thiserror/lib.rs
1//! # Notice: This is a modified version of [`thiserror`](https://crates.io/crates/thiserror) with `no_std` support.
2//!
3//! [![github]](https://github.com/dtolnay/thiserror) [![crates-io]](https://crates.io/crates/thiserror) [![docs-rs]](https://docs.rs/thiserror)
4//!
5//! [github]: https://img.shields.io/badge/github-8da0cb?style=for-the-badge&labelColor=555555&logo=github
6//! [crates-io]: https://img.shields.io/badge/crates.io-fc8d62?style=for-the-badge&labelColor=555555&logo=rust
7//! [docs-rs]: https://img.shields.io/badge/docs.rs-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs
8//!
9//! <br>
10//!
11//! This library provides a convenient derive macro for the standard library's
12//! [`std::error::Error`] trait.
13//!
14//! [`std::error::Error`]: https://doc.rust-lang.org/std/error/trait.Error.html
15//!
16//! <br>
17//!
18//! # Example
19//!
20//! ```rust
21//! # use std::io;
22//! use thiserror::Error;
23//!
24//! #[derive(Error, Debug)]
25//! pub enum DataStoreError {
26//! #[error("data store disconnected")]
27//! Disconnect(#[from] io::Error),
28//! #[error("the data for key `{0}` is not available")]
29//! Redaction(String),
30//! #[error("invalid header (expected {expected:?}, found {found:?})")]
31//! InvalidHeader {
32//! expected: String,
33//! found: String,
34//! },
35//! #[error("unknown data store error")]
36//! Unknown,
37//! }
38//! ```
39//!
40//! <br>
41//!
42//! # Details
43//!
44//! - Thiserror deliberately does not appear in your public API. You get the
45//! same thing as if you had written an implementation of `std::error::Error`
46//! by hand, and switching from handwritten impls to thiserror or vice versa
47//! is not a breaking change.
48//!
49//! - Errors may be enums, structs with named fields, tuple structs, or unit
50//! structs.
51//!
52//! - A `Display` impl is generated for your error if you provide
53//! `#[error("...")]` messages on the struct or each variant of your enum, as
54//! shown above in the example.
55//!
56//! The messages support a shorthand for interpolating fields from the error.
57//!
58//! - `#[error("{var}")]` ⟶ `write!("{}", self.var)`
59//! - `#[error("{0}")]` ⟶ `write!("{}", self.0)`
60//! - `#[error("{var:?}")]` ⟶ `write!("{:?}", self.var)`
61//! - `#[error("{0:?}")]` ⟶ `write!("{:?}", self.0)`
62//!
63//! These shorthands can be used together with any additional format args,
64//! which may be arbitrary expressions. For example:
65//!
66//! ```rust
67//! # use std::i32;
68//! # use thiserror::Error;
69//! #
70//! #[derive(Error, Debug)]
71//! pub enum Error {
72//! #[error("invalid rdo_lookahead_frames {0} (expected < {})", i32::MAX)]
73//! InvalidLookahead(u32),
74//! }
75//! ```
76//!
77//! If one of the additional expression arguments needs to refer to a field of
78//! the struct or enum, then refer to named fields as `.var` and tuple fields
79//! as `.0`.
80//!
81//! ```rust
82//! # use thiserror::Error;
83//! #
84//! # fn first_char(s: &String) -> char {
85//! # s.chars().next().unwrap()
86//! # }
87//! #
88//! # #[derive(Debug)]
89//! # struct Limits {
90//! # lo: usize,
91//! # hi: usize,
92//! # }
93//! #
94//! #[derive(Error, Debug)]
95//! pub enum Error {
96//! #[error("first letter must be lowercase but was {:?}", first_char(.0))]
97//! WrongCase(String),
98//! #[error("invalid index {idx}, expected at least {} and at most {}", .limits.lo, .limits.hi)]
99//! OutOfBounds { idx: usize, limits: Limits },
100//! }
101//! ```
102//!
103//! - A `From` impl is generated for each variant containing a `#[from]`
104//! attribute.
105//!
106//! Note that the variant must not contain any other fields beyond the source
107//! error and possibly a backtrace. A backtrace is captured from within the
108//! `From` impl if there is a field for it.
109//!
110//! ```rust
111//! # const IGNORE: &str = stringify! {
112//! #[derive(Error, Debug)]
113//! pub enum MyError {
114//! Io {
115//! #[from]
116//! source: io::Error,
117//! backtrace: Backtrace,
118//! },
119//! }
120//! # };
121//! ```
122//!
123//! - The Error trait's `source()` method is implemented to return whichever
124//! field has a `#[source]` attribute or is named `source`, if any. This is
125//! for identifying the underlying lower level error that caused your error.
126//!
127//! The `#[from]` attribute always implies that the same field is `#[source]`,
128//! so you don't ever need to specify both attributes.
129//!
130//! Any error type that implements `std::error::Error` or dereferences to `dyn
131//! std::error::Error` will work as a source.
132//!
133//! ```rust
134//! # use std::fmt::{self, Display};
135//! # use thiserror::Error;
136//! #
137//! #[derive(Error, Debug)]
138//! pub struct MyError {
139//! msg: String,
140//! #[source] // optional if field name is `source`
141//! source: anyhow::Error,
142//! }
143//! #
144//! # impl Display for MyError {
145//! # fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
146//! # unimplemented!()
147//! # }
148//! # }
149//! ```
150//!
151//! - The Error trait's `provide()` method is implemented to provide whichever
152//! field has a type named `Backtrace`, if any, as a
153//! `std::backtrace::Backtrace`.
154//!
155//! ```rust
156//! # const IGNORE: &str = stringify! {
157//! use std::backtrace::Backtrace;
158//!
159//! #[derive(Error, Debug)]
160//! pub struct MyError {
161//! msg: String,
162//! backtrace: Backtrace, // automatically detected
163//! }
164//! # };
165//! ```
166//!
167//! - If a field is both a source (named `source`, or has `#[source]` or
168//! `#[from]` attribute) *and* is marked `#[backtrace]`, then the Error
169//! trait's `provide()` method is forwarded to the source's `provide` so that
170//! both layers of the error share the same backtrace.
171//!
172//! ```rust
173//! # const IGNORE: &str = stringify! {
174//! #[derive(Error, Debug)]
175//! pub enum MyError {
176//! Io {
177//! #[backtrace]
178//! source: io::Error,
179//! },
180//! }
181//! # };
182//! ```
183//!
184//! - Errors may use `error(transparent)` to forward the source and Display
185//! methods straight through to an underlying error without adding an
186//! additional message. This would be appropriate for enums that need an
187//! "anything else" variant.
188//!
189//! ```
190//! # use thiserror::Error;
191//! #
192//! #[derive(Error, Debug)]
193//! pub enum MyError {
194//! # /*
195//! ...
196//! # */
197//!
198//! #[error(transparent)]
199//! Other(#[from] anyhow::Error), // source and Display delegate to anyhow::Error
200//! }
201//! ```
202//!
203//! Another use case is hiding implementation details of an error
204//! representation behind an opaque error type, so that the representation is
205//! able to evolve without breaking the crate's public API.
206//!
207//! ```
208//! # use thiserror::Error;
209//! #
210//! // PublicError is public, but opaque and easy to keep compatible.
211//! #[derive(Error, Debug)]
212//! #[error(transparent)]
213//! pub struct PublicError(#[from] ErrorRepr);
214//!
215//! impl PublicError {
216//! // Accessors for anything we do want to expose publicly.
217//! }
218//!
219//! // Private and free to change across minor version of the crate.
220//! #[derive(Error, Debug)]
221//! enum ErrorRepr {
222//! # /*
223//! ...
224//! # */
225//! }
226//! ```
227//!
228//! - See also the [`anyhow`] library for a convenient single error type to use
229//! in application code.
230//!
231//! [`anyhow`]: https://github.com/dtolnay/anyhow
232
233#![doc(html_root_url = "https://docs.rs/thiserror/1.0.56")]
234#![allow(
235 clippy::module_name_repetitions,
236 clippy::needless_lifetimes,
237 clippy::return_self_not_must_use,
238 clippy::wildcard_imports
239)]
240#![cfg_attr(error_generic_member_access, feature(error_generic_member_access))]
241
242#[cfg(all(thiserror_nightly_testing, not(error_generic_member_access)))]
243compile_error!("Build script probe failed to compile.");
244
245mod aserror;
246mod display;
247#[cfg(error_generic_member_access)]
248mod provide;
249
250pub use thiserror_impl::*;
251
252// Not public API.
253#[doc(hidden)]
254pub mod __private {
255 #[doc(hidden)]
256 pub use crate::aserror::AsDynError;
257 #[doc(hidden)]
258 pub use crate::display::AsDisplay;
259 #[cfg(error_generic_member_access)]
260 #[doc(hidden)]
261 pub use crate::provide::ThiserrorProvide;
262}