blob: 15c3b12ab315313ac27c637bca205787c1b5042e [file] [log] [blame]
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() {
} 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 {
/// 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 {
/// 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
_ => 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 {
/// 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> {
/// 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> {
/// 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)
mod test {
fn assert_just_data<T: Send + Sync + 'static>() { }
fn assert_error_is_just_data() {