1//! [![github]](https://github.com/dtolnay/anyhow) [![crates-io]](https://crates.io/crates/anyhow) [![docs-rs]](https://docs.rs/anyhow)
2//!
3//! [github]: https://img.shields.io/badge/github-8da0cb?style=for-the-badge&labelColor=555555&logo=github
4//! [crates-io]: https://img.shields.io/badge/crates.io-fc8d62?style=for-the-badge&labelColor=555555&logo=rust
5//! [docs-rs]: https://img.shields.io/badge/docs.rs-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs
6//!
7//! <br>
8//!
9//! This library provides [`anyhow::Error`][Error], a trait object based error
10//! type for easy idiomatic error handling in Rust applications.
11//!
12//! <br>
13//!
14//! # Details
15//!
16//! - Use `Result<T, anyhow::Error>`, or equivalently `anyhow::Result<T>`, as
17//! the return type of any fallible function.
18//!
19//! Within the function, use `?` to easily propagate any error that implements
20//! the [`std::error::Error`] trait.
21//!
22//! ```
23//! # pub trait Deserialize {}
24//! #
25//! # mod serde_json {
26//! # use super::Deserialize;
27//! # use std::io;
28//! #
29//! # pub fn from_str<T: Deserialize>(json: &str) -> io::Result<T> {
30//! # unimplemented!()
31//! # }
32//! # }
33//! #
34//! # struct ClusterMap;
35//! #
36//! # impl Deserialize for ClusterMap {}
37//! #
38//! use anyhow::Result;
39//!
40//! fn get_cluster_info() -> Result<ClusterMap> {
41//! let config = std::fs::read_to_string("cluster.json")?;
42//! let map: ClusterMap = serde_json::from_str(&config)?;
43//! Ok(map)
44//! }
45//! #
46//! # fn main() {}
47//! ```
48//!
49//! - Attach context to help the person troubleshooting the error understand
50//! where things went wrong. A low-level error like "No such file or
51//! directory" can be annoying to debug without more context about what higher
52//! level step the application was in the middle of.
53//!
54//! ```
55//! # struct It;
56//! #
57//! # impl It {
58//! # fn detach(&self) -> Result<()> {
59//! # unimplemented!()
60//! # }
61//! # }
62//! #
63//! use anyhow::{Context, Result};
64//!
65//! fn main() -> Result<()> {
66//! # return Ok(());
67//! #
68//! # const _: &str = stringify! {
69//! ...
70//! # };
71//! #
72//! # let it = It;
73//! # let path = "./path/to/instrs.json";
74//! #
75//! it.detach().context("Failed to detach the important thing")?;
76//!
77//! let content = std::fs::read(path)
78//! .with_context(|| format!("Failed to read instrs from {}", path))?;
79//! #
80//! # const _: &str = stringify! {
81//! ...
82//! # };
83//! #
84//! # Ok(())
85//! }
86//! ```
87//!
88//! ```console
89//! Error: Failed to read instrs from ./path/to/instrs.json
90//!
91//! Caused by:
92//! No such file or directory (os error 2)
93//! ```
94//!
95//! - Downcasting is supported and can be by value, by shared reference, or by
96//! mutable reference as needed.
97//!
98//! ```
99//! # use anyhow::anyhow;
100//! # use std::fmt::{self, Display};
101//! # use std::task::Poll;
102//! #
103//! # #[derive(Debug)]
104//! # enum DataStoreError {
105//! # Censored(()),
106//! # }
107//! #
108//! # impl Display for DataStoreError {
109//! # fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
110//! # unimplemented!()
111//! # }
112//! # }
113//! #
114//! # impl std::error::Error for DataStoreError {}
115//! #
116//! # const REDACTED_CONTENT: () = ();
117//! #
118//! # let error = anyhow!("...");
119//! # let root_cause = &error;
120//! #
121//! # let ret =
122//! // If the error was caused by redaction, then return a
123//! // tombstone instead of the content.
124//! match root_cause.downcast_ref::<DataStoreError>() {
125//! Some(DataStoreError::Censored(_)) => Ok(Poll::Ready(REDACTED_CONTENT)),
126//! None => Err(error),
127//! }
128//! # ;
129//! ```
130//!
131//! - If using Rust &ge; 1.65, a backtrace is captured and printed with the
132//! error if the underlying error type does not already provide its own. In
133//! order to see backtraces, they must be enabled through the environment
134//! variables described in [`std::backtrace`]:
135//!
136//! - If you want panics and errors to both have backtraces, set
137//! `RUST_BACKTRACE=1`;
138//! - If you want only errors to have backtraces, set `RUST_LIB_BACKTRACE=1`;
139//! - If you want only panics to have backtraces, set `RUST_BACKTRACE=1` and
140//! `RUST_LIB_BACKTRACE=0`.
141//!
142//! [`std::backtrace`]: std::backtrace#environment-variables
143//!
144//! - Anyhow works with any error type that has an impl of `std::error::Error`,
145//! including ones defined in your crate. We do not bundle a `derive(Error)`
146//! macro but you can write the impls yourself or use a standalone macro like
147//! [thiserror].
148//!
149//! [thiserror]: https://github.com/dtolnay/thiserror
150//!
151//! ```
152//! use thiserror::Error;
153//!
154//! #[derive(Error, Debug)]
155//! pub enum FormatError {
156//! #[error("Invalid header (expected {expected:?}, got {found:?})")]
157//! InvalidHeader {
158//! expected: String,
159//! found: String,
160//! },
161//! #[error("Missing attribute: {0}")]
162//! MissingAttribute(String),
163//! }
164//! ```
165//!
166//! - One-off error messages can be constructed using the `anyhow!` macro, which
167//! supports string interpolation and produces an `anyhow::Error`.
168//!
169//! ```
170//! # use anyhow::{anyhow, Result};
171//! #
172//! # fn demo() -> Result<()> {
173//! # let missing = "...";
174//! return Err(anyhow!("Missing attribute: {}", missing));
175//! # Ok(())
176//! # }
177//! ```
178//!
179//! A `bail!` macro is provided as a shorthand for the same early return.
180//!
181//! ```
182//! # use anyhow::{bail, Result};
183//! #
184//! # fn demo() -> Result<()> {
185//! # let missing = "...";
186//! bail!("Missing attribute: {}", missing);
187//! # Ok(())
188//! # }
189//! ```
190//!
191//! <br>
192//!
193//! # No-std support
194//!
195//! In no_std mode, almost all of the same API is available and works the same
196//! way. To depend on Anyhow in no_std mode, disable our default enabled "std"
197//! feature in Cargo.toml. A global allocator is required.
198//!
199//! ```toml
200//! [dependencies]
201//! anyhow = { version = "1.0", default-features = false }
202//! ```
203//!
204//! With versions of Rust older than 1.81, no_std mode may require an additional
205//! `.map_err(Error::msg)` when working with a non-Anyhow error type inside a
206//! function that returns Anyhow's error type, as the trait that `?`-based error
207//! conversions are defined by is only available in std in those old versions.
208
209#![doc(html_root_url = "https://docs.rs/anyhow/1.0.98")]
210#![cfg_attr(error_generic_member_access, feature(error_generic_member_access))]
211#![no_std]
212#![deny(dead_code, unused_imports, unused_mut)]
213#![cfg_attr(
214 not(anyhow_no_unsafe_op_in_unsafe_fn_lint),
215 deny(unsafe_op_in_unsafe_fn)
216)]
217#![cfg_attr(anyhow_no_unsafe_op_in_unsafe_fn_lint, allow(unused_unsafe))]
218#![allow(
219 clippy::doc_markdown,
220 clippy::elidable_lifetime_names,
221 clippy::enum_glob_use,
222 clippy::explicit_auto_deref,
223 clippy::extra_unused_type_parameters,
224 clippy::incompatible_msrv,
225 clippy::let_underscore_untyped,
226 clippy::missing_errors_doc,
227 clippy::missing_panics_doc,
228 clippy::module_name_repetitions,
229 clippy::must_use_candidate,
230 clippy::needless_doctest_main,
231 clippy::needless_lifetimes,
232 clippy::new_ret_no_self,
233 clippy::redundant_else,
234 clippy::return_self_not_must_use,
235 clippy::struct_field_names,
236 clippy::unused_self,
237 clippy::used_underscore_binding,
238 clippy::wildcard_imports,
239 clippy::wrong_self_convention
240)]
241
242#[cfg(all(
243 anyhow_nightly_testing,
244 feature = "std",
245 not(error_generic_member_access)
246))]
247compile_error!("Build script probe failed to compile.");
248
249extern crate alloc;
250
251#[cfg(feature = "std")]
252extern crate std;
253
254#[macro_use]
255mod backtrace;
256mod chain;
257mod context;
258mod ensure;
259mod error;
260mod fmt;
261mod kind;
262mod macros;
263#[cfg(error_generic_member_access)]
264mod nightly;
265mod ptr;
266mod wrapper;
267
268use crate::error::ErrorImpl;
269use crate::ptr::Own;
270use core::fmt::Display;
271
272#[cfg(all(not(feature = "std"), anyhow_no_core_error))]
273use core::fmt::Debug;
274
275#[cfg(feature = "std")]
276use std::error::Error as StdError;
277
278#[cfg(not(any(feature = "std", anyhow_no_core_error)))]
279use core::error::Error as StdError;
280
281#[cfg(all(not(feature = "std"), anyhow_no_core_error))]
282trait StdError: Debug + Display {
283 fn source(&self) -> Option<&(dyn StdError + 'static)> {
284 None
285 }
286}
287
288#[doc(no_inline)]
289pub use anyhow as format_err;
290
291/// The `Error` type, a wrapper around a dynamic error type.
292///
293/// `Error` works a lot like `Box<dyn std::error::Error>`, but with these
294/// differences:
295///
296/// - `Error` requires that the error is `Send`, `Sync`, and `'static`.
297/// - `Error` guarantees that a backtrace is available, even if the underlying
298/// error type does not provide one.
299/// - `Error` is represented as a narrow pointer &mdash; exactly one word in
300/// size instead of two.
301///
302/// <br>
303///
304/// # Display representations
305///
306/// When you print an error object using "{}" or to_string(), only the outermost
307/// underlying error or context is printed, not any of the lower level causes.
308/// This is exactly as if you had called the Display impl of the error from
309/// which you constructed your anyhow::Error.
310///
311/// ```console
312/// Failed to read instrs from ./path/to/instrs.json
313/// ```
314///
315/// To print causes as well using anyhow's default formatting of causes, use the
316/// alternate selector "{:#}".
317///
318/// ```console
319/// Failed to read instrs from ./path/to/instrs.json: No such file or directory (os error 2)
320/// ```
321///
322/// The Debug format "{:?}" includes your backtrace if one was captured. Note
323/// that this is the representation you get by default if you return an error
324/// from `fn main` instead of printing it explicitly yourself.
325///
326/// ```console
327/// Error: Failed to read instrs from ./path/to/instrs.json
328///
329/// Caused by:
330/// No such file or directory (os error 2)
331/// ```
332///
333/// and if there is a backtrace available:
334///
335/// ```console
336/// Error: Failed to read instrs from ./path/to/instrs.json
337///
338/// Caused by:
339/// No such file or directory (os error 2)
340///
341/// Stack backtrace:
342/// 0: <E as anyhow::context::ext::StdError>::ext_context
343/// at /git/anyhow/src/backtrace.rs:26
344/// 1: core::result::Result<T,E>::map_err
345/// at /git/rustc/src/libcore/result.rs:596
346/// 2: anyhow::context::<impl anyhow::Context<T,E> for core::result::Result<T,E>>::with_context
347/// at /git/anyhow/src/context.rs:58
348/// 3: testing::main
349/// at src/main.rs:5
350/// 4: std::rt::lang_start
351/// at /git/rustc/src/libstd/rt.rs:61
352/// 5: main
353/// 6: __libc_start_main
354/// 7: _start
355/// ```
356///
357/// To see a conventional struct-style Debug representation, use "{:#?}".
358///
359/// ```console
360/// Error {
361/// context: "Failed to read instrs from ./path/to/instrs.json",
362/// source: Os {
363/// code: 2,
364/// kind: NotFound,
365/// message: "No such file or directory",
366/// },
367/// }
368/// ```
369///
370/// If none of the built-in representations are appropriate and you would prefer
371/// to render the error and its cause chain yourself, it can be done something
372/// like this:
373///
374/// ```
375/// use anyhow::{Context, Result};
376///
377/// fn main() {
378/// if let Err(err) = try_main() {
379/// eprintln!("ERROR: {}", err);
380/// err.chain().skip(1).for_each(|cause| eprintln!("because: {}", cause));
381/// std::process::exit(1);
382/// }
383/// }
384///
385/// fn try_main() -> Result<()> {
386/// # const IGNORE: &str = stringify! {
387/// ...
388/// # };
389/// # Ok(())
390/// }
391/// ```
392#[repr(transparent)]
393pub struct Error {
394 inner: Own<ErrorImpl>,
395}
396
397/// Iterator of a chain of source errors.
398///
399/// This type is the iterator returned by [`Error::chain`].
400///
401/// # Example
402///
403/// ```
404/// use anyhow::Error;
405/// use std::io;
406///
407/// pub fn underlying_io_error_kind(error: &Error) -> Option<io::ErrorKind> {
408/// for cause in error.chain() {
409/// if let Some(io_error) = cause.downcast_ref::<io::Error>() {
410/// return Some(io_error.kind());
411/// }
412/// }
413/// None
414/// }
415/// ```
416#[cfg(any(feature = "std", not(anyhow_no_core_error)))]
417#[derive(Clone)]
418pub struct Chain<'a> {
419 state: crate::chain::ChainState<'a>,
420}
421
422/// `Result<T, Error>`
423///
424/// This is a reasonable return type to use throughout your application but also
425/// for `fn main`; if you do, failures will be printed along with any
426/// [context][Context] and a backtrace if one was captured.
427///
428/// `anyhow::Result` may be used with one *or* two type parameters.
429///
430/// ```rust
431/// use anyhow::Result;
432///
433/// # const IGNORE: &str = stringify! {
434/// fn demo1() -> Result<T> {...}
435/// // ^ equivalent to std::result::Result<T, anyhow::Error>
436///
437/// fn demo2() -> Result<T, OtherError> {...}
438/// // ^ equivalent to std::result::Result<T, OtherError>
439/// # };
440/// ```
441///
442/// # Example
443///
444/// ```
445/// # pub trait Deserialize {}
446/// #
447/// # mod serde_json {
448/// # use super::Deserialize;
449/// # use std::io;
450/// #
451/// # pub fn from_str<T: Deserialize>(json: &str) -> io::Result<T> {
452/// # unimplemented!()
453/// # }
454/// # }
455/// #
456/// # #[derive(Debug)]
457/// # struct ClusterMap;
458/// #
459/// # impl Deserialize for ClusterMap {}
460/// #
461/// use anyhow::Result;
462///
463/// fn main() -> Result<()> {
464/// # return Ok(());
465/// let config = std::fs::read_to_string("cluster.json")?;
466/// let map: ClusterMap = serde_json::from_str(&config)?;
467/// println!("cluster info: {:#?}", map);
468/// Ok(())
469/// }
470/// ```
471pub type Result<T, E = Error> = core::result::Result<T, E>;
472
473/// Provides the `context` method for `Result`.
474///
475/// This trait is sealed and cannot be implemented for types outside of
476/// `anyhow`.
477///
478/// <br>
479///
480/// # Example
481///
482/// ```
483/// use anyhow::{Context, Result};
484/// use std::fs;
485/// use std::path::PathBuf;
486///
487/// pub struct ImportantThing {
488/// path: PathBuf,
489/// }
490///
491/// impl ImportantThing {
492/// # const IGNORE: &'static str = stringify! {
493/// pub fn detach(&mut self) -> Result<()> {...}
494/// # };
495/// # fn detach(&mut self) -> Result<()> {
496/// # unimplemented!()
497/// # }
498/// }
499///
500/// pub fn do_it(mut it: ImportantThing) -> Result<Vec<u8>> {
501/// it.detach().context("Failed to detach the important thing")?;
502///
503/// let path = &it.path;
504/// let content = fs::read(path)
505/// .with_context(|| format!("Failed to read instrs from {}", path.display()))?;
506///
507/// Ok(content)
508/// }
509/// ```
510///
511/// When printed, the outermost context would be printed first and the lower
512/// level underlying causes would be enumerated below.
513///
514/// ```console
515/// Error: Failed to read instrs from ./path/to/instrs.json
516///
517/// Caused by:
518/// No such file or directory (os error 2)
519/// ```
520///
521/// Refer to the [Display representations] documentation for other forms in
522/// which this context chain can be rendered.
523///
524/// [Display representations]: Error#display-representations
525///
526/// <br>
527///
528/// # Effect on downcasting
529///
530/// After attaching context of type `C` onto an error of type `E`, the resulting
531/// `anyhow::Error` may be downcast to `C` **or** to `E`.
532///
533/// That is, in codebases that rely on downcasting, Anyhow's context supports
534/// both of the following use cases:
535///
536/// - **Attaching context whose type is insignificant onto errors whose type
537/// is used in downcasts.**
538///
539/// In other error libraries whose context is not designed this way, it can
540/// be risky to introduce context to existing code because new context might
541/// break existing working downcasts. In Anyhow, any downcast that worked
542/// before adding context will continue to work after you add a context, so
543/// you should freely add human-readable context to errors wherever it would
544/// be helpful.
545///
546/// ```
547/// # use anyhow::bail;
548/// # use thiserror::Error;
549/// #
550/// # #[derive(Error, Debug)]
551/// # #[error("???")]
552/// # struct SuspiciousError;
553/// #
554/// # fn helper() -> Result<()> {
555/// # bail!(SuspiciousError);
556/// # }
557/// #
558/// use anyhow::{Context, Result};
559///
560/// fn do_it() -> Result<()> {
561/// helper().context("Failed to complete the work")?;
562/// # const IGNORE: &str = stringify! {
563/// ...
564/// # };
565/// # unreachable!()
566/// }
567///
568/// fn main() {
569/// let err = do_it().unwrap_err();
570/// if let Some(e) = err.downcast_ref::<SuspiciousError>() {
571/// // If helper() returned SuspiciousError, this downcast will
572/// // correctly succeed even with the context in between.
573/// # return;
574/// }
575/// # panic!("expected downcast to succeed");
576/// }
577/// ```
578///
579/// - **Attaching context whose type is used in downcasts onto errors whose
580/// type is insignificant.**
581///
582/// Some codebases prefer to use machine-readable context to categorize
583/// lower level errors in a way that will be actionable to higher levels of
584/// the application.
585///
586/// ```
587/// # use anyhow::bail;
588/// # use thiserror::Error;
589/// #
590/// # #[derive(Error, Debug)]
591/// # #[error("???")]
592/// # struct HelperFailed;
593/// #
594/// # fn helper() -> Result<()> {
595/// # bail!("no such file or directory");
596/// # }
597/// #
598/// use anyhow::{Context, Result};
599///
600/// fn do_it() -> Result<()> {
601/// helper().context(HelperFailed)?;
602/// # const IGNORE: &str = stringify! {
603/// ...
604/// # };
605/// # unreachable!()
606/// }
607///
608/// fn main() {
609/// let err = do_it().unwrap_err();
610/// if let Some(e) = err.downcast_ref::<HelperFailed>() {
611/// // If helper failed, this downcast will succeed because
612/// // HelperFailed is the context that has been attached to
613/// // that error.
614/// # return;
615/// }
616/// # panic!("expected downcast to succeed");
617/// }
618/// ```
619pub trait Context<T, E>: context::private::Sealed {
620 /// Wrap the error value with additional context.
621 fn context<C>(self, context: C) -> Result<T, Error>
622 where
623 C: Display + Send + Sync + 'static;
624
625 /// Wrap the error value with additional context that is evaluated lazily
626 /// only once an error does occur.
627 fn with_context<C, F>(self, f: F) -> Result<T, Error>
628 where
629 C: Display + Send + Sync + 'static,
630 F: FnOnce() -> C;
631}
632
633/// Equivalent to `Ok::<_, anyhow::Error>(value)`.
634///
635/// This simplifies creation of an `anyhow::Result` in places where type
636/// inference cannot deduce the `E` type of the result &mdash; without needing
637/// to write`Ok::<_, anyhow::Error>(value)`.
638///
639/// One might think that `anyhow::Result::Ok(value)` would work in such cases
640/// but it does not.
641///
642/// ```console
643/// error[E0282]: type annotations needed for `std::result::Result<i32, E>`
644/// --> src/main.rs:11:13
645/// |
646/// 11 | let _ = anyhow::Result::Ok(1);
647/// | - ^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `E` declared on the enum `Result`
648/// | |
649/// | consider giving this pattern the explicit type `std::result::Result<i32, E>`, where the type parameter `E` is specified
650/// ```
651#[allow(non_snake_case)]
652pub fn Ok<T>(value: T) -> Result<T> {
653 Result::Ok(value)
654}
655
656// Not public API. Referenced by macro-generated code.
657#[doc(hidden)]
658pub mod __private {
659 use self::not::Bool;
660 use crate::Error;
661 use alloc::fmt;
662 use core::fmt::Arguments;
663
664 #[doc(hidden)]
665 pub use crate::ensure::{BothDebug, NotBothDebug};
666 #[doc(hidden)]
667 pub use alloc::format;
668 #[doc(hidden)]
669 pub use core::result::Result::Err;
670 #[doc(hidden)]
671 pub use core::{concat, format_args, stringify};
672
673 #[doc(hidden)]
674 pub mod kind {
675 #[doc(hidden)]
676 pub use crate::kind::{AdhocKind, TraitKind};
677
678 #[cfg(any(feature = "std", not(anyhow_no_core_error)))]
679 #[doc(hidden)]
680 pub use crate::kind::BoxedKind;
681 }
682
683 #[doc(hidden)]
684 #[inline]
685 #[cold]
686 pub fn format_err(args: Arguments) -> Error {
687 #[cfg(anyhow_no_fmt_arguments_as_str)]
688 let fmt_arguments_as_str = None::<&str>;
689 #[cfg(not(anyhow_no_fmt_arguments_as_str))]
690 let fmt_arguments_as_str = args.as_str();
691
692 if let Some(message) = fmt_arguments_as_str {
693 // anyhow!("literal"), can downcast to &'static str
694 Error::msg(message)
695 } else {
696 // anyhow!("interpolate {var}"), can downcast to String
697 Error::msg(fmt::format(args))
698 }
699 }
700
701 #[doc(hidden)]
702 #[inline]
703 #[cold]
704 #[must_use]
705 pub fn must_use(error: Error) -> Error {
706 error
707 }
708
709 #[doc(hidden)]
710 #[inline]
711 pub fn not(cond: impl Bool) -> bool {
712 cond.not()
713 }
714
715 mod not {
716 #[doc(hidden)]
717 pub trait Bool {
718 fn not(self) -> bool;
719 }
720
721 impl Bool for bool {
722 #[inline]
723 fn not(self) -> bool {
724 !self
725 }
726 }
727
728 impl Bool for &bool {
729 #[inline]
730 fn not(self) -> bool {
731 !*self
732 }
733 }
734 }
735}
736