pub trait Error: Debug + Display {
fn source(&self) -> Option<&(dyn Error + 'static)> { ... }
fn backtrace(&self) -> Option<&Backtrace> { ... }
fn description(&self) -> &str { ... }
fn cause(&self) -> Option<&dyn Error> { ... }
}
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");
RunErrors may provide cause chain 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 via source
chains.
Provided Methods
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"),
}
}
RunReturns a stack backtrace, if available, of where this error occurred.
This function allows inspecting the location, in code, of where an error
happened. The returned Backtrace
contains information about the stack
trace of the OS thread of execution of where the error originated from.
Note that not all errors contain a Backtrace
. Also note that a
Backtrace
may actually be empty. For more information consult the
Backtrace
type itself.
fn description(&self) -> &str
fn description(&self) -> &str
use the Display impl or to_string()
if let Err(e) = "xc".parse::<u32>() {
// Print `e` itself, no need for description().
eprintln!("Error: {e}");
}
RunImplementations
sourceimpl dyn Error + 'static
impl dyn Error + 'static
1.3.0 · sourcepub fn is<T: Error + 'static>(&self) -> bool
pub fn is<T: Error + 'static>(&self) -> bool
Returns true
if the inner type is the same as T
.
1.3.0 · sourcepub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T>
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 · sourcepub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T>
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.
sourceimpl dyn Error + Send + 'static
impl dyn Error + Send + 'static
1.3.0 · sourcepub fn is<T: Error + 'static>(&self) -> bool
pub fn is<T: Error + 'static>(&self) -> bool
Forwards to the method defined on the type dyn Error
.
1.3.0 · sourcepub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T>
pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T>
Forwards to the method defined on the type dyn Error
.
1.3.0 · sourcepub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T>
pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T>
Forwards to the method defined on the type dyn Error
.
sourceimpl dyn Error + Send + Sync + 'static
impl dyn Error + Send + Sync + 'static
1.3.0 · sourcepub fn is<T: Error + 'static>(&self) -> bool
pub fn is<T: Error + 'static>(&self) -> bool
Forwards to the method defined on the type dyn Error
.
1.3.0 · sourcepub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T>
pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T>
Forwards to the method defined on the type dyn Error
.
1.3.0 · sourcepub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T>
pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T>
Forwards to the method defined on the type dyn Error
.
sourceimpl dyn Error
impl dyn Error
1.3.0 · sourcepub fn downcast<T: Error + 'static>(
self: Box<Self>
) -> Result<Box<T>, Box<dyn Error>>
pub fn downcast<T: Error + 'static>(
self: Box<Self>
) -> Result<Box<T>, Box<dyn Error>>
Attempts to downcast the box to a concrete type.
sourcepub fn chain(&self) -> Chain<'_>ⓘNotable traits for Chain<'a>impl<'a> Iterator for Chain<'a> type Item = &'a (dyn Error + 'static);
pub fn chain(&self) -> Chain<'_>ⓘNotable traits for Chain<'a>impl<'a> Iterator for Chain<'a> type Item = &'a (dyn Error + 'static);
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.chain();
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());
RunImplementations on Foreign Types
sourceimpl Error for NulError
impl Error for NulError
sourcefn description(&self) -> &str
fn description(&self) -> &str
use the Display impl or to_string()
1.17.0 · sourceimpl Error for FromBytesWithNulError
impl Error for FromBytesWithNulError
sourcefn description(&self) -> &str
fn description(&self) -> &str
use the Display impl or to_string()