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#![doc = include_str!("error.md")]
#![unstable(feature = "error_in_core", issue = "103765")]
#[cfg(test)]
mod tests;
use crate::any::{Demand, Provider, TypeId};
use crate::fmt::{Debug, Display};
/// `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. Error messages are typically concise lowercase sentences without
/// trailing punctuation:
///
/// ```
/// let err = "NaN".parse::<u32>().unwrap_err();
/// assert_eq!(err.to_string(), "invalid digit found in string");
/// ```
///
/// Errors may provide cause information. [`Error::source()`] 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
/// accessing that error via [`Error::source()`]. This makes it possible for the
/// high-level module to provide its own errors while also revealing some of the
/// implementation for debugging.
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), rustc_diagnostic_item = "Error")]
#[rustc_has_incoherent_inherent_impls]
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 {
/// source: 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.source)
/// }
/// }
///
/// #[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 { source: 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>()
}
/// ```
/// if let Err(e) = "xc".parse::<u32>() {
/// // Print `e` itself, no need for description().
/// eprintln!("Error: {e}");
/// }
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[deprecated(since = "1.42.0", note = "use the Display impl or to_string()")]
fn description(&self) -> &str {
"description() is deprecated; use Display"
}
#[stable(feature = "rust1", since = "1.0.0")]
#[deprecated(
since = "1.33.0",
note = "replaced by Error::source, which can support downcasting"
)]
#[allow(missing_docs)]
fn cause(&self) -> Option<&dyn Error> {
self.source()
}
/// Provides type based access to context intended for error reports.
///
/// Used in conjunction with [`Demand::provide_value`] and [`Demand::provide_ref`] to extract
/// references to member variables from `dyn Error` trait objects.
///
/// # Example
///
/// ```rust
/// #![feature(provide_any)]
/// #![feature(error_generic_member_access)]
/// use core::fmt;
/// use core::any::Demand;
///
/// #[derive(Debug)]
/// struct MyBacktrace {
/// // ...
/// }
///
/// impl MyBacktrace {
/// fn new() -> MyBacktrace {
/// // ...
/// # MyBacktrace {}
/// }
/// }
///
/// #[derive(Debug)]
/// struct SourceError {
/// // ...
/// }
///
/// impl fmt::Display for SourceError {
/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
/// write!(f, "Example Source Error")
/// }
/// }
///
/// impl std::error::Error for SourceError {}
///
/// #[derive(Debug)]
/// struct Error {
/// source: SourceError,
/// backtrace: MyBacktrace,
/// }
///
/// impl fmt::Display for Error {
/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
/// write!(f, "Example Error")
/// }
/// }
///
/// impl std::error::Error for Error {
/// fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
/// demand
/// .provide_ref::<MyBacktrace>(&self.backtrace)
/// .provide_ref::<dyn std::error::Error + 'static>(&self.source);
/// }
/// }
///
/// fn main() {
/// let backtrace = MyBacktrace::new();
/// let source = SourceError {};
/// let error = Error { source, backtrace };
/// let dyn_error = &error as &dyn std::error::Error;
/// let backtrace_ref = dyn_error.request_ref::<MyBacktrace>().unwrap();
///
/// assert!(core::ptr::eq(&error.backtrace, backtrace_ref));
/// }
/// ```
#[unstable(feature = "error_generic_member_access", issue = "99301")]
#[allow(unused_variables)]
fn provide<'a>(&'a self, demand: &mut Demand<'a>) {}
}
#[unstable(feature = "error_generic_member_access", issue = "99301")]
impl<E> Provider for E
where
E: Error + ?Sized,
{
fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
self.provide(demand)
}
}
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;
}
#[unstable(feature = "never_type", issue = "35121")]
impl Error for ! {}
impl<'a> dyn Error + 'a {
/// Request a reference of type `T` as context about this error.
#[unstable(feature = "error_generic_member_access", issue = "99301")]
pub fn request_ref<T: ?Sized + 'static>(&'a self) -> Option<&'a T> {
core::any::request_ref(self)
}
/// Request a value of type `T` as context about this error.
#[unstable(feature = "error_generic_member_access", issue = "99301")]
pub fn request_value<T: 'static>(&'a self) -> Option<T> {
core::any::request_value(self)
}
}
// Copied from `any.rs`.
impl dyn Error + 'static {
/// Returns `true` if the inner 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 (`self`).
let concrete = self.type_id(private::Internal);
// Compare both `TypeId`s on equality.
t == concrete
}
/// Returns some reference to the inner 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>() {
// SAFETY: `is` ensures this type cast is correct
unsafe { Some(&*(self as *const dyn Error as *const T)) }
} else {
None
}
}
/// Returns some mutable reference to the inner 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>() {
// SAFETY: `is` ensures this type cast is correct
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)
}
/// Request a reference of type `T` as context about this error.
#[unstable(feature = "error_generic_member_access", issue = "99301")]
pub fn request_ref<T: ?Sized + 'static>(&self) -> Option<&T> {
<dyn Error>::request_ref(self)
}
/// Request a value of type `T` as context about this error.
#[unstable(feature = "error_generic_member_access", issue = "99301")]
pub fn request_value<T: 'static>(&self) -> Option<T> {
<dyn Error>::request_value(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)
}
/// Request a reference of type `T` as context about this error.
#[unstable(feature = "error_generic_member_access", issue = "99301")]
pub fn request_ref<T: ?Sized + 'static>(&self) -> Option<&T> {
<dyn Error>::request_ref(self)
}
/// Request a value of type `T` as context about this error.
#[unstable(feature = "error_generic_member_access", issue = "99301")]
pub fn request_value<T: 'static>(&self) -> Option<T> {
<dyn Error>::request_value(self)
}
}
impl dyn Error {
/// Returns an iterator starting with the current error and continuing with
/// recursively calling [`Error::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.sources();
///
/// 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());
/// ```
#[unstable(feature = "error_iter", issue = "58520")]
#[inline]
pub fn sources(&self) -> Source<'_> {
// You may think this method would be better in the Error trait, and you'd be right.
// Unfortunately that doesn't work, not because of the object safety rules but because we
// save a reference to self in Sources below as a trait object. If this method was
// declared in Error, then self would have the type &T where T is some concrete type which
// implements Error. We would need to coerce self to have type &dyn Error, but that requires
// that Self has a known size (i.e., Self: Sized). We can't put that bound on Error
// since that would forbid Error trait objects, and we can't put that bound on the method
// because that means the method can't be called on trait objects (we'd also need the
// 'static bound, but that isn't allowed because methods with bounds on Self other than
// Sized are not object-safe). Requiring an Unsize bound is not backwards compatible.
Source { 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)`.
#[unstable(feature = "error_iter", issue = "58520")]
#[derive(Clone, Debug)]
pub struct Source<'a> {
current: Option<&'a (dyn Error + 'static)>,
}
#[unstable(feature = "error_iter", issue = "58520")]
impl<'a> Iterator for Source<'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
}
}
#[stable(feature = "error_by_ref", since = "1.51.0")]
impl<'a, T: Error + ?Sized> Error for &'a 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)
}
fn provide<'b>(&'b self, demand: &mut Demand<'b>) {
Error::provide(&**self, demand);
}
}
#[stable(feature = "fmt_error", since = "1.11.0")]
impl Error for crate::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 crate::cell::BorrowError {
#[allow(deprecated)]
fn description(&self) -> &str {
"already mutably borrowed"
}
}
#[stable(feature = "try_borrow", since = "1.13.0")]
impl Error for crate::cell::BorrowMutError {
#[allow(deprecated)]
fn description(&self) -> &str {
"already borrowed"
}
}
#[stable(feature = "try_from", since = "1.34.0")]
impl Error for crate::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 crate::char::ParseCharError {
#[allow(deprecated)]
fn description(&self) -> &str {
self.__description()
}
}
#[stable(feature = "duration_checked_float", since = "1.66.0")]
impl Error for crate::time::TryFromFloatSecsError {}
#[stable(feature = "frombyteswithnulerror_impls", since = "1.17.0")]
impl Error for crate::ffi::FromBytesWithNulError {
#[allow(deprecated)]
fn description(&self) -> &str {
self.__description()
}
}
#[unstable(feature = "cstr_from_bytes_until_nul", issue = "95027")]
impl Error for crate::ffi::FromBytesUntilNulError {}
#[unstable(feature = "get_many_mut", issue = "104642")]
impl<const N: usize> Error for crate::slice::GetManyMutError<N> {}