underlying error's backtrace method // is nondeterministic, which would only happen in maliciously // constructed code. this.deref() .backtrace .as_ref() .or_else(|| { #[cfg(backtrace)] return Self::error(this).request_ref::(); #[cfg(not(backtrace))] return (vtable(this.ptr).object_backtrace)(this); }) .expect("backtrace capture failed") } #[cfg(backtrace)] unsafe fn provide<'a>(this: Ref<'a, Self>, demand: &mut Demand<'a>) { if let Some(backtrace) = &this.deref().backtrace { demand.provide_ref(backtrace); } Self::error(this).provide(demand); } #[cold] pub(crate) unsafe fn chain(this: Ref) -> Chain { Chain::new(Self::error(this)) } } impl StdError for ErrorImpl where E: StdError, { fn source(&self) -> Option<&(dyn StdError + 'static)> { unsafe { ErrorImpl::error(self.erase()).source() } } #[cfg(backtrace)] fn provide<'a>(&'a self, demand: &mut Demand<'a>) { unsafe { ErrorImpl::provide(self.erase(), demand) } } } impl Debug for ErrorImpl where E: Debug, { fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result { unsafe { ErrorImpl::debug(self.erase(), formatter) } } } impl Display for ErrorImpl where E: Display, { fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result { unsafe { Display::fmt(ErrorImpl::error(self.erase()), formatter) } } } impl From for Box { #[cold] fn from(error: Error) -> Self { let outer = ManuallyDrop::new(error); unsafe { // Use vtable to attach ErrorImpl's native StdError vtable for // the right original type E. (vtable(outer.inner.ptr).object_boxed)(outer.inner) } } } impl From for Box { fn from(error: Error) -> Self { Box::::from(error) } } impl From for Box { fn from(error: Error) -> Self { Box::::from(error) } } #[cfg(feature = "std")] impl AsRef for Error { fn as_ref(&self) -> &(dyn StdError + Send + Sync + 'static) { &**self } } #[cfg(feature = "std")] impl AsRef for Error { fn as_ref(&self) -> &(dyn StdError + 'static) { &**self } }