1 //! [![github]](https://github.com/dtolnay/thiserror) [![crates-io]](https://crates.io/crates/thiserror) [![docs-rs]](https://docs.rs/thiserror) 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&logoColor=white&logo=data:image/svg+xml;base64,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 6 //! 7 //! <br> 8 //! 9 //! This library provides a convenient derive macro for the standard library's 10 //! [`std::error::Error`] trait. 11 //! 12 //! [`std::error::Error`]: https://doc.rust-lang.org/std/error/trait.Error.html 13 //! 14 //! <br> 15 //! 16 //! # Example 17 //! 18 //! ```rust 19 //! # use std::io; 20 //! use thiserror::Error; 21 //! 22 //! #[derive(Error, Debug)] 23 //! pub enum DataStoreError { 24 //! #[error("data store disconnected")] 25 //! Disconnect(#[from] io::Error), 26 //! #[error("the data for key `{0}` is not available")] 27 //! Redaction(String), 28 //! #[error("invalid header (expected {expected:?}, found {found:?})")] 29 //! InvalidHeader { 30 //! expected: String, 31 //! found: String, 32 //! }, 33 //! #[error("unknown data store error")] 34 //! Unknown, 35 //! } 36 //! ``` 37 //! 38 //! <br> 39 //! 40 //! # Details 41 //! 42 //! - Thiserror deliberately does not appear in your public API. You get the 43 //! same thing as if you had written an implementation of `std::error::Error` 44 //! by hand, and switching from handwritten impls to thiserror or vice versa 45 //! is not a breaking change. 46 //! 47 //! - Errors may be enums, structs with named fields, tuple structs, or unit 48 //! structs. 49 //! 50 //! - A `Display` impl is generated for your error if you provide 51 //! `#[error("...")]` messages on the struct or each variant of your enum, as 52 //! shown above in the example. 53 //! 54 //! The messages support a shorthand for interpolating fields from the error. 55 //! 56 //! - `#[error("{var}")]` ⟶ `write!("{}", self.var)` 57 //! - `#[error("{0}")]` ⟶ `write!("{}", self.0)` 58 //! - `#[error("{var:?}")]` ⟶ `write!("{:?}", self.var)` 59 //! - `#[error("{0:?}")]` ⟶ `write!("{:?}", self.0)` 60 //! 61 //! These shorthands can be used together with any additional format args, 62 //! which may be arbitrary expressions. For example: 63 //! 64 //! ```rust 65 //! # use std::i32; 66 //! # use thiserror::Error; 67 //! # 68 //! #[derive(Error, Debug)] 69 //! pub enum Error { 70 //! #[error("invalid rdo_lookahead_frames {0} (expected < {})", i32::MAX)] 71 //! InvalidLookahead(u32), 72 //! } 73 //! ``` 74 //! 75 //! If one of the additional expression arguments needs to refer to a field of 76 //! the struct or enum, then refer to named fields as `.var` and tuple fields 77 //! as `.0`. 78 //! 79 //! ```rust 80 //! # use thiserror::Error; 81 //! # 82 //! # fn first_char(s: &String) -> char { 83 //! # s.chars().next().unwrap() 84 //! # } 85 //! # 86 //! # #[derive(Debug)] 87 //! # struct Limits { 88 //! # lo: usize, 89 //! # hi: usize, 90 //! # } 91 //! # 92 //! #[derive(Error, Debug)] 93 //! pub enum Error { 94 //! #[error("first letter must be lowercase but was {:?}", first_char(.0))] 95 //! WrongCase(String), 96 //! #[error("invalid index {idx}, expected at least {} and at most {}", .limits.lo, .limits.hi)] 97 //! OutOfBounds { idx: usize, limits: Limits }, 98 //! } 99 //! ``` 100 //! 101 //! - A `From` impl is generated for each variant containing a `#[from]` 102 //! attribute. 103 //! 104 //! Note that the variant must not contain any other fields beyond the source 105 //! error and possibly a backtrace. A backtrace is captured from within the 106 //! `From` impl if there is a field for it. 107 //! 108 //! ```rust 109 //! # const IGNORE: &str = stringify! { 110 //! #[derive(Error, Debug)] 111 //! pub enum MyError { 112 //! Io { 113 //! #[from] 114 //! source: io::Error, 115 //! backtrace: Backtrace, 116 //! }, 117 //! } 118 //! # }; 119 //! ``` 120 //! 121 //! - The Error trait's `source()` method is implemented to return whichever 122 //! field has a `#[source]` attribute or is named `source`, if any. This is 123 //! for identifying the underlying lower level error that caused your error. 124 //! 125 //! The `#[from]` attribute always implies that the same field is `#[source]`, 126 //! so you don't ever need to specify both attributes. 127 //! 128 //! Any error type that implements `std::error::Error` or dereferences to `dyn 129 //! std::error::Error` will work as a source. 130 //! 131 //! ```rust 132 //! # use std::fmt::{self, Display}; 133 //! # use thiserror::Error; 134 //! # 135 //! #[derive(Error, Debug)] 136 //! pub struct MyError { 137 //! msg: String, 138 //! #[source] // optional if field name is `source` 139 //! source: anyhow::Error, 140 //! } 141 //! # 142 //! # impl Display for MyError { 143 //! # fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result { 144 //! # unimplemented!() 145 //! # } 146 //! # } 147 //! ``` 148 //! 149 //! - The Error trait's `backtrace()` method is implemented to return whichever 150 //! field has a type named `Backtrace`, if any. 151 //! 152 //! ```rust 153 //! # const IGNORE: &str = stringify! { 154 //! use std::backtrace::Backtrace; 155 //! 156 //! #[derive(Error, Debug)] 157 //! pub struct MyError { 158 //! msg: String, 159 //! backtrace: Backtrace, // automatically detected 160 //! } 161 //! # }; 162 //! ``` 163 //! 164 //! - Errors may use `error(transparent)` to forward the source and Display 165 //! methods straight through to an underlying error without adding an 166 //! additional message. This would be appropriate for enums that need an 167 //! "anything else" variant. 168 //! 169 //! ``` 170 //! # use thiserror::Error; 171 //! # 172 //! #[derive(Error, Debug)] 173 //! pub enum MyError { 174 //! # /* 175 //! ... 176 //! # */ 177 //! 178 //! #[error(transparent)] 179 //! Other(#[from] anyhow::Error), // source and Display delegate to anyhow::Error 180 //! } 181 //! ``` 182 //! 183 //! - See also the [`anyhow`] library for a convenient single error type to use 184 //! in application code. 185 //! 186 //! [`anyhow`]: https://github.com/dtolnay/anyhow 187 188 mod aserror; 189 mod display; 190 191 pub use thiserror_impl::*; 192 193 // Not public API. 194 #[doc(hidden)] 195 pub mod private { 196 pub use crate::aserror::AsDynError; 197 pub use crate::display::{DisplayAsDisplay, PathAsDisplay}; 198 } 199