Trait core::error::Error

1.0.0 · source ·

pub trait Error: Debug + Display {
    // Provided methods
    fn source(&self) -> Option<&(dyn Error + 'static)> { ... }
    fn type_id(&self, _: Internal) -> TypeId
       where Self: 'static { ... }
    fn description(&self) -> &str { ... }
    fn cause(&self) -> Option<&dyn Error> { ... }
    fn provide<'a>(&'a self, demand: &mut Demand<'a>) { ... }
}

Expand description

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");

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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.

Provided Methods§

1.30.0 · source

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

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"),
    }
}

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fn type_id(&self, _: Internal) -> TypeIdwhere Self: 'static,

🔬This is a nightly-only experimental API. (error_type_id #60784)

Gets the TypeId of self.

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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}");
}

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source

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

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

source

fn provide<'a>(&'a self, demand: &mut Demand<'a>)

🔬This is a nightly-only experimental API. (error_generic_member_access #99301)

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

#![feature(provide_any)]
#![feature(error_generic_member_access)]
use core::fmt;
use core::any::Demand;

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

impl MyBacktrace {
    fn new() -> 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));
}

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Implementations§

source §

impl<'a> dyn Error + 'a

source

pub fn request_ref<T: ?Sized + 'static>(&'a self) -> Option<&'a T>

🔬This is a nightly-only experimental API. (error_generic_member_access #99301)

Request a reference of type T as context about this error.

source

pub fn request_value<T: 'static>(&'a self) -> Option<T>

🔬This is a nightly-only experimental API. (error_generic_member_access #99301)

Request a value of type T as context about this error.

source §

impl dyn Error + 'static

1.3.0 · source

pub fn is<T: Error + 'static>(&self) -> bool

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

1.3.0 · source

pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T>

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

1.3.0 · source

pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T>

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

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impl dyn Error + Send + 'static

1.3.0 · source

pub fn is<T: Error + 'static>(&self) -> bool

Forwards to the method defined on the type dyn Error.

1.3.0 · source

pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T>

Forwards to the method defined on the type dyn Error.

1.3.0 · source

pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T>

Forwards to the method defined on the type dyn Error.

source

pub fn request_ref<T: ?Sized + 'static>(&self) -> Option<&T>

🔬This is a nightly-only experimental API. (error_generic_member_access #99301)

Request a reference of type T as context about this error.

source

pub fn request_value<T: 'static>(&self) -> Option<T>

🔬This is a nightly-only experimental API. (error_generic_member_access #99301)

Request a value of type T as context about this error.

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impl dyn Error + Send + Sync + 'static

1.3.0 · source

pub fn is<T: Error + 'static>(&self) -> bool

Forwards to the method defined on the type dyn Error.

1.3.0 · source

pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T>

Forwards to the method defined on the type dyn Error.

1.3.0 · source

pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T>

Forwards to the method defined on the type dyn Error.

source

pub fn request_ref<T: ?Sized + 'static>(&self) -> Option<&T>

🔬This is a nightly-only experimental API. (error_generic_member_access #99301)

Request a reference of type T as context about this error.

source

pub fn request_value<T: 'static>(&self) -> Option<T>

🔬This is a nightly-only experimental API. (error_generic_member_access #99301)

Request a value of type T as context about this error.

source §

impl dyn Error

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pub fn sources(&self) -> Source<'_> ⓘ

🔬This is a nightly-only experimental API. (error_iter #58520)

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());