1 //! Traits for working with Errors.
3 #![stable(feature = "rust1", since = "1.0.0")]
5 // A note about crates and the facade:
7 // Originally, the `Error` trait was defined in libcore, and the impls
8 // were scattered about. However, coherence objected to this
9 // arrangement, because to create the blanket impls for `Box` required
10 // knowing that `&str: !Error`, and we have no means to deal with that
11 // sort of conflict just now. Therefore, for the time being, we have
12 // moved the `Error` trait into libstd. As we evolve a sol'n to the
13 // coherence challenge (e.g., specialization, neg impls, etc) we can
14 // reconsider what crate these items belong in.
20 use core::convert::Infallible;
22 use crate::alloc::{AllocError, LayoutError};
23 use crate::any::TypeId;
24 use crate::backtrace::Backtrace;
25 use crate::borrow::Cow;
28 use crate::fmt::{self, Debug, Display};
29 use crate::mem::transmute;
35 /// `Error` is a trait representing the basic expectations for error values,
36 /// i.e., values of type `E` in [`Result<T, E>`].
38 /// Errors must describe themselves through the [`Display`] and [`Debug`]
39 /// traits. Error messages are typically concise lowercase sentences without
40 /// trailing punctuation:
43 /// let err = "NaN".parse::<u32>().unwrap_err();
44 /// assert_eq!(err.to_string(), "invalid digit found in string");
47 /// Errors may provide cause chain information. [`Error::source()`] is generally
48 /// used when errors cross "abstraction boundaries". If one module must report
49 /// an error that is caused by an error from a lower-level module, it can allow
50 /// accessing that error via [`Error::source()`]. This makes it possible for the
51 /// high-level module to provide its own errors while also revealing some of the
52 /// implementation for debugging via `source` chains.
53 #[stable(feature = "rust1", since = "1.0.0")]
54 pub trait Error: Debug + Display {
55 /// The lower-level source of this error, if any.
60 /// use std::error::Error;
64 /// struct SuperError {
65 /// side: SuperErrorSideKick,
68 /// impl fmt::Display for SuperError {
69 /// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
70 /// write!(f, "SuperError is here!")
74 /// impl Error for SuperError {
75 /// fn source(&self) -> Option<&(dyn Error + 'static)> {
81 /// struct SuperErrorSideKick;
83 /// impl fmt::Display for SuperErrorSideKick {
84 /// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
85 /// write!(f, "SuperErrorSideKick is here!")
89 /// impl Error for SuperErrorSideKick {}
91 /// fn get_super_error() -> Result<(), SuperError> {
92 /// Err(SuperError { side: SuperErrorSideKick })
96 /// match get_super_error() {
98 /// println!("Error: {}", e);
99 /// println!("Caused by: {}", e.source().unwrap());
101 /// _ => println!("No error"),
105 #[stable(feature = "error_source", since = "1.30.0")]
106 fn source(&self) -> Option<&(dyn Error + 'static)> {
110 /// Gets the `TypeId` of `self`.
113 feature = "error_type_id",
114 reason = "this is memory-unsafe to override in user code",
117 fn type_id(&self, _: private::Internal) -> TypeId
124 /// Returns a stack backtrace, if available, of where this error occurred.
126 /// This function allows inspecting the location, in code, of where an error
127 /// happened. The returned `Backtrace` contains information about the stack
128 /// trace of the OS thread of execution of where the error originated from.
130 /// Note that not all errors contain a `Backtrace`. Also note that a
131 /// `Backtrace` may actually be empty. For more information consult the
132 /// `Backtrace` type itself.
133 #[unstable(feature = "backtrace", issue = "53487")]
134 fn backtrace(&self) -> Option<&Backtrace> {
139 /// if let Err(e) = "xc".parse::<u32>() {
140 /// // Print `e` itself, no need for description().
141 /// eprintln!("Error: {}", e);
144 #[stable(feature = "rust1", since = "1.0.0")]
145 #[rustc_deprecated(since = "1.42.0", reason = "use the Display impl or to_string()")]
146 fn description(&self) -> &str {
147 "description() is deprecated; use Display"
150 #[stable(feature = "rust1", since = "1.0.0")]
153 reason = "replaced by Error::source, which can support downcasting"
155 #[allow(missing_docs)]
156 fn cause(&self) -> Option<&dyn Error> {
162 // This is a hack to prevent `type_id` from being overridden by `Error`
163 // implementations, since that can enable unsound downcasting.
164 #[unstable(feature = "error_type_id", issue = "60784")]
169 #[stable(feature = "rust1", since = "1.0.0")]
170 impl<'a, E: Error + 'a> From<E> for Box<dyn Error + 'a> {
171 /// Converts a type of [`Error`] into a box of dyn [`Error`].
176 /// use std::error::Error;
183 /// impl fmt::Display for AnError {
184 /// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
185 /// write!(f , "An error")
189 /// impl Error for AnError {}
191 /// let an_error = AnError;
192 /// assert!(0 == mem::size_of_val(&an_error));
193 /// let a_boxed_error = Box::<dyn Error>::from(an_error);
194 /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
196 fn from(err: E) -> Box<dyn Error + 'a> {
201 #[stable(feature = "rust1", since = "1.0.0")]
202 impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<dyn Error + Send + Sync + 'a> {
203 /// Converts a type of [`Error`] + [`Send`] + [`Sync`] into a box of
204 /// dyn [`Error`] + [`Send`] + [`Sync`].
209 /// use std::error::Error;
216 /// impl fmt::Display for AnError {
217 /// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
218 /// write!(f , "An error")
222 /// impl Error for AnError {}
224 /// unsafe impl Send for AnError {}
226 /// unsafe impl Sync for AnError {}
228 /// let an_error = AnError;
229 /// assert!(0 == mem::size_of_val(&an_error));
230 /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(an_error);
232 /// mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
234 fn from(err: E) -> Box<dyn Error + Send + Sync + 'a> {
239 #[stable(feature = "rust1", since = "1.0.0")]
240 impl From<String> for Box<dyn Error + Send + Sync> {
241 /// Converts a [`String`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
246 /// use std::error::Error;
249 /// let a_string_error = "a string error".to_string();
250 /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_string_error);
252 /// mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
255 fn from(err: String) -> Box<dyn Error + Send + Sync> {
256 struct StringError(String);
258 impl Error for StringError {
260 fn description(&self) -> &str {
265 impl Display for StringError {
266 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
267 Display::fmt(&self.0, f)
271 // Purposefully skip printing "StringError(..)"
272 impl Debug for StringError {
273 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
274 Debug::fmt(&self.0, f)
278 Box::new(StringError(err))
282 #[stable(feature = "string_box_error", since = "1.6.0")]
283 impl From<String> for Box<dyn Error> {
284 /// Converts a [`String`] into a box of dyn [`Error`].
289 /// use std::error::Error;
292 /// let a_string_error = "a string error".to_string();
293 /// let a_boxed_error = Box::<dyn Error>::from(a_string_error);
294 /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
296 fn from(str_err: String) -> Box<dyn Error> {
297 let err1: Box<dyn Error + Send + Sync> = From::from(str_err);
298 let err2: Box<dyn Error> = err1;
303 #[stable(feature = "rust1", since = "1.0.0")]
304 impl<'a> From<&str> for Box<dyn Error + Send + Sync + 'a> {
305 /// Converts a [`str`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
307 /// [`str`]: prim@str
312 /// use std::error::Error;
315 /// let a_str_error = "a str error";
316 /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_str_error);
318 /// mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
321 fn from(err: &str) -> Box<dyn Error + Send + Sync + 'a> {
322 From::from(String::from(err))
326 #[stable(feature = "string_box_error", since = "1.6.0")]
327 impl From<&str> for Box<dyn Error> {
328 /// Converts a [`str`] into a box of dyn [`Error`].
330 /// [`str`]: prim@str
335 /// use std::error::Error;
338 /// let a_str_error = "a str error";
339 /// let a_boxed_error = Box::<dyn Error>::from(a_str_error);
340 /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
342 fn from(err: &str) -> Box<dyn Error> {
343 From::from(String::from(err))
347 #[stable(feature = "cow_box_error", since = "1.22.0")]
348 impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + Send + Sync + 'a> {
349 /// Converts a [`Cow`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
354 /// use std::error::Error;
356 /// use std::borrow::Cow;
358 /// let a_cow_str_error = Cow::from("a str error");
359 /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_cow_str_error);
361 /// mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
363 fn from(err: Cow<'b, str>) -> Box<dyn Error + Send + Sync + 'a> {
364 From::from(String::from(err))
368 #[stable(feature = "cow_box_error", since = "1.22.0")]
369 impl<'a> From<Cow<'a, str>> for Box<dyn Error> {
370 /// Converts a [`Cow`] into a box of dyn [`Error`].
375 /// use std::error::Error;
377 /// use std::borrow::Cow;
379 /// let a_cow_str_error = Cow::from("a str error");
380 /// let a_boxed_error = Box::<dyn Error>::from(a_cow_str_error);
381 /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
383 fn from(err: Cow<'a, str>) -> Box<dyn Error> {
384 From::from(String::from(err))
388 #[unstable(feature = "never_type", issue = "35121")]
392 feature = "allocator_api",
393 reason = "the precise API and guarantees it provides may be tweaked.",
396 impl Error for AllocError {}
398 #[stable(feature = "alloc_layout", since = "1.28.0")]
399 impl Error for LayoutError {}
401 #[stable(feature = "rust1", since = "1.0.0")]
402 impl Error for str::ParseBoolError {
404 fn description(&self) -> &str {
405 "failed to parse bool"
409 #[stable(feature = "rust1", since = "1.0.0")]
410 impl Error for str::Utf8Error {
412 fn description(&self) -> &str {
413 "invalid utf-8: corrupt contents"
417 #[stable(feature = "rust1", since = "1.0.0")]
418 impl Error for num::ParseIntError {
420 fn description(&self) -> &str {
425 #[stable(feature = "try_from", since = "1.34.0")]
426 impl Error for num::TryFromIntError {
428 fn description(&self) -> &str {
433 #[stable(feature = "try_from", since = "1.34.0")]
434 impl Error for array::TryFromSliceError {
436 fn description(&self) -> &str {
441 #[stable(feature = "rust1", since = "1.0.0")]
442 impl Error for num::ParseFloatError {
444 fn description(&self) -> &str {
449 #[stable(feature = "rust1", since = "1.0.0")]
450 impl Error for string::FromUtf8Error {
452 fn description(&self) -> &str {
457 #[stable(feature = "rust1", since = "1.0.0")]
458 impl Error for string::FromUtf16Error {
460 fn description(&self) -> &str {
465 #[stable(feature = "str_parse_error2", since = "1.8.0")]
466 impl Error for Infallible {
467 fn description(&self) -> &str {
472 #[stable(feature = "decode_utf16", since = "1.9.0")]
473 impl Error for char::DecodeUtf16Error {
475 fn description(&self) -> &str {
476 "unpaired surrogate found"
480 #[unstable(feature = "map_try_insert", issue = "82766")]
481 impl<'a, K: Debug + Ord, V: Debug> Error
482 for crate::collections::btree_map::OccupiedError<'a, K, V>
485 fn description(&self) -> &str {
490 #[unstable(feature = "map_try_insert", issue = "82766")]
491 impl<'a, K: Debug, V: Debug> Error for crate::collections::hash_map::OccupiedError<'a, K, V> {
493 fn description(&self) -> &str {
498 #[stable(feature = "box_error", since = "1.8.0")]
499 impl<T: Error> Error for Box<T> {
500 #[allow(deprecated, deprecated_in_future)]
501 fn description(&self) -> &str {
502 Error::description(&**self)
506 fn cause(&self) -> Option<&dyn Error> {
507 Error::cause(&**self)
510 fn source(&self) -> Option<&(dyn Error + 'static)> {
511 Error::source(&**self)
515 #[stable(feature = "error_by_ref", since = "1.51.0")]
516 impl<'a, T: Error + ?Sized> Error for &'a T {
517 #[allow(deprecated, deprecated_in_future)]
518 fn description(&self) -> &str {
519 Error::description(&**self)
523 fn cause(&self) -> Option<&dyn Error> {
524 Error::cause(&**self)
527 fn source(&self) -> Option<&(dyn Error + 'static)> {
528 Error::source(&**self)
531 fn backtrace(&self) -> Option<&Backtrace> {
532 Error::backtrace(&**self)
536 #[stable(feature = "arc_error", since = "1.52.0")]
537 impl<T: Error + ?Sized> Error for Arc<T> {
538 #[allow(deprecated, deprecated_in_future)]
539 fn description(&self) -> &str {
540 Error::description(&**self)
544 fn cause(&self) -> Option<&dyn Error> {
545 Error::cause(&**self)
548 fn source(&self) -> Option<&(dyn Error + 'static)> {
549 Error::source(&**self)
552 fn backtrace(&self) -> Option<&Backtrace> {
553 Error::backtrace(&**self)
557 #[stable(feature = "fmt_error", since = "1.11.0")]
558 impl Error for fmt::Error {
560 fn description(&self) -> &str {
561 "an error occurred when formatting an argument"
565 #[stable(feature = "try_borrow", since = "1.13.0")]
566 impl Error for cell::BorrowError {
568 fn description(&self) -> &str {
569 "already mutably borrowed"
573 #[stable(feature = "try_borrow", since = "1.13.0")]
574 impl Error for cell::BorrowMutError {
576 fn description(&self) -> &str {
581 #[stable(feature = "try_from", since = "1.34.0")]
582 impl Error for char::CharTryFromError {
584 fn description(&self) -> &str {
585 "converted integer out of range for `char`"
589 #[stable(feature = "char_from_str", since = "1.20.0")]
590 impl Error for char::ParseCharError {
592 fn description(&self) -> &str {
597 #[unstable(feature = "try_reserve", reason = "new API", issue = "48043")]
598 impl Error for alloc::collections::TryReserveError {}
600 // Copied from `any.rs`.
601 impl dyn Error + 'static {
602 /// Returns `true` if the boxed type is the same as `T`
603 #[stable(feature = "error_downcast", since = "1.3.0")]
605 pub fn is<T: Error + 'static>(&self) -> bool {
606 // Get `TypeId` of the type this function is instantiated with.
607 let t = TypeId::of::<T>();
609 // Get `TypeId` of the type in the trait object.
610 let boxed = self.type_id(private::Internal);
612 // Compare both `TypeId`s on equality.
616 /// Returns some reference to the boxed value if it is of type `T`, or
617 /// `None` if it isn't.
618 #[stable(feature = "error_downcast", since = "1.3.0")]
620 pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
622 unsafe { Some(&*(self as *const dyn Error as *const T)) }
628 /// Returns some mutable reference to the boxed value if it is of type `T`, or
629 /// `None` if it isn't.
630 #[stable(feature = "error_downcast", since = "1.3.0")]
632 pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
634 unsafe { Some(&mut *(self as *mut dyn Error as *mut T)) }
641 impl dyn Error + 'static + Send {
642 /// Forwards to the method defined on the type `dyn Error`.
643 #[stable(feature = "error_downcast", since = "1.3.0")]
645 pub fn is<T: Error + 'static>(&self) -> bool {
646 <dyn Error + 'static>::is::<T>(self)
649 /// Forwards to the method defined on the type `dyn Error`.
650 #[stable(feature = "error_downcast", since = "1.3.0")]
652 pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
653 <dyn Error + 'static>::downcast_ref::<T>(self)
656 /// Forwards to the method defined on the type `dyn Error`.
657 #[stable(feature = "error_downcast", since = "1.3.0")]
659 pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
660 <dyn Error + 'static>::downcast_mut::<T>(self)
664 impl dyn Error + 'static + Send + Sync {
665 /// Forwards to the method defined on the type `dyn Error`.
666 #[stable(feature = "error_downcast", since = "1.3.0")]
668 pub fn is<T: Error + 'static>(&self) -> bool {
669 <dyn Error + 'static>::is::<T>(self)
672 /// Forwards to the method defined on the type `dyn Error`.
673 #[stable(feature = "error_downcast", since = "1.3.0")]
675 pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
676 <dyn Error + 'static>::downcast_ref::<T>(self)
679 /// Forwards to the method defined on the type `dyn Error`.
680 #[stable(feature = "error_downcast", since = "1.3.0")]
682 pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
683 <dyn Error + 'static>::downcast_mut::<T>(self)
689 #[stable(feature = "error_downcast", since = "1.3.0")]
690 /// Attempts to downcast the box to a concrete type.
691 pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<dyn Error>> {
694 let raw: *mut dyn Error = Box::into_raw(self);
695 Ok(Box::from_raw(raw as *mut T))
702 /// Returns an iterator starting with the current error and continuing with
703 /// recursively calling [`Error::source`].
705 /// If you want to omit the current error and only use its sources,
711 /// #![feature(error_iter)]
712 /// use std::error::Error;
719 /// struct B(Option<Box<dyn Error + 'static>>);
721 /// impl fmt::Display for A {
722 /// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
727 /// impl fmt::Display for B {
728 /// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
733 /// impl Error for A {}
735 /// impl Error for B {
736 /// fn source(&self) -> Option<&(dyn Error + 'static)> {
737 /// self.0.as_ref().map(|e| e.as_ref())
741 /// let b = B(Some(Box::new(A)));
743 /// // let err : Box<Error> = b.into(); // or
744 /// let err = &b as &(dyn Error);
746 /// let mut iter = err.chain();
748 /// assert_eq!("B".to_string(), iter.next().unwrap().to_string());
749 /// assert_eq!("A".to_string(), iter.next().unwrap().to_string());
750 /// assert!(iter.next().is_none());
751 /// assert!(iter.next().is_none());
753 #[unstable(feature = "error_iter", issue = "58520")]
755 pub fn chain(&self) -> Chain<'_> {
756 Chain { current: Some(self) }
760 /// An iterator over an [`Error`] and its sources.
762 /// If you want to omit the initial error and only process
763 /// its sources, use `skip(1)`.
764 #[unstable(feature = "error_iter", issue = "58520")]
765 #[derive(Clone, Debug)]
766 pub struct Chain<'a> {
767 current: Option<&'a (dyn Error + 'static)>,
770 #[unstable(feature = "error_iter", issue = "58520")]
771 impl<'a> Iterator for Chain<'a> {
772 type Item = &'a (dyn Error + 'static);
774 fn next(&mut self) -> Option<Self::Item> {
775 let current = self.current;
776 self.current = self.current.and_then(Error::source);
781 impl dyn Error + Send {
783 #[stable(feature = "error_downcast", since = "1.3.0")]
784 /// Attempts to downcast the box to a concrete type.
785 pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<dyn Error + Send>> {
786 let err: Box<dyn Error> = self;
787 <dyn Error>::downcast(err).map_err(|s| unsafe {
788 // Reapply the `Send` marker.
789 transmute::<Box<dyn Error>, Box<dyn Error + Send>>(s)
794 impl dyn Error + Send + Sync {
796 #[stable(feature = "error_downcast", since = "1.3.0")]
797 /// Attempts to downcast the box to a concrete type.
798 pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<Self>> {
799 let err: Box<dyn Error> = self;
800 <dyn Error>::downcast(err).map_err(|s| unsafe {
801 // Reapply the `Send + Sync` marker.
802 transmute::<Box<dyn Error>, Box<dyn Error + Send + Sync>>(s)