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use core::fmt::{self, Display, Debug}; use core::mem; use core::ptr; use {Causes, Fail}; use backtrace::Backtrace; use context::Context; use compat::Compat; /// The `Error` type, which can contain any failure. /// /// Functions which accumulate many kinds of errors should return this type. /// All failures can be converted into it, so functions which catch those /// errors can be tried with `?` inside of a function that returns this kind /// of error. /// /// In addition to implementing `Debug` and `Display`, this type carries `Backtrace` /// information, and can be downcast into the failure that underlies it for /// more detailed inspection. pub struct Error { pub(crate) inner: Box<Inner<Fail>>, } pub(crate) struct Inner<F: ?Sized + Fail> { backtrace: Backtrace, pub(crate) failure: F, } impl<F: Fail> From<F> for Error { fn from(failure: F) -> Error { let inner: Inner<F> = { let backtrace = if failure.backtrace().is_none() { Backtrace::new() } else { Backtrace::none() }; Inner { failure, backtrace } }; Error { inner: Box::new(inner) } } } impl Error { /// Returns a reference to the underlying cause of this `Error`. Unlike the /// method on `Fail`, this does not return an `Option`. The `Error` type /// always has an underlying failure. pub fn cause(&self) -> &Fail { &self.inner.failure } /// Gets a reference to the `Backtrace` for this `Error`. /// /// If the failure this wrapped carried a backtrace, that backtrace will /// be returned. Otherwise, the backtrace will have been constructed at /// the point that failure was cast into the `Error` type. pub fn backtrace(&self) -> &Backtrace { self.inner.failure.backtrace().unwrap_or(&self.inner.backtrace) } /// Provides context for this `Error`. /// /// This can provide additional information about this error, appropriate /// to the semantics of the current layer. That is, if you have a /// lower-level error, such as an IO error, you can provide additional context /// about what that error means in the context of your function. This /// gives users of this function more information about what has gone /// wrong. /// /// This takes any type that implements `Display`, as well as /// `Send`/`Sync`/`'static`. In practice, this means it can take a `String` /// or a string literal, or a failure, or some other custom context-carrying /// type. pub fn context<D: Display + Send + Sync + 'static>(self, context: D) -> Context<D> { Context::with_err(context, self) } /// Wraps `Error` in a compatibility type. /// /// This type implements the `Error` trait from `std::error`. If you need /// to pass failure's `Error` to an interface that takes any `Error`, you /// can use this method to get a compatible type. pub fn compat(self) -> Compat<Error> { Compat { error: self } } /// Attempts to downcast this `Error` to a particular `Fail` type. /// /// This downcasts by value, returning an owned `T` if the underlying /// failure is of the type `T`. For this reason it returns a `Result` - in /// the case that the underlying error is of a different type, the /// original `Error` is returned. pub fn downcast<T: Fail>(self) -> Result<T, Error> { let ret: Option<T> = self.downcast_ref().map(|fail| { unsafe { // drop the backtrace let _ = ptr::read(&self.inner.backtrace as *const Backtrace); // read out the fail type ptr::read(fail as *const T) } }); match ret { Some(ret) => { // forget self (backtrace is dropped, failure is moved mem::forget(self); Ok(ret) } _ => Err(self) } } /// Returns the "root cause" of this error - the last value in the /// cause chain which does not return an underlying `cause`. pub fn root_cause(&self) -> &Fail { ::find_root_cause(self.cause()) } /// Attempts to downcast this `Error` to a particular `Fail` type by /// reference. /// /// If the underlying error is not of type `T`, this will return `None`. pub fn downcast_ref<T: Fail>(&self) -> Option<&T> { self.inner.failure.downcast_ref() } /// Attempts to downcast this `Error` to a particular `Fail` type by /// mutable reference. /// /// If the underlying error is not of type `T`, this will return `None`. pub fn downcast_mut<T: Fail>(&mut self) -> Option<&mut T> { self.inner.failure.downcast_mut() } /// Returns a iterator over the causes of the `Error`, beginning with /// the failure returned by the `cause` method and ending with the failure /// returned by `root_cause`. pub fn causes(&self) -> Causes { Causes { fail: Some(self.cause()) } } } impl Display for Error { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { Display::fmt(&self.inner.failure, f) } } impl Debug for Error { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { if self.inner.backtrace.is_none() { Debug::fmt(&self.inner.failure, f) } else { write!(f, "{:?}\n\n{:?}", &self.inner.failure, self.inner.backtrace) } } } #[cfg(test)] mod test { fn assert_just_data<T: Send + Sync + 'static>() { } #[test] fn assert_error_is_just_data() { assert_just_data::<super::Error>(); } }