Trait Error 

pub trait Error: Debug + Display {
    // Provided methods
    fn source(&self) -> Option<&(dyn Error + 'static)> { ... }
    fn description(&self) -> &str { ... }
    fn cause(&self) -> Option<&dyn Error> { ... }
    fn provide<'a>(&'a self, request: &mut Request<'a>) { ... }
}

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");

Error source

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.

In error types that wrap an underlying error, the underlying error should be either returned by the outer error’s Error::source(), or rendered by the outer error’s Display implementation, but not both.

Example

Implementing the Error trait only requires that Debug and Display are implemented too.

use std::error::Error;
use std::fmt;
use std::path::PathBuf;

#[derive(Debug)]
struct ReadConfigError {
    path: PathBuf
}

impl fmt::Display for ReadConfigError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let path = self.path.display();
        write!(f, "unable to read configuration at {path}")
    }
}

impl Error for ReadConfigError {}

Provided Methods

fn source(&self) -> Option<&(dyn Error + 'static)>

Returns 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"),
    }
}

fn description(&self) -> &str

👎Deprecated since 1.42.0: use the Display impl or to_string()

if let Err(e) = "xc".parse::<u32>() {
    // Print `e` itself, no need for description().
    eprintln!("Error: {e}");
}

fn cause(&self) -> Option<&dyn Error>

👎Deprecated since 1.33.0: replaced by Error::source, which can support downcasting

fn provide<'a>(&'a self, request: &mut Request<'a>)

🔬This is a nightly-only experimental API. (error_generic_member_access)

Provides type-based access to context intended for error reports.

Used in conjunction with Request::provide_value and Request::provide_ref to extract references to member variables from dyn Error trait objects.

Example

#![feature(error_generic_member_access)]
use core::fmt;
use core::error::{request_ref, Request};

#[derive(Debug)]
enum MyLittleTeaPot {
    Empty,
}

#[derive(Debug)]
struct MyBacktrace {
    // ...
}

impl MyBacktrace {
    fn new() -> MyBacktrace {
        // ...
    }
}

#[derive(Debug)]
struct Error {
    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, request: &mut Request<'a>) {
        request
            .provide_ref::<MyBacktrace>(&self.backtrace);
    }
}

fn main() {
    let backtrace = MyBacktrace::new();
    let error = Error { backtrace };
    let dyn_error = &error as &dyn std::error::Error;
    let backtrace_ref = request_ref::<MyBacktrace>(dyn_error).unwrap();

    assert!(core::ptr::eq(&error.backtrace, backtrace_ref));
    assert!(request_ref::<MyLittleTeaPot>(dyn_error).is_none());
}

Implementations

impl dyn Error

pub fn is<T>(&self) -> bool

where T: Error + 'static,

Returns true if the inner type is the same as T.

pub fn downcast_ref<T>(&self) -> Option<&T>

where T: Error + 'static,

Returns some reference to the inner value if it is of type T, or None if it isn’t.

pub fn downcast_mut<T>(&mut self) -> Option<&mut T>

where T: Error + 'static,

Returns some mutable reference to the inner value if it is of type T, or None if it isn’t.

impl dyn Error + Send

pub fn is<T>(&self) -> bool

where T: Error + 'static,

Forwards to the method defined on the type dyn Error.

pub fn downcast_ref<T>(&self) -> Option<&T>

where T: Error + 'static,

Forwards to the method defined on the type dyn Error.

pub fn downcast_mut<T>(&mut self) -> Option<&mut T>

where T: Error + 'static,

Forwards to the method defined on the type dyn Error.

impl dyn Error + Send + Sync

pub fn is<T>(&self) -> bool

where T: Error + 'static,

Forwards to the method defined on the type dyn Error.

pub fn downcast_ref<T>(&self) -> Option<&T>

where T: Error + 'static,

Forwards to the method defined on the type dyn Error.

pub fn downcast_mut<T>(&mut self) -> Option<&mut T>

where T: Error + 'static,

Forwards to the method defined on the type dyn Error.

impl dyn Error

pub fn sources(&self) -> Source<'_>

🔬This is a nightly-only experimental API. (error_iter)

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());

impl dyn Error

pub fn downcast<T>(self: Box<dyn Error>) -> Result<Box<T>, Box<dyn Error>>

where T: Error + 'static,

Attempts to downcast the box to a concrete type.

impl dyn Error + Send

pub fn downcast<T>( self: Box<dyn Error + Send>, ) -> Result<Box<T>, Box<dyn Error + Send>>

where T: Error + 'static,

Attempts to downcast the box to a concrete type.

impl dyn Error + Send + Sync

pub fn downcast<T>( self: Box<dyn Error + Send + Sync>, ) -> Result<Box<T>, Box<dyn Error + Send + Sync>>

where T: Error + 'static,

Attempts to downcast the box to a concrete type.

Implementors

impl !Error for &str

impl Error for Infallible

impl Error for FromBytesWithNulError

impl Error for core::slice::GetDisjointMutError

impl Error for VarError

impl Error for std::fs::TryLockError

impl Error for std::sync::mpsc::RecvTimeoutError

impl Error for std::sync::mpsc::TryRecvError

impl Error for async_channel::TryRecvError

Available on crate feature std only.

impl Error for async_channel::TryRecvError

impl Error for PopError

Available on crate feature std only.

impl Error for crossbeam_channel::err::RecvTimeoutError

impl Error for crossbeam_channel::err::TryRecvError

impl Error for indexmap::GetDisjointMutError

impl Error for slab::GetDisjointMutError

Available on crate feature std only.

impl Error for TryAcquireError

impl Error for tokio::sync::broadcast::error::RecvError

impl Error for tokio::sync::broadcast::error::TryRecvError

impl Error for tokio::sync::mpsc::error::TryRecvError

impl Error for tokio::sync::oneshot::error::TryRecvError

impl Error for WaterMarkError

Available on crate feature std only.

impl Error for !

impl Error for UnorderedKeyError

impl Error for alloc::collections::TryReserveError

impl Error for FromVecWithNulError

impl Error for IntoStringError

impl Error for NulError

impl Error for FromUtf8Error

impl Error for FromUtf16Error

impl Error for LayoutError

impl Error for AllocError

impl Error for TryFromSliceError

impl Error for BorrowError

impl Error for BorrowMutError

impl Error for CharTryFromError

impl Error for ParseCharError

impl Error for DecodeUtf16Error

impl Error for TryFromCharError

impl Error for FromBytesUntilNulError

impl Error for core::fmt::Error

impl Error for AddrParseError

impl Error for ParseFloatError

impl Error for ParseIntError

impl Error for core::num::error::TryFromIntError

impl Error for ParseBoolError

impl Error for Utf8Error

impl Error for TryFromFloatSecsError

impl Error for JoinPathsError

impl Error for WriterPanicked

impl Error for std::io::error::Error

impl Error for NormalizeError

impl Error for StripPrefixError

impl Error for ExitStatusError

impl Error for std::sync::mpsc::RecvError

impl Error for std::thread::local::AccessError

impl Error for SystemTimeError

impl Error for agnostic_lite::time::delay::Aborted

impl Error for agnostic_lite::time::timeout::Elapsed

impl Error for async_channel::RecvError

Available on crate feature std only.

impl Error for async_channel::RecvError

impl Error for async_std::future::timeout::TimeoutError

impl Error for async_std::stream::stream::timeout::TimeoutError

impl Error for async_std::task::task_local::AccessError

impl Error for ParseError

Available on crate feature std only.

impl Error for crossbeam_channel::err::RecvError

impl Error for SelectTimeoutError

impl Error for TrySelectError

impl Error for futures_channel::mpsc::SendError

impl Error for futures_channel::mpsc::TryRecvError

impl Error for Canceled

Available on crate feature std only.

impl Error for EnterError

impl Error for SpawnError

Available on crate feature std only.

impl Error for futures_util::abortable::Aborted

Available on crate feature std only.

impl Error for indexmap::TryReserveError

impl Error for js_sys::TryFromIntError

Available on crate feature std only.

impl Error for Errno

Available on crate feature std or error_in_core only.

impl Error for TryCurrentError

impl Error for JoinError

impl Error for AcquireError

impl Error for tokio::sync::mutex::TryLockError

impl Error for tokio::sync::oneshot::error::RecvError

impl Error for tokio::sync::watch::error::RecvError

impl Error for tokio::time::error::Elapsed

impl Error for tokio::time::error::Error

impl<'a, K, V> Error for alloc::collections::btree::map::entry::OccupiedError<'a, K, V>

where K: Debug + Ord, V: Debug,

impl<'a, K, V> Error for std::collections::hash::map::OccupiedError<'a, K, V>

where K: Debug, V: Debug,

impl<'a, T> Error for &'a T

where T: Error + ?Sized,

impl<C, P> Error for TransactionError<C, P>

where C: Error, P: Error, TransactionError<C, P>: Debug + Display,

impl<C, P, E> Error for WtmError<C, P, E>

where C: Error, P: Error, E: Error,

impl<E> Error for AfterHandleError<E>

where E: Error,

Available on crate feature time only.

impl<E> Error for Box<E>

where E: Error,

impl<T> Error for std::sync::mpmc::error::SendTimeoutError<T>

impl<T> Error for std::sync::mpsc::TrySendError<T>

impl<T> Error for std::sync::poison::TryLockError<T>

impl<T> Error for async_channel::TrySendError<T>

Available on crate feature std only.

impl<T> Error for async_channel::TrySendError<T>

impl<T> Error for PushError<T>

where T: Debug,

Available on crate feature std only.

impl<T> Error for crossbeam_channel::err::SendTimeoutError<T>

where T: Send,

impl<T> Error for crossbeam_channel::err::TrySendError<T>

where T: Send,

impl<T> Error for tokio::sync::mpsc::error::SendTimeoutError<T>

Available on crate feature time only.

impl<T> Error for tokio::sync::mpsc::error::TrySendError<T>

impl<T> Error for SetError<T>

where T: Debug,

impl<T> Error for ThinBox<T>

where T: Error + ?Sized,

impl<T> Error for Arc<T>

where T: Error + ?Sized,

impl<T> Error for std::sync::mpsc::SendError<T>

impl<T> Error for PoisonError<T>

impl<T> Error for async_channel::SendError<T>

Available on crate feature std only.

impl<T> Error for async_channel::SendError<T>

impl<T> Error for ForcePushError<T>

where T: Debug,

Available on crate feature std only.

impl<T> Error for crossbeam_channel::err::SendError<T>

where T: Send,

impl<T> Error for futures_channel::mpsc::TrySendError<T>

where T: Any,

impl<T> Error for futures_util::io::split::ReuniteError<T>

where T: Any,

Available on crate feature std only.

impl<T> Error for tokio::sync::broadcast::error::SendError<T>

where T: Debug,

impl<T> Error for tokio::sync::mpsc::error::SendError<T>

impl<T> Error for SetOnceError<T>

where T: Debug,

impl<T> Error for tokio::sync::watch::error::SendError<T>

impl<T, E> Error for TryChunksError<T, E>

where E: Debug + Display,

Available on crate feature std only.

impl<T, E> Error for TryReadyChunksError<T, E>

where E: Debug + Display,

Available on crate feature std only.

impl<T, Item> Error for futures_util::stream::stream::split::ReuniteError<T, Item>

where T: Any,

Available on crate feature std only.

impl<W> Error for IntoInnerError<W>

where W: Send + Debug,