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`]: https://doc.rust-lang.org/std/backtrace/index.html#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/1.0.95")]
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::enum_glob_use,
221 clippy::explicit_auto_deref,
222 clippy::extra_unused_type_parameters,
223 clippy::incompatible_msrv,
224 clippy::let_underscore_untyped,
225 clippy::missing_errors_doc,
226 clippy::missing_panics_doc,
227 clippy::module_name_repetitions,
228 clippy::must_use_candidate,
229 clippy::needless_doctest_main,
230 clippy::needless_lifetimes,
231 clippy::new_ret_no_self,
232 clippy::redundant_else,
233 clippy::return_self_not_must_use,
234 clippy::struct_field_names,
235 clippy::unused_self,
236 clippy::used_underscore_binding,
237 clippy::wildcard_imports,
238 clippy::wrong_self_convention
239)]
240
241#[cfg(all(
242 anyhow_nightly_testing,
243 feature = "std",
244 not(error_generic_member_access)
245))]
246compile_error!("Build script probe failed to compile.");
247
248extern crate alloc;
249
250#[cfg(feature = "std")]
251extern crate std;
252
253#[macro_use]
254mod backtrace;
255mod chain;
256mod context;
257mod ensure;
258mod error;
259mod fmt;
260mod kind;
261mod macros;
262mod ptr;
263mod wrapper;
264
265use crate::error::ErrorImpl;
266use crate::ptr::Own;
267use core::fmt::Display;
268
269#[cfg(all(not(feature = "std"), anyhow_no_core_error))]
270use core::fmt::Debug;
271
272#[cfg(feature = "std")]
273use std::error::Error as StdError;
274
275#[cfg(not(any(feature = "std", anyhow_no_core_error)))]
276use core::error::Error as StdError;
277
278#[cfg(all(not(feature = "std"), anyhow_no_core_error))]
279trait StdError: Debug + Display {
280 fn source(&self) -> Option<&(dyn StdError + 'static)> {
281 None
282 }
283}
284
285#[doc(no_inline)]
286pub use anyhow as format_err;
287
288/// The `Error` type, a wrapper around a dynamic error type.
289///
290/// `Error` works a lot like `Box<dyn std::error::Error>`, but with these
291/// differences:
292///
293/// - `Error` requires that the error is `Send`, `Sync`, and `'static`.
294/// - `Error` guarantees that a backtrace is available, even if the underlying
295/// error type does not provide one.
296/// - `Error` is represented as a narrow pointer &mdash; exactly one word in
297/// size instead of two.
298///
299/// <br>
300///
301/// # Display representations
302///
303/// When you print an error object using "{}" or to_string(), only the outermost
304/// underlying error or context is printed, not any of the lower level causes.
305/// This is exactly as if you had called the Display impl of the error from
306/// which you constructed your anyhow::Error.
307///
308/// ```console
309/// Failed to read instrs from ./path/to/instrs.json
310/// ```
311///
312/// To print causes as well using anyhow's default formatting of causes, use the
313/// alternate selector "{:#}".
314///
315/// ```console
316/// Failed to read instrs from ./path/to/instrs.json: No such file or directory (os error 2)
317/// ```
318///
319/// The Debug format "{:?}" includes your backtrace if one was captured. Note
320/// that this is the representation you get by default if you return an error
321/// from `fn main` instead of printing it explicitly yourself.
322///
323/// ```console
324/// Error: Failed to read instrs from ./path/to/instrs.json
325///
326/// Caused by:
327/// No such file or directory (os error 2)
328/// ```
329///
330/// and if there is a backtrace available:
331///
332/// ```console
333/// Error: Failed to read instrs from ./path/to/instrs.json
334///
335/// Caused by:
336/// No such file or directory (os error 2)
337///
338/// Stack backtrace:
339/// 0: <E as anyhow::context::ext::StdError>::ext_context
340/// at /git/anyhow/src/backtrace.rs:26
341/// 1: core::result::Result<T,E>::map_err
342/// at /git/rustc/src/libcore/result.rs:596
343/// 2: anyhow::context::<impl anyhow::Context<T,E> for core::result::Result<T,E>>::with_context
344/// at /git/anyhow/src/context.rs:58
345/// 3: testing::main
346/// at src/main.rs:5
347/// 4: std::rt::lang_start
348/// at /git/rustc/src/libstd/rt.rs:61
349/// 5: main
350/// 6: __libc_start_main
351/// 7: _start
352/// ```
353///
354/// To see a conventional struct-style Debug representation, use "{:#?}".
355///
356/// ```console
357/// Error {
358/// context: "Failed to read instrs from ./path/to/instrs.json",
359/// source: Os {
360/// code: 2,
361/// kind: NotFound,
362/// message: "No such file or directory",
363/// },
364/// }
365/// ```
366///
367/// If none of the built-in representations are appropriate and you would prefer
368/// to render the error and its cause chain yourself, it can be done something
369/// like this:
370///
371/// ```
372/// use anyhow::{Context, Result};
373///
374/// fn main() {
375/// if let Err(err) = try_main() {
376/// eprintln!("ERROR: {}", err);
377/// err.chain().skip(1).for_each(|cause| eprintln!("because: {}", cause));
378/// std::process::exit(1);
379/// }
380/// }
381///
382/// fn try_main() -> Result<()> {
383/// # const IGNORE: &str = stringify! {
384/// ...
385/// # };
386/// # Ok(())
387/// }
388/// ```
389#[repr(transparent)]
390pub struct Error {
391 inner: Own<ErrorImpl>,
392}
393
394/// Iterator of a chain of source errors.
395///
396/// This type is the iterator returned by [`Error::chain`].
397///
398/// # Example
399///
400/// ```
401/// use anyhow::Error;
402/// use std::io;
403///
404/// pub fn underlying_io_error_kind(error: &Error) -> Option<io::ErrorKind> {
405/// for cause in error.chain() {
406/// if let Some(io_error) = cause.downcast_ref::<io::Error>() {
407/// return Some(io_error.kind());
408/// }
409/// }
410/// None
411/// }
412/// ```
413#[cfg(any(feature = "std", not(anyhow_no_core_error)))]
414#[derive(Clone)]
415pub struct Chain<'a> {
416 state: crate::chain::ChainState<'a>,
417}
418
419/// `Result<T, Error>`
420///
421/// This is a reasonable return type to use throughout your application but also
422/// for `fn main`; if you do, failures will be printed along with any
423/// [context][Context] and a backtrace if one was captured.
424///
425/// `anyhow::Result` may be used with one *or* two type parameters.
426///
427/// ```rust
428/// use anyhow::Result;
429///
430/// # const IGNORE: &str = stringify! {
431/// fn demo1() -> Result<T> {...}
432/// // ^ equivalent to std::result::Result<T, anyhow::Error>
433///
434/// fn demo2() -> Result<T, OtherError> {...}
435/// // ^ equivalent to std::result::Result<T, OtherError>
436/// # };
437/// ```
438///
439/// # Example
440///
441/// ```
442/// # pub trait Deserialize {}
443/// #
444/// # mod serde_json {
445/// # use super::Deserialize;
446/// # use std::io;
447/// #
448/// # pub fn from_str<T: Deserialize>(json: &str) -> io::Result<T> {
449/// # unimplemented!()
450/// # }
451/// # }
452/// #
453/// # #[derive(Debug)]
454/// # struct ClusterMap;
455/// #
456/// # impl Deserialize for ClusterMap {}
457/// #
458/// use anyhow::Result;
459///
460/// fn main() -> Result<()> {
461/// # return Ok(());
462/// let config = std::fs::read_to_string("cluster.json")?;
463/// let map: ClusterMap = serde_json::from_str(&config)?;
464/// println!("cluster info: {:#?}", map);
465/// Ok(())
466/// }
467/// ```
468pub type Result<T, E = Error> = core::result::Result<T, E>;
469
470/// Provides the `context` method for `Result`.
471///
472/// This trait is sealed and cannot be implemented for types outside of
473/// `anyhow`.
474///
475/// <br>
476///
477/// # Example
478///
479/// ```
480/// use anyhow::{Context, Result};
481/// use std::fs;
482/// use std::path::PathBuf;
483///
484/// pub struct ImportantThing {
485/// path: PathBuf,
486/// }
487///
488/// impl ImportantThing {
489/// # const IGNORE: &'static str = stringify! {
490/// pub fn detach(&mut self) -> Result<()> {...}
491/// # };
492/// # fn detach(&mut self) -> Result<()> {
493/// # unimplemented!()
494/// # }
495/// }
496///
497/// pub fn do_it(mut it: ImportantThing) -> Result<Vec<u8>> {
498/// it.detach().context("Failed to detach the important thing")?;
499///
500/// let path = &it.path;
501/// let content = fs::read(path)
502/// .with_context(|| format!("Failed to read instrs from {}", path.display()))?;
503///
504/// Ok(content)
505/// }
506/// ```
507///
508/// When printed, the outermost context would be printed first and the lower
509/// level underlying causes would be enumerated below.
510///
511/// ```console
512/// Error: Failed to read instrs from ./path/to/instrs.json
513///
514/// Caused by:
515/// No such file or directory (os error 2)
516/// ```
517///
518/// Refer to the [Display representations] documentation for other forms in
519/// which this context chain can be rendered.
520///
521/// [Display representations]: Error#display-representations
522///
523/// <br>
524///
525/// # Effect on downcasting
526///
527/// After attaching context of type `C` onto an error of type `E`, the resulting
528/// `anyhow::Error` may be downcast to `C` **or** to `E`.
529///
530/// That is, in codebases that rely on downcasting, Anyhow's context supports
531/// both of the following use cases:
532///
533/// - **Attaching context whose type is insignificant onto errors whose type
534/// is used in downcasts.**
535///
536/// In other error libraries whose context is not designed this way, it can
537/// be risky to introduce context to existing code because new context might
538/// break existing working downcasts. In Anyhow, any downcast that worked
539/// before adding context will continue to work after you add a context, so
540/// you should freely add human-readable context to errors wherever it would
541/// be helpful.
542///
543/// ```
544/// # use anyhow::bail;
545/// # use thiserror::Error;
546/// #
547/// # #[derive(Error, Debug)]
548/// # #[error("???")]
549/// # struct SuspiciousError;
550/// #
551/// # fn helper() -> Result<()> {
552/// # bail!(SuspiciousError);
553/// # }
554/// #
555/// use anyhow::{Context, Result};
556///
557/// fn do_it() -> Result<()> {
558/// helper().context("Failed to complete the work")?;
559/// # const IGNORE: &str = stringify! {
560/// ...
561/// # };
562/// # unreachable!()
563/// }
564///
565/// fn main() {
566/// let err = do_it().unwrap_err();
567/// if let Some(e) = err.downcast_ref::<SuspiciousError>() {
568/// // If helper() returned SuspiciousError, this downcast will
569/// // correctly succeed even with the context in between.
570/// # return;
571/// }
572/// # panic!("expected downcast to succeed");
573/// }
574/// ```
575///
576/// - **Attaching context whose type is used in downcasts onto errors whose
577/// type is insignificant.**
578///
579/// Some codebases prefer to use machine-readable context to categorize
580/// lower level errors in a way that will be actionable to higher levels of
581/// the application.
582///
583/// ```
584/// # use anyhow::bail;
585/// # use thiserror::Error;
586/// #
587/// # #[derive(Error, Debug)]
588/// # #[error("???")]
589/// # struct HelperFailed;
590/// #
591/// # fn helper() -> Result<()> {
592/// # bail!("no such file or directory");
593/// # }
594/// #
595/// use anyhow::{Context, Result};
596///
597/// fn do_it() -> Result<()> {
598/// helper().context(HelperFailed)?;
599/// # const IGNORE: &str = stringify! {
600/// ...
601/// # };
602/// # unreachable!()
603/// }
604///
605/// fn main() {
606/// let err = do_it().unwrap_err();
607/// if let Some(e) = err.downcast_ref::<HelperFailed>() {
608/// // If helper failed, this downcast will succeed because
609/// // HelperFailed is the context that has been attached to
610/// // that error.
611/// # return;
612/// }
613/// # panic!("expected downcast to succeed");
614/// }
615/// ```
616pub trait Context<T, E>: context::private::Sealed {
617 /// Wrap the error value with additional context.
618 fn context<C>(self, context: C) -> Result<T, Error>
619 where
620 C: Display + Send + Sync + 'static;
621
622 /// Wrap the error value with additional context that is evaluated lazily
623 /// only once an error does occur.
624 fn with_context<C, F>(self, f: F) -> Result<T, Error>
625 where
626 C: Display + Send + Sync + 'static,
627 F: FnOnce() -> C;
628}
629
630/// Equivalent to `Ok::<_, anyhow::Error>(value)`.
631///
632/// This simplifies creation of an `anyhow::Result` in places where type
633/// inference cannot deduce the `E` type of the result &mdash; without needing
634/// to write`Ok::<_, anyhow::Error>(value)`.
635///
636/// One might think that `anyhow::Result::Ok(value)` would work in such cases
637/// but it does not.
638///
639/// ```console
640/// error[E0282]: type annotations needed for `std::result::Result<i32, E>`
641/// --> src/main.rs:11:13
642/// |
643/// 11 | let _ = anyhow::Result::Ok(1);
644/// | - ^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `E` declared on the enum `Result`
645/// | |
646/// | consider giving this pattern the explicit type `std::result::Result<i32, E>`, where the type parameter `E` is specified
647/// ```
648#[allow(non_snake_case)]
649pub fn Ok<T>(value: T) -> Result<T> {
650 Result::Ok(value)
651}
652
653// Not public API. Referenced by macro-generated code.
654#[doc(hidden)]
655pub mod __private {
656 use self::not::Bool;
657 use crate::Error;
658 use alloc::fmt;
659 use core::fmt::Arguments;
660
661 #[doc(hidden)]
662 pub use crate::ensure::{BothDebug, NotBothDebug};
663 #[doc(hidden)]
664 pub use alloc::format;
665 #[doc(hidden)]
666 pub use core::result::Result::Err;
667 #[doc(hidden)]
668 pub use core::{concat, format_args, stringify};
669
670 #[doc(hidden)]
671 pub mod kind {
672 #[doc(hidden)]
673 pub use crate::kind::{AdhocKind, TraitKind};
674
675 #[cfg(any(feature = "std", not(anyhow_no_core_error)))]
676 #[doc(hidden)]
677 pub use crate::kind::BoxedKind;
678 }
679
680 #[doc(hidden)]
681 #[inline]
682 #[cold]
683 pub fn format_err(args: Arguments) -> Error {
684 #[cfg(anyhow_no_fmt_arguments_as_str)]
685 let fmt_arguments_as_str = None::<&str>;
686 #[cfg(not(anyhow_no_fmt_arguments_as_str))]
687 let fmt_arguments_as_str = args.as_str();
688
689 if let Some(message) = fmt_arguments_as_str {
690 // anyhow!("literal"), can downcast to &'static str
691 Error::msg(message)
692 } else {
693 // anyhow!("interpolate {var}"), can downcast to String
694 Error::msg(fmt::format(args))
695 }
696 }
697
698 #[doc(hidden)]
699 #[inline]
700 #[cold]
701 #[must_use]
702 pub fn must_use(error: Error) -> Error {
703 error
704 }
705
706 #[doc(hidden)]
707 #[inline]
708 pub fn not(cond: impl Bool) -> bool {
709 cond.not()
710 }
711
712 mod not {
713 #[doc(hidden)]
714 pub trait Bool {
715 fn not(self) -> bool;
716 }
717
718 impl Bool for bool {
719 #[inline]
720 fn not(self) -> bool {
721 !self
722 }
723 }
724
725 impl Bool for &bool {
726 #[inline]
727 fn not(self) -> bool {
728 !*self
729 }
730 }
731 }
732}
733