anyerr/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 [`anyerr::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, anyerr::Error>`, or equivalently `anyerr::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 anyerr::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 anyerr::{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 anyerr::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 `anyerr::Error`.
168//!
169//! ```
170//! # use anyerr::{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 anyerr::{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//! 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/1.0.85")]
210#![cfg_attr(error_generic_member_access, feature(error_generic_member_access))]
211#![cfg_attr(docsrs, feature(doc_cfg))]
212#![no_std]
213#![deny(dead_code, unused_imports, unused_mut)]
214#![deny(unsafe_op_in_unsafe_fn)]
215#![allow(
216 clippy::doc_markdown,
217 clippy::enum_glob_use,
218 clippy::explicit_auto_deref,
219 clippy::extra_unused_type_parameters,
220 clippy::incompatible_msrv,
221 clippy::let_underscore_untyped,
222 clippy::missing_errors_doc,
223 clippy::missing_panics_doc,
224 clippy::module_name_repetitions,
225 clippy::must_use_candidate,
226 clippy::needless_doctest_main,
227 clippy::new_ret_no_self,
228 clippy::redundant_else,
229 clippy::return_self_not_must_use,
230 clippy::struct_field_names,
231 clippy::unused_self,
232 clippy::used_underscore_binding,
233 clippy::wildcard_imports,
234 clippy::wrong_self_convention
235)]
236
237#[cfg(all(anyhow_nightly_testing, feature = "std", not(error_generic_member_access)))]
238compile_error!("Build script probe failed to compile.");
239
240extern crate alloc;
241
242#[cfg(feature = "std")]
243extern crate std;
244
245#[macro_use]
246mod backtrace;
247mod chain;
248mod context;
249mod ensure;
250mod error;
251mod fmt;
252mod kind;
253mod macros;
254mod ptr;
255mod wrapper;
256
257#[cfg(not(feature = "std"))]
258use core::fmt::Debug;
259use core::fmt::Display;
260#[cfg(feature = "std")]
261use std::error::Error as StdError;
262
263use crate::{error::ErrorImpl, ptr::Own};
264
265#[cfg(not(feature = "std"))]
266trait StdError: Debug + Display {
267 fn source(&self) -> Option<&(dyn StdError + 'static)> {
268 None
269 }
270}
271
272#[doc(no_inline)]
273pub use anyhow as format_err;
274
275/// The `Error` type, a wrapper around a dynamic error type.
276///
277/// `Error` works a lot like `Box<dyn std::error::Error>`, but with these
278/// differences:
279///
280/// - `Error` requires that the error is `Send`, `Sync`, and `'static`.
281/// - `Error` guarantees that a backtrace is available, even if the underlying
282/// error type does not provide one.
283/// - `Error` is represented as a narrow pointer — exactly one word in
284/// size instead of two.
285///
286/// <br>
287///
288/// # Display representations
289///
290/// When you print an error object using "{}" or to_string(), only the outermost
291/// underlying error or context is printed, not any of the lower level causes.
292/// This is exactly as if you had called the Display impl of the error from
293/// which you constructed your anyerr::Error.
294///
295/// ```console
296/// Failed to read instrs from ./path/to/instrs.json
297/// ```
298///
299/// To print causes as well using anyhow's default formatting of causes, use the
300/// alternate selector "{:#}".
301///
302/// ```console
303/// Failed to read instrs from ./path/to/instrs.json: No such file or directory (os error 2)
304/// ```
305///
306/// The Debug format "{:?}" includes your backtrace if one was captured. Note
307/// that this is the representation you get by default if you return an error
308/// from `fn main` instead of printing it explicitly yourself.
309///
310/// ```console
311/// Error: Failed to read instrs from ./path/to/instrs.json
312///
313/// Caused by:
314/// No such file or directory (os error 2)
315/// ```
316///
317/// and if there is a backtrace available:
318///
319/// ```console
320/// Error: Failed to read instrs from ./path/to/instrs.json
321///
322/// Caused by:
323/// No such file or directory (os error 2)
324///
325/// Stack backtrace:
326/// 0: <E as anyerr::context::ext::StdError>::ext_context
327/// at /git/anyhow/src/backtrace.rs:26
328/// 1: core::result::Result<T,E>::map_err
329/// at /git/rustc/src/libcore/result.rs:596
330/// 2: anyerr::context::<impl anyerr::Context<T,E> for core::result::Result<T,E>>::with_context
331/// at /git/anyhow/src/context.rs:58
332/// 3: testing::main
333/// at src/main.rs:5
334/// 4: std::rt::lang_start
335/// at /git/rustc/src/libstd/rt.rs:61
336/// 5: main
337/// 6: __libc_start_main
338/// 7: _start
339/// ```
340///
341/// To see a conventional struct-style Debug representation, use "{:#?}".
342///
343/// ```console
344/// Error {
345/// context: "Failed to read instrs from ./path/to/instrs.json",
346/// source: Os {
347/// code: 2,
348/// kind: NotFound,
349/// message: "No such file or directory",
350/// },
351/// }
352/// ```
353///
354/// If none of the built-in representations are appropriate and you would prefer
355/// to render the error and its cause chain yourself, it can be done something
356/// like this:
357///
358/// ```
359/// use anyerr::{Context, Result};
360///
361/// fn main() {
362/// if let Err(err) = try_main() {
363/// eprintln!("ERROR: {}", err);
364/// err.chain().skip(1).for_each(|cause| eprintln!("because: {}", cause));
365/// std::process::exit(1);
366/// }
367/// }
368///
369/// fn try_main() -> Result<()> {
370/// # const IGNORE: &str = stringify! {
371/// ...
372/// # };
373/// # Ok(())
374/// }
375/// ```
376#[repr(transparent)]
377pub struct Error {
378 inner: Own<ErrorImpl>,
379}
380
381/// Iterator of a chain of source errors.
382///
383/// This type is the iterator returned by [`Error::chain`].
384///
385/// # Example
386///
387/// ```
388/// use anyerr::Error;
389/// use std::io;
390///
391/// pub fn underlying_io_error_kind(error: &Error) -> Option<io::ErrorKind> {
392/// for cause in error.chain() {
393/// if let Some(io_error) = cause.downcast_ref::<io::Error>() {
394/// return Some(io_error.kind());
395/// }
396/// }
397/// None
398/// }
399/// ```
400#[cfg(feature = "std")]
401#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
402#[derive(Clone)]
403pub struct Chain<'a> {
404 state: crate::chain::ChainState<'a>,
405}
406
407/// `Result<T, Error>`
408///
409/// This is a reasonable return type to use throughout your application but also
410/// for `fn main`; if you do, failures will be printed along with any
411/// [context][Context] and a backtrace if one was captured.
412///
413/// `anyerr::Result` may be used with one *or* two type parameters.
414///
415/// ```rust
416/// use anyerr::Result;
417///
418/// # const IGNORE: &str = stringify! {
419/// fn demo1() -> Result<T> {...}
420/// // ^ equivalent to std::result::Result<T, anyerr::Error>
421///
422/// fn demo2() -> Result<T, OtherError> {...}
423/// // ^ equivalent to std::result::Result<T, OtherError>
424/// # };
425/// ```
426///
427/// # Example
428///
429/// ```
430/// # pub trait Deserialize {}
431/// #
432/// # mod serde_json {
433/// # use super::Deserialize;
434/// # use std::io;
435/// #
436/// # pub fn from_str<T: Deserialize>(json: &str) -> io::Result<T> {
437/// # unimplemented!()
438/// # }
439/// # }
440/// #
441/// # #[derive(Debug)]
442/// # struct ClusterMap;
443/// #
444/// # impl Deserialize for ClusterMap {}
445/// #
446/// use anyerr::Result;
447///
448/// fn main() -> Result<()> {
449/// # return Ok(());
450/// let config = std::fs::read_to_string("cluster.json")?;
451/// let map: ClusterMap = serde_json::from_str(&config)?;
452/// println!("cluster info: {:#?}", map);
453/// Ok(())
454/// }
455/// ```
456pub type Result<T, E = Error> = core::result::Result<T, E>;
457
458/// Provides the `context` method for `Result`.
459///
460/// This trait is sealed and cannot be implemented for types outside of
461/// `anyhow`.
462///
463/// <br>
464///
465/// # Example
466///
467/// ```
468/// use anyerr::{Context, Result};
469/// use std::fs;
470/// use std::path::PathBuf;
471///
472/// pub struct ImportantThing {
473/// path: PathBuf,
474/// }
475///
476/// impl ImportantThing {
477/// # const IGNORE: &'static str = stringify! {
478/// pub fn detach(&mut self) -> Result<()> {...}
479/// # };
480/// # fn detach(&mut self) -> Result<()> {
481/// # unimplemented!()
482/// # }
483/// }
484///
485/// pub fn do_it(mut it: ImportantThing) -> Result<Vec<u8>> {
486/// it.detach().context("Failed to detach the important thing")?;
487///
488/// let path = &it.path;
489/// let content = fs::read(path)
490/// .with_context(|| format!("Failed to read instrs from {}", path.display()))?;
491///
492/// Ok(content)
493/// }
494/// ```
495///
496/// When printed, the outermost context would be printed first and the lower
497/// level underlying causes would be enumerated below.
498///
499/// ```console
500/// Error: Failed to read instrs from ./path/to/instrs.json
501///
502/// Caused by:
503/// No such file or directory (os error 2)
504/// ```
505///
506/// Refer to the [Display representations] documentation for other forms in
507/// which this context chain can be rendered.
508///
509/// [Display representations]: Error#display-representations
510///
511/// <br>
512///
513/// # Effect on downcasting
514///
515/// After attaching context of type `C` onto an error of type `E`, the resulting
516/// `anyerr::Error` may be downcast to `C` **or** to `E`.
517///
518/// That is, in codebases that rely on downcasting, Anyhow's context supports
519/// both of the following use cases:
520///
521/// - **Attaching context whose type is insignificant onto errors whose type
522/// is used in downcasts.**
523///
524/// In other error libraries whose context is not designed this way, it can
525/// be risky to introduce context to existing code because new context might
526/// break existing working downcasts. In Anyhow, any downcast that worked
527/// before adding context will continue to work after you add a context, so
528/// you should freely add human-readable context to errors wherever it would
529/// be helpful.
530///
531/// ```
532/// # use anyerr::bail;
533/// # use thiserror::Error;
534/// #
535/// # #[derive(Error, Debug)]
536/// # #[error("???")]
537/// # struct SuspiciousError;
538/// #
539/// # fn helper() -> Result<()> {
540/// # bail!(SuspiciousError);
541/// # }
542/// #
543/// use anyerr::{Context, Result};
544///
545/// fn do_it() -> Result<()> {
546/// helper().context("Failed to complete the work")?;
547/// # const IGNORE: &str = stringify! {
548/// ...
549/// # };
550/// # unreachable!()
551/// }
552///
553/// fn main() {
554/// let err = do_it().unwrap_err();
555/// if let Some(e) = err.downcast_ref::<SuspiciousError>() {
556/// // If helper() returned SuspiciousError, this downcast will
557/// // correctly succeed even with the context in between.
558/// # return;
559/// }
560/// # panic!("expected downcast to succeed");
561/// }
562/// ```
563///
564/// - **Attaching context whose type is used in downcasts onto errors whose
565/// type is insignificant.**
566///
567/// Some codebases prefer to use machine-readable context to categorize
568/// lower level errors in a way that will be actionable to higher levels of
569/// the application.
570///
571/// ```
572/// # use anyerr::bail;
573/// # use thiserror::Error;
574/// #
575/// # #[derive(Error, Debug)]
576/// # #[error("???")]
577/// # struct HelperFailed;
578/// #
579/// # fn helper() -> Result<()> {
580/// # bail!("no such file or directory");
581/// # }
582/// #
583/// use anyerr::{Context, Result};
584///
585/// fn do_it() -> Result<()> {
586/// helper().context(HelperFailed)?;
587/// # const IGNORE: &str = stringify! {
588/// ...
589/// # };
590/// # unreachable!()
591/// }
592///
593/// fn main() {
594/// let err = do_it().unwrap_err();
595/// if let Some(e) = err.downcast_ref::<HelperFailed>() {
596/// // If helper failed, this downcast will succeed because
597/// // HelperFailed is the context that has been attached to
598/// // that error.
599/// # return;
600/// }
601/// # panic!("expected downcast to succeed");
602/// }
603/// ```
604pub trait Context<T, E>: context::private::Sealed {
605 /// Wrap the error value with additional context.
606 fn context<C>(self, context: C) -> Result<T, Error>
607 where
608 C: Display + Send + Sync + 'static;
609
610 /// Wrap the error value with additional context that is evaluated lazily
611 /// only once an error does occur.
612 fn with_context<C, F>(self, f: F) -> Result<T, Error>
613 where
614 C: Display + Send + Sync + 'static,
615 F: FnOnce() -> C;
616}
617
618/// Equivalent to Ok::<_, anyerr::Error>(value).
619///
620/// This simplifies creation of an anyerr::Result in places where type inference
621/// cannot deduce the `E` type of the result — without needing to write
622/// `Ok::<_, anyerr::Error>(value)`.
623///
624/// One might think that `anyerr::Result::Ok(value)` would work in such cases
625/// but it does not.
626///
627/// ```console
628/// error[E0282]: type annotations needed for `std::result::Result<i32, E>`
629/// --> src/main.rs:11:13
630/// |
631/// 11 | let _ = anyerr::Result::Ok(1);
632/// | - ^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `E` declared on the enum `Result`
633/// | |
634/// | consider giving this pattern the explicit type `std::result::Result<i32, E>`, where the type parameter `E` is specified
635/// ```
636#[allow(non_snake_case)]
637pub fn Ok<T>(t: T) -> Result<T> {
638 Result::Ok(t)
639}
640
641// Not public API. Referenced by macro-generated code.
642#[doc(hidden)]
643pub mod __private {
644 use alloc::fmt;
645 #[doc(hidden)]
646 pub use alloc::format;
647 use core::fmt::Arguments;
648 #[doc(hidden)]
649 pub use core::result::Result::Err;
650 #[doc(hidden)]
651 pub use core::{concat, format_args, stringify};
652
653 use self::not::Bool;
654 use crate::Error;
655 #[doc(hidden)]
656 pub use crate::ensure::{BothDebug, NotBothDebug};
657
658 #[doc(hidden)]
659 pub mod kind {
660 #[cfg(feature = "std")]
661 #[doc(hidden)]
662 pub use crate::kind::BoxedKind;
663 #[doc(hidden)]
664 pub use crate::kind::{AdhocKind, TraitKind};
665 }
666
667 #[doc(hidden)]
668 #[inline]
669 #[cold]
670 pub fn format_err(args: Arguments) -> Error {
671 let fmt_arguments_as_str = args.as_str();
672
673 if let Some(message) = fmt_arguments_as_str {
674 // anyhow!("literal"), can downcast to &'static str
675 Error::msg(message)
676 } else {
677 // anyhow!("interpolate {var}"), can downcast to String
678 Error::msg(fmt::format(args))
679 }
680 }
681
682 #[doc(hidden)]
683 #[inline]
684 #[cold]
685 #[must_use]
686 pub fn must_use(error: Error) -> Error {
687 error
688 }
689
690 #[doc(hidden)]
691 #[inline]
692 pub fn not(cond: impl Bool) -> bool {
693 cond.not()
694 }
695
696 mod not {
697 #[doc(hidden)]
698 pub trait Bool {
699 fn not(self) -> bool;
700 }
701
702 impl Bool for bool {
703 #[inline]
704 fn not(self) -> bool {
705 !self
706 }
707 }
708
709 impl Bool for &bool {
710 #[inline]
711 fn not(self) -> bool {
712 !*self
713 }
714 }
715 }
716}