anyhow/lib.rs
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 ≥ 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, the same API is almost all available and works the same way.
196//! 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//! Since the `?`-based error conversions would normally rely on the
205//! `std::error::Error` trait which is only available through std, no_std mode
206//! will require an explicit `.map_err(Error::msg)` when working with a
207//! non-Anyhow error type inside a function that returns Anyhow's error type.
208
209#![doc(html_root_url = "https://docs.rs/anyhow/1.0.82")]
210#![cfg_attr(error_generic_member_access, feature(error_generic_member_access))]
211#![cfg_attr(doc_cfg, feature(doc_cfg))]
212#![no_std]
213#![deny(dead_code, unused_imports, unused_mut)]
214#![cfg_attr(
215 not(anyhow_no_unsafe_op_in_unsafe_fn_lint),
216 deny(unsafe_op_in_unsafe_fn)
217)]
218#![cfg_attr(anyhow_no_unsafe_op_in_unsafe_fn_lint, allow(unused_unsafe))]
219#![allow(
220 clippy::doc_markdown,
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::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(not(feature = "std"))]
270use core::fmt::Debug;
271
272#[cfg(feature = "std")]
273use std::error::Error as StdError;
274
275#[cfg(not(feature = "std"))]
276trait StdError: Debug + Display {
277 fn source(&self) -> Option<&(dyn StdError + 'static)> {
278 None
279 }
280}
281
282#[doc(no_inline)]
283pub use anyhow as format_err;
284
285/// The `Error` type, a wrapper around a dynamic error type.
286///
287/// `Error` works a lot like `Box<dyn std::error::Error>`, but with these
288/// differences:
289///
290/// - `Error` requires that the error is `Send`, `Sync`, and `'static`.
291/// - `Error` guarantees that a backtrace is available, even if the underlying
292/// error type does not provide one.
293/// - `Error` is represented as a narrow pointer — exactly one word in
294/// size instead of two.
295///
296/// <br>
297///
298/// # Display representations
299///
300/// When you print an error object using "{}" or to_string(), only the outermost
301/// underlying error or context is printed, not any of the lower level causes.
302/// This is exactly as if you had called the Display impl of the error from
303/// which you constructed your anyhow::Error.
304///
305/// ```console
306/// Failed to read instrs from ./path/to/instrs.json
307/// ```
308///
309/// To print causes as well using anyhow's default formatting of causes, use the
310/// alternate selector "{:#}".
311///
312/// ```console
313/// Failed to read instrs from ./path/to/instrs.json: No such file or directory (os error 2)
314/// ```
315///
316/// The Debug format "{:?}" includes your backtrace if one was captured. Note
317/// that this is the representation you get by default if you return an error
318/// from `fn main` instead of printing it explicitly yourself.
319///
320/// ```console
321/// Error: Failed to read instrs from ./path/to/instrs.json
322///
323/// Caused by:
324/// No such file or directory (os error 2)
325/// ```
326///
327/// and if there is a backtrace available:
328///
329/// ```console
330/// Error: Failed to read instrs from ./path/to/instrs.json
331///
332/// Caused by:
333/// No such file or directory (os error 2)
334///
335/// Stack backtrace:
336/// 0: <E as anyhow::context::ext::StdError>::ext_context
337/// at /git/anyhow/src/backtrace.rs:26
338/// 1: core::result::Result<T,E>::map_err
339/// at /git/rustc/src/libcore/result.rs:596
340/// 2: anyhow::context::<impl anyhow::Context<T,E> for core::result::Result<T,E>>::with_context
341/// at /git/anyhow/src/context.rs:58
342/// 3: testing::main
343/// at src/main.rs:5
344/// 4: std::rt::lang_start
345/// at /git/rustc/src/libstd/rt.rs:61
346/// 5: main
347/// 6: __libc_start_main
348/// 7: _start
349/// ```
350///
351/// To see a conventional struct-style Debug representation, use "{:#?}".
352///
353/// ```console
354/// Error {
355/// context: "Failed to read instrs from ./path/to/instrs.json",
356/// source: Os {
357/// code: 2,
358/// kind: NotFound,
359/// message: "No such file or directory",
360/// },
361/// }
362/// ```
363///
364/// If none of the built-in representations are appropriate and you would prefer
365/// to render the error and its cause chain yourself, it can be done something
366/// like this:
367///
368/// ```
369/// use anyhow::{Context, Result};
370///
371/// fn main() {
372/// if let Err(err) = try_main() {
373/// eprintln!("ERROR: {}", err);
374/// err.chain().skip(1).for_each(|cause| eprintln!("because: {}", cause));
375/// std::process::exit(1);
376/// }
377/// }
378///
379/// fn try_main() -> Result<()> {
380/// # const IGNORE: &str = stringify! {
381/// ...
382/// # };
383/// # Ok(())
384/// }
385/// ```
386#[repr(transparent)]
387pub struct Error {
388 inner: Own<ErrorImpl>,
389}
390
391/// Iterator of a chain of source errors.
392///
393/// This type is the iterator returned by [`Error::chain`].
394///
395/// # Example
396///
397/// ```
398/// use anyhow::Error;
399/// use std::io;
400///
401/// pub fn underlying_io_error_kind(error: &Error) -> Option<io::ErrorKind> {
402/// for cause in error.chain() {
403/// if let Some(io_error) = cause.downcast_ref::<io::Error>() {
404/// return Some(io_error.kind());
405/// }
406/// }
407/// None
408/// }
409/// ```
410#[cfg(feature = "std")]
411#[cfg_attr(doc_cfg, doc(cfg(feature = "std")))]
412#[derive(Clone)]
413pub struct Chain<'a> {
414 state: crate::chain::ChainState<'a>,
415}
416
417/// `Result<T, Error>`
418///
419/// This is a reasonable return type to use throughout your application but also
420/// for `fn main`; if you do, failures will be printed along with any
421/// [context][Context] and a backtrace if one was captured.
422///
423/// `anyhow::Result` may be used with one *or* two type parameters.
424///
425/// ```rust
426/// use anyhow::Result;
427///
428/// # const IGNORE: &str = stringify! {
429/// fn demo1() -> Result<T> {...}
430/// // ^ equivalent to std::result::Result<T, anyhow::Error>
431///
432/// fn demo2() -> Result<T, OtherError> {...}
433/// // ^ equivalent to std::result::Result<T, OtherError>
434/// # };
435/// ```
436///
437/// # Example
438///
439/// ```
440/// # pub trait Deserialize {}
441/// #
442/// # mod serde_json {
443/// # use super::Deserialize;
444/// # use std::io;
445/// #
446/// # pub fn from_str<T: Deserialize>(json: &str) -> io::Result<T> {
447/// # unimplemented!()
448/// # }
449/// # }
450/// #
451/// # #[derive(Debug)]
452/// # struct ClusterMap;
453/// #
454/// # impl Deserialize for ClusterMap {}
455/// #
456/// use anyhow::Result;
457///
458/// fn main() -> Result<()> {
459/// # return Ok(());
460/// let config = std::fs::read_to_string("cluster.json")?;
461/// let map: ClusterMap = serde_json::from_str(&config)?;
462/// println!("cluster info: {:#?}", map);
463/// Ok(())
464/// }
465/// ```
466pub type Result<T, E = Error> = core::result::Result<T, E>;
467
468/// Provides the `context` method for `Result`.
469///
470/// This trait is sealed and cannot be implemented for types outside of
471/// `anyhow`.
472///
473/// <br>
474///
475/// # Example
476///
477/// ```
478/// use anyhow::{Context, Result};
479/// use std::fs;
480/// use std::path::PathBuf;
481///
482/// pub struct ImportantThing {
483/// path: PathBuf,
484/// }
485///
486/// impl ImportantThing {
487/// # const IGNORE: &'static str = stringify! {
488/// pub fn detach(&mut self) -> Result<()> {...}
489/// # };
490/// # fn detach(&mut self) -> Result<()> {
491/// # unimplemented!()
492/// # }
493/// }
494///
495/// pub fn do_it(mut it: ImportantThing) -> Result<Vec<u8>> {
496/// it.detach().context("Failed to detach the important thing")?;
497///
498/// let path = &it.path;
499/// let content = fs::read(path)
500/// .with_context(|| format!("Failed to read instrs from {}", path.display()))?;
501///
502/// Ok(content)
503/// }
504/// ```
505///
506/// When printed, the outermost context would be printed first and the lower
507/// level underlying causes would be enumerated below.
508///
509/// ```console
510/// Error: Failed to read instrs from ./path/to/instrs.json
511///
512/// Caused by:
513/// No such file or directory (os error 2)
514/// ```
515///
516/// Refer to the [Display representations] documentation for other forms in
517/// which this context chain can be rendered.
518///
519/// [Display representations]: Error#display-representations
520///
521/// <br>
522///
523/// # Effect on downcasting
524///
525/// After attaching context of type `C` onto an error of type `E`, the resulting
526/// `anyhow::Error` may be downcast to `C` **or** to `E`.
527///
528/// That is, in codebases that rely on downcasting, Anyhow's context supports
529/// both of the following use cases:
530///
531/// - **Attaching context whose type is insignificant onto errors whose type
532/// is used in downcasts.**
533///
534/// In other error libraries whose context is not designed this way, it can
535/// be risky to introduce context to existing code because new context might
536/// break existing working downcasts. In Anyhow, any downcast that worked
537/// before adding context will continue to work after you add a context, so
538/// you should freely add human-readable context to errors wherever it would
539/// be helpful.
540///
541/// ```
542/// # use anyhow::bail;
543/// # use thiserror::Error;
544/// #
545/// # #[derive(Error, Debug)]
546/// # #[error("???")]
547/// # struct SuspiciousError;
548/// #
549/// # fn helper() -> Result<()> {
550/// # bail!(SuspiciousError);
551/// # }
552/// #
553/// use anyhow::{Context, Result};
554///
555/// fn do_it() -> Result<()> {
556/// helper().context("Failed to complete the work")?;
557/// # const IGNORE: &str = stringify! {
558/// ...
559/// # };
560/// # unreachable!()
561/// }
562///
563/// fn main() {
564/// let err = do_it().unwrap_err();
565/// if let Some(e) = err.downcast_ref::<SuspiciousError>() {
566/// // If helper() returned SuspiciousError, this downcast will
567/// // correctly succeed even with the context in between.
568/// # return;
569/// }
570/// # panic!("expected downcast to succeed");
571/// }
572/// ```
573///
574/// - **Attaching context whose type is used in downcasts onto errors whose
575/// type is insignificant.**
576///
577/// Some codebases prefer to use machine-readable context to categorize
578/// lower level errors in a way that will be actionable to higher levels of
579/// the application.
580///
581/// ```
582/// # use anyhow::bail;
583/// # use thiserror::Error;
584/// #
585/// # #[derive(Error, Debug)]
586/// # #[error("???")]
587/// # struct HelperFailed;
588/// #
589/// # fn helper() -> Result<()> {
590/// # bail!("no such file or directory");
591/// # }
592/// #
593/// use anyhow::{Context, Result};
594///
595/// fn do_it() -> Result<()> {
596/// helper().context(HelperFailed)?;
597/// # const IGNORE: &str = stringify! {
598/// ...
599/// # };
600/// # unreachable!()
601/// }
602///
603/// fn main() {
604/// let err = do_it().unwrap_err();
605/// if let Some(e) = err.downcast_ref::<HelperFailed>() {
606/// // If helper failed, this downcast will succeed because
607/// // HelperFailed is the context that has been attached to
608/// // that error.
609/// # return;
610/// }
611/// # panic!("expected downcast to succeed");
612/// }
613/// ```
614pub trait Context<T, E>: context::private::Sealed {
615 /// Wrap the error value with additional context.
616 fn context<C>(self, context: C) -> Result<T, Error>
617 where
618 C: Display + Send + Sync + 'static;
619
620 /// Wrap the error value with additional context that is evaluated lazily
621 /// only once an error does occur.
622 fn with_context<C, F>(self, f: F) -> Result<T, Error>
623 where
624 C: Display + Send + Sync + 'static,
625 F: FnOnce() -> C;
626}
627
628/// Equivalent to Ok::<_, anyhow::Error>(value).
629///
630/// This simplifies creation of an anyhow::Result in places where type inference
631/// cannot deduce the `E` type of the result — without needing to write
632/// `Ok::<_, anyhow::Error>(value)`.
633///
634/// One might think that `anyhow::Result::Ok(value)` would work in such cases
635/// but it does not.
636///
637/// ```console
638/// error[E0282]: type annotations needed for `std::result::Result<i32, E>`
639/// --> src/main.rs:11:13
640/// |
641/// 11 | let _ = anyhow::Result::Ok(1);
642/// | - ^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `E` declared on the enum `Result`
643/// | |
644/// | consider giving this pattern the explicit type `std::result::Result<i32, E>`, where the type parameter `E` is specified
645/// ```
646#[allow(non_snake_case)]
647pub fn Ok<T>(t: T) -> Result<T> {
648 Result::Ok(t)
649}
650
651// Not public API. Referenced by macro-generated code.
652#[doc(hidden)]
653pub mod __private {
654 use crate::Error;
655 use alloc::fmt;
656 use core::fmt::Arguments;
657
658 #[doc(hidden)]
659 pub use crate::ensure::{BothDebug, NotBothDebug};
660 #[doc(hidden)]
661 pub use alloc::format;
662 #[doc(hidden)]
663 pub use core::result::Result::Err;
664 #[doc(hidden)]
665 pub use core::{concat, format_args, stringify};
666
667 #[doc(hidden)]
668 pub mod kind {
669 #[doc(hidden)]
670 pub use crate::kind::{AdhocKind, TraitKind};
671
672 #[cfg(feature = "std")]
673 #[doc(hidden)]
674 pub use crate::kind::BoxedKind;
675 }
676
677 #[doc(hidden)]
678 #[inline]
679 #[cold]
680 pub fn format_err(args: Arguments) -> Error {
681 #[cfg(anyhow_no_fmt_arguments_as_str)]
682 let fmt_arguments_as_str = None::<&str>;
683 #[cfg(not(anyhow_no_fmt_arguments_as_str))]
684 let fmt_arguments_as_str = args.as_str();
685
686 if let Some(message) = fmt_arguments_as_str {
687 // anyhow!("literal"), can downcast to &'static str
688 Error::msg(message)
689 } else {
690 // anyhow!("interpolate {var}"), can downcast to String
691 Error::msg(fmt::format(args))
692 }
693 }
694
695 #[doc(hidden)]
696 #[inline]
697 #[cold]
698 #[must_use]
699 pub fn must_use(error: Error) -> Error {
700 error
701 }
702}