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;
36 /// `Error` is a trait representing the basic expectations for error values,
37 /// i.e., values of type `E` in [`Result<T, E>`].
39 /// Errors must describe themselves through the [`Display`] and [`Debug`]
40 /// traits. Error messages are typically concise lowercase sentences without
41 /// trailing punctuation:
44 /// let err = "NaN".parse::<u32>().unwrap_err();
45 /// assert_eq!(err.to_string(), "invalid digit found in string");
48 /// Errors may provide cause chain information. [`Error::source()`] is generally
49 /// used when errors cross "abstraction boundaries". If one module must report
50 /// an error that is caused by an error from a lower-level module, it can allow
51 /// accessing that error via [`Error::source()`]. This makes it possible for the
52 /// high-level module to provide its own errors while also revealing some of the
53 /// implementation for debugging via `source` chains.
54 #[stable(feature = "rust1", since = "1.0.0")]
55 pub trait Error: Debug + Display {
56 /// The lower-level source of this error, if any.
61 /// use std::error::Error;
65 /// struct SuperError {
66 /// side: SuperErrorSideKick,
69 /// impl fmt::Display for SuperError {
70 /// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
71 /// write!(f, "SuperError is here!")
75 /// impl Error for SuperError {
76 /// fn source(&self) -> Option<&(dyn Error + 'static)> {
82 /// struct SuperErrorSideKick;
84 /// impl fmt::Display for SuperErrorSideKick {
85 /// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
86 /// write!(f, "SuperErrorSideKick is here!")
90 /// impl Error for SuperErrorSideKick {}
92 /// fn get_super_error() -> Result<(), SuperError> {
93 /// Err(SuperError { side: SuperErrorSideKick })
97 /// match get_super_error() {
99 /// println!("Error: {}", e);
100 /// println!("Caused by: {}", e.source().unwrap());
102 /// _ => println!("No error"),
106 #[stable(feature = "error_source", since = "1.30.0")]
107 fn source(&self) -> Option<&(dyn Error + 'static)> {
111 /// Gets the `TypeId` of `self`.
114 feature = "error_type_id",
115 reason = "this is memory-unsafe to override in user code",
118 fn type_id(&self, _: private::Internal) -> TypeId
125 /// Returns a stack backtrace, if available, of where this error occurred.
127 /// This function allows inspecting the location, in code, of where an error
128 /// happened. The returned `Backtrace` contains information about the stack
129 /// trace of the OS thread of execution of where the error originated from.
131 /// Note that not all errors contain a `Backtrace`. Also note that a
132 /// `Backtrace` may actually be empty. For more information consult the
133 /// `Backtrace` type itself.
134 #[unstable(feature = "backtrace", issue = "53487")]
135 fn backtrace(&self) -> Option<&Backtrace> {
140 /// if let Err(e) = "xc".parse::<u32>() {
141 /// // Print `e` itself, no need for description().
142 /// eprintln!("Error: {}", e);
145 #[stable(feature = "rust1", since = "1.0.0")]
146 #[rustc_deprecated(since = "1.42.0", reason = "use the Display impl or to_string()")]
147 fn description(&self) -> &str {
148 "description() is deprecated; use Display"
151 #[stable(feature = "rust1", since = "1.0.0")]
154 reason = "replaced by Error::source, which can support downcasting"
156 #[allow(missing_docs)]
157 fn cause(&self) -> Option<&dyn Error> {
163 // This is a hack to prevent `type_id` from being overridden by `Error`
164 // implementations, since that can enable unsound downcasting.
165 #[unstable(feature = "error_type_id", issue = "60784")]
170 #[stable(feature = "rust1", since = "1.0.0")]
171 impl<'a, E: Error + 'a> From<E> for Box<dyn Error + 'a> {
172 /// Converts a type of [`Error`] into a box of dyn [`Error`].
177 /// use std::error::Error;
184 /// impl fmt::Display for AnError {
185 /// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
186 /// write!(f, "An error")
190 /// impl Error for AnError {}
192 /// let an_error = AnError;
193 /// assert!(0 == mem::size_of_val(&an_error));
194 /// let a_boxed_error = Box::<dyn Error>::from(an_error);
195 /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
197 fn from(err: E) -> Box<dyn Error + 'a> {
202 #[stable(feature = "rust1", since = "1.0.0")]
203 impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<dyn Error + Send + Sync + 'a> {
204 /// Converts a type of [`Error`] + [`Send`] + [`Sync`] into a box of
205 /// dyn [`Error`] + [`Send`] + [`Sync`].
210 /// use std::error::Error;
217 /// impl fmt::Display for AnError {
218 /// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
219 /// write!(f, "An error")
223 /// impl Error for AnError {}
225 /// unsafe impl Send for AnError {}
227 /// unsafe impl Sync for AnError {}
229 /// let an_error = AnError;
230 /// assert!(0 == mem::size_of_val(&an_error));
231 /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(an_error);
233 /// mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
235 fn from(err: E) -> Box<dyn Error + Send + Sync + 'a> {
240 #[stable(feature = "rust1", since = "1.0.0")]
241 impl From<String> for Box<dyn Error + Send + Sync> {
242 /// Converts a [`String`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
247 /// use std::error::Error;
250 /// let a_string_error = "a string error".to_string();
251 /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_string_error);
253 /// mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
256 fn from(err: String) -> Box<dyn Error + Send + Sync> {
257 struct StringError(String);
259 impl Error for StringError {
261 fn description(&self) -> &str {
266 impl Display for StringError {
267 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
268 Display::fmt(&self.0, f)
272 // Purposefully skip printing "StringError(..)"
273 impl Debug for StringError {
274 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
275 Debug::fmt(&self.0, f)
279 Box::new(StringError(err))
283 #[stable(feature = "string_box_error", since = "1.6.0")]
284 impl From<String> for Box<dyn Error> {
285 /// Converts a [`String`] into a box of dyn [`Error`].
290 /// use std::error::Error;
293 /// let a_string_error = "a string error".to_string();
294 /// let a_boxed_error = Box::<dyn Error>::from(a_string_error);
295 /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
297 fn from(str_err: String) -> Box<dyn Error> {
298 let err1: Box<dyn Error + Send + Sync> = From::from(str_err);
299 let err2: Box<dyn Error> = err1;
304 #[stable(feature = "rust1", since = "1.0.0")]
305 impl<'a> From<&str> for Box<dyn Error + Send + Sync + 'a> {
306 /// Converts a [`str`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
308 /// [`str`]: prim@str
313 /// use std::error::Error;
316 /// let a_str_error = "a str error";
317 /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_str_error);
319 /// mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
322 fn from(err: &str) -> Box<dyn Error + Send + Sync + 'a> {
323 From::from(String::from(err))
327 #[stable(feature = "string_box_error", since = "1.6.0")]
328 impl From<&str> for Box<dyn Error> {
329 /// Converts a [`str`] into a box of dyn [`Error`].
331 /// [`str`]: prim@str
336 /// use std::error::Error;
339 /// let a_str_error = "a str error";
340 /// let a_boxed_error = Box::<dyn Error>::from(a_str_error);
341 /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
343 fn from(err: &str) -> Box<dyn Error> {
344 From::from(String::from(err))
348 #[stable(feature = "cow_box_error", since = "1.22.0")]
349 impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + Send + Sync + 'a> {
350 /// Converts a [`Cow`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
355 /// use std::error::Error;
357 /// use std::borrow::Cow;
359 /// let a_cow_str_error = Cow::from("a str error");
360 /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_cow_str_error);
362 /// mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
364 fn from(err: Cow<'b, str>) -> Box<dyn Error + Send + Sync + 'a> {
365 From::from(String::from(err))
369 #[stable(feature = "cow_box_error", since = "1.22.0")]
370 impl<'a> From<Cow<'a, str>> for Box<dyn Error> {
371 /// Converts a [`Cow`] into a box of dyn [`Error`].
376 /// use std::error::Error;
378 /// use std::borrow::Cow;
380 /// let a_cow_str_error = Cow::from("a str error");
381 /// let a_boxed_error = Box::<dyn Error>::from(a_cow_str_error);
382 /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
384 fn from(err: Cow<'a, str>) -> Box<dyn Error> {
385 From::from(String::from(err))
389 #[unstable(feature = "never_type", issue = "35121")]
393 feature = "allocator_api",
394 reason = "the precise API and guarantees it provides may be tweaked.",
397 impl Error for AllocError {}
399 #[stable(feature = "alloc_layout", since = "1.28.0")]
400 impl Error for LayoutError {}
402 #[stable(feature = "rust1", since = "1.0.0")]
403 impl Error for str::ParseBoolError {
405 fn description(&self) -> &str {
406 "failed to parse bool"
410 #[stable(feature = "rust1", since = "1.0.0")]
411 impl Error for str::Utf8Error {
413 fn description(&self) -> &str {
414 "invalid utf-8: corrupt contents"
418 #[stable(feature = "rust1", since = "1.0.0")]
419 impl Error for num::ParseIntError {
421 fn description(&self) -> &str {
426 #[stable(feature = "try_from", since = "1.34.0")]
427 impl Error for num::TryFromIntError {
429 fn description(&self) -> &str {
434 #[stable(feature = "try_from", since = "1.34.0")]
435 impl Error for array::TryFromSliceError {
437 fn description(&self) -> &str {
442 #[stable(feature = "rust1", since = "1.0.0")]
443 impl Error for num::ParseFloatError {
445 fn description(&self) -> &str {
450 #[stable(feature = "rust1", since = "1.0.0")]
451 impl Error for string::FromUtf8Error {
453 fn description(&self) -> &str {
458 #[stable(feature = "rust1", since = "1.0.0")]
459 impl Error for string::FromUtf16Error {
461 fn description(&self) -> &str {
466 #[stable(feature = "str_parse_error2", since = "1.8.0")]
467 impl Error for Infallible {
468 fn description(&self) -> &str {
473 #[stable(feature = "decode_utf16", since = "1.9.0")]
474 impl Error for char::DecodeUtf16Error {
476 fn description(&self) -> &str {
477 "unpaired surrogate found"
481 #[stable(feature = "u8_from_char", since = "1.59.0")]
482 impl Error for char::TryFromCharError {}
484 #[unstable(feature = "map_try_insert", issue = "82766")]
485 impl<'a, K: Debug + Ord, V: Debug> Error
486 for crate::collections::btree_map::OccupiedError<'a, K, V>
489 fn description(&self) -> &str {
494 #[unstable(feature = "map_try_insert", issue = "82766")]
495 impl<'a, K: Debug, V: Debug> Error for crate::collections::hash_map::OccupiedError<'a, K, V> {
497 fn description(&self) -> &str {
502 #[stable(feature = "box_error", since = "1.8.0")]
503 impl<T: Error> Error for Box<T> {
504 #[allow(deprecated, deprecated_in_future)]
505 fn description(&self) -> &str {
506 Error::description(&**self)
510 fn cause(&self) -> Option<&dyn Error> {
511 Error::cause(&**self)
514 fn source(&self) -> Option<&(dyn Error + 'static)> {
515 Error::source(&**self)
519 #[stable(feature = "error_by_ref", since = "1.51.0")]
520 impl<'a, T: Error + ?Sized> Error for &'a T {
521 #[allow(deprecated, deprecated_in_future)]
522 fn description(&self) -> &str {
523 Error::description(&**self)
527 fn cause(&self) -> Option<&dyn Error> {
528 Error::cause(&**self)
531 fn source(&self) -> Option<&(dyn Error + 'static)> {
532 Error::source(&**self)
535 fn backtrace(&self) -> Option<&Backtrace> {
536 Error::backtrace(&**self)
540 #[stable(feature = "arc_error", since = "1.52.0")]
541 impl<T: Error + ?Sized> Error for Arc<T> {
542 #[allow(deprecated, deprecated_in_future)]
543 fn description(&self) -> &str {
544 Error::description(&**self)
548 fn cause(&self) -> Option<&dyn Error> {
549 Error::cause(&**self)
552 fn source(&self) -> Option<&(dyn Error + 'static)> {
553 Error::source(&**self)
556 fn backtrace(&self) -> Option<&Backtrace> {
557 Error::backtrace(&**self)
561 #[stable(feature = "fmt_error", since = "1.11.0")]
562 impl Error for fmt::Error {
564 fn description(&self) -> &str {
565 "an error occurred when formatting an argument"
569 #[stable(feature = "try_borrow", since = "1.13.0")]
570 impl Error for cell::BorrowError {
572 fn description(&self) -> &str {
573 "already mutably borrowed"
577 #[stable(feature = "try_borrow", since = "1.13.0")]
578 impl Error for cell::BorrowMutError {
580 fn description(&self) -> &str {
585 #[stable(feature = "try_from", since = "1.34.0")]
586 impl Error for char::CharTryFromError {
588 fn description(&self) -> &str {
589 "converted integer out of range for `char`"
593 #[stable(feature = "char_from_str", since = "1.20.0")]
594 impl Error for char::ParseCharError {
596 fn description(&self) -> &str {
601 #[stable(feature = "try_reserve", since = "1.57.0")]
602 impl Error for alloc::collections::TryReserveError {}
604 #[unstable(feature = "duration_checked_float", issue = "83400")]
605 impl Error for time::FromSecsError {}
607 // Copied from `any.rs`.
608 impl dyn Error + 'static {
609 /// Returns `true` if the inner type is the same as `T`.
610 #[stable(feature = "error_downcast", since = "1.3.0")]
612 pub fn is<T: Error + 'static>(&self) -> bool {
613 // Get `TypeId` of the type this function is instantiated with.
614 let t = TypeId::of::<T>();
616 // Get `TypeId` of the type in the trait object (`self`).
617 let concrete = self.type_id(private::Internal);
619 // Compare both `TypeId`s on equality.
623 /// Returns some reference to the inner value if it is of type `T`, or
624 /// `None` if it isn't.
625 #[stable(feature = "error_downcast", since = "1.3.0")]
627 pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
629 unsafe { Some(&*(self as *const dyn Error as *const T)) }
635 /// Returns some mutable reference to the inner value if it is of type `T`, or
636 /// `None` if it isn't.
637 #[stable(feature = "error_downcast", since = "1.3.0")]
639 pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
641 unsafe { Some(&mut *(self as *mut dyn Error as *mut T)) }
648 impl dyn Error + 'static + Send {
649 /// Forwards to the method defined on the type `dyn Error`.
650 #[stable(feature = "error_downcast", since = "1.3.0")]
652 pub fn is<T: Error + 'static>(&self) -> bool {
653 <dyn Error + 'static>::is::<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_ref<T: Error + 'static>(&self) -> Option<&T> {
660 <dyn Error + 'static>::downcast_ref::<T>(self)
663 /// Forwards to the method defined on the type `dyn Error`.
664 #[stable(feature = "error_downcast", since = "1.3.0")]
666 pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
667 <dyn Error + 'static>::downcast_mut::<T>(self)
671 impl dyn Error + 'static + Send + Sync {
672 /// Forwards to the method defined on the type `dyn Error`.
673 #[stable(feature = "error_downcast", since = "1.3.0")]
675 pub fn is<T: Error + 'static>(&self) -> bool {
676 <dyn Error + 'static>::is::<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_ref<T: Error + 'static>(&self) -> Option<&T> {
683 <dyn Error + 'static>::downcast_ref::<T>(self)
686 /// Forwards to the method defined on the type `dyn Error`.
687 #[stable(feature = "error_downcast", since = "1.3.0")]
689 pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
690 <dyn Error + 'static>::downcast_mut::<T>(self)
696 #[stable(feature = "error_downcast", since = "1.3.0")]
697 /// Attempts to downcast the box to a concrete type.
698 pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<dyn Error>> {
701 let raw: *mut dyn Error = Box::into_raw(self);
702 Ok(Box::from_raw(raw as *mut T))
709 /// Returns an iterator starting with the current error and continuing with
710 /// recursively calling [`Error::source`].
712 /// If you want to omit the current error and only use its sources,
718 /// #![feature(error_iter)]
719 /// use std::error::Error;
726 /// struct B(Option<Box<dyn Error + 'static>>);
728 /// impl fmt::Display for A {
729 /// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
734 /// impl fmt::Display for B {
735 /// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
740 /// impl Error for A {}
742 /// impl Error for B {
743 /// fn source(&self) -> Option<&(dyn Error + 'static)> {
744 /// self.0.as_ref().map(|e| e.as_ref())
748 /// let b = B(Some(Box::new(A)));
750 /// // let err : Box<Error> = b.into(); // or
751 /// let err = &b as &(dyn Error);
753 /// let mut iter = err.chain();
755 /// assert_eq!("B".to_string(), iter.next().unwrap().to_string());
756 /// assert_eq!("A".to_string(), iter.next().unwrap().to_string());
757 /// assert!(iter.next().is_none());
758 /// assert!(iter.next().is_none());
760 #[unstable(feature = "error_iter", issue = "58520")]
762 pub fn chain(&self) -> Chain<'_> {
763 Chain { current: Some(self) }
767 /// An iterator over an [`Error`] and its sources.
769 /// If you want to omit the initial error and only process
770 /// its sources, use `skip(1)`.
771 #[unstable(feature = "error_iter", issue = "58520")]
772 #[derive(Clone, Debug)]
773 pub struct Chain<'a> {
774 current: Option<&'a (dyn Error + 'static)>,
777 #[unstable(feature = "error_iter", issue = "58520")]
778 impl<'a> Iterator for Chain<'a> {
779 type Item = &'a (dyn Error + 'static);
781 fn next(&mut self) -> Option<Self::Item> {
782 let current = self.current;
783 self.current = self.current.and_then(Error::source);
788 impl dyn Error + Send {
790 #[stable(feature = "error_downcast", since = "1.3.0")]
791 /// Attempts to downcast the box to a concrete type.
792 pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<dyn Error + Send>> {
793 let err: Box<dyn Error> = self;
794 <dyn Error>::downcast(err).map_err(|s| unsafe {
795 // Reapply the `Send` marker.
796 transmute::<Box<dyn Error>, Box<dyn Error + Send>>(s)
801 impl dyn Error + Send + Sync {
803 #[stable(feature = "error_downcast", since = "1.3.0")]
804 /// Attempts to downcast the box to a concrete type.
805 pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<Self>> {
806 let err: Box<dyn Error> = self;
807 <dyn Error>::downcast(err).map_err(|s| unsafe {
808 // Reapply the `Send + Sync` marker.
809 transmute::<Box<dyn Error>, Box<dyn Error + Send + Sync>>(s)