1derive(Error) 2============= 3 4[<img alt="github" src="https://img.shields.io/badge/github-dtolnay/thiserror-8da0cb?style=for-the-badge&labelColor=555555&logo=github" height="20">](https://github.com/dtolnay/thiserror) 5[<img alt="crates.io" src="https://img.shields.io/crates/v/thiserror.svg?style=for-the-badge&color=fc8d62&logo=rust" height="20">](https://crates.io/crates/thiserror) 6[<img alt="docs.rs" src="https://img.shields.io/badge/docs.rs-thiserror-66c2a5?style=for-the-badge&labelColor=555555&logoColor=white&logo=data:image/svg+xml;base64,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" height="20">](https://docs.rs/thiserror) 7[<img alt="build status" src="https://img.shields.io/github/workflow/status/dtolnay/thiserror/CI/master?style=for-the-badge" height="20">](https://github.com/dtolnay/thiserror/actions?query=branch%3Amaster) 8 9This 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```toml 15[dependencies] 16thiserror = "1.0" 17``` 18 19*Compiler support: requires rustc 1.31+* 20 21<br> 22 23## Example 24 25```rust 26use thiserror::Error; 27 28#[derive(Error, Debug)] 29pub enum DataStoreError { 30 #[error("data store disconnected")] 31 Disconnect(#[from] io::Error), 32 #[error("the data for key `{0}` is not available")] 33 Redaction(String), 34 #[error("invalid header (expected {expected:?}, found {found:?})")] 35 InvalidHeader { 36 expected: String, 37 found: String, 38 }, 39 #[error("unknown data store error")] 40 Unknown, 41} 42``` 43 44<br> 45 46## Details 47 48- Thiserror deliberately does not appear in your public API. You get the same 49 thing as if you had written an implementation of `std::error::Error` by hand, 50 and switching from handwritten impls to thiserror or vice versa is not a 51 breaking change. 52 53- Errors may be enums, structs with named fields, tuple structs, or unit 54 structs. 55 56- A `Display` impl is generated for your error if you provide `#[error("...")]` 57 messages on the struct or each variant of your enum, as shown above in the 58 example. 59 60 The messages support a shorthand for interpolating fields from the error. 61 62 - `#[error("{var}")]` ⟶ `write!("{}", self.var)` 63 - `#[error("{0}")]` ⟶ `write!("{}", self.0)` 64 - `#[error("{var:?}")]` ⟶ `write!("{:?}", self.var)` 65 - `#[error("{0:?}")]` ⟶ `write!("{:?}", self.0)` 66 67 These shorthands can be used together with any additional format args, which 68 may be arbitrary expressions. For example: 69 70 ```rust 71 #[derive(Error, Debug)] 72 pub enum Error { 73 #[error("invalid rdo_lookahead_frames {0} (expected < {})", i32::MAX)] 74 InvalidLookahead(u32), 75 } 76 ``` 77 78 If one of the additional expression arguments needs to refer to a field of the 79 struct or enum, then refer to named fields as `.var` and tuple fields as `.0`. 80 81 ```rust 82 #[derive(Error, Debug)] 83 pub enum Error { 84 #[error("first letter must be lowercase but was {:?}", first_char(.0))] 85 WrongCase(String), 86 #[error("invalid index {idx}, expected at least {} and at most {}", .limits.lo, .limits.hi)] 87 OutOfBounds { idx: usize, limits: Limits }, 88 } 89 ``` 90 91- A `From` impl is generated for each variant containing a `#[from]` attribute. 92 93 Note that the variant must not contain any other fields beyond the source 94 error and possibly a backtrace. A backtrace is captured from within the `From` 95 impl if there is a field for it. 96 97 ```rust 98 #[derive(Error, Debug)] 99 pub enum MyError { 100 Io { 101 #[from] 102 source: io::Error, 103 backtrace: Backtrace, 104 }, 105 } 106 ``` 107 108- The Error trait's `source()` method is implemented to return whichever field 109 has a `#[source]` attribute or is named `source`, if any. This is for 110 identifying the underlying lower level error that caused your error. 111 112 The `#[from]` attribute always implies that the same field is `#[source]`, so 113 you don't ever need to specify both attributes. 114 115 Any error type that implements `std::error::Error` or dereferences to `dyn 116 std::error::Error` will work as a source. 117 118 ```rust 119 #[derive(Error, Debug)] 120 pub struct MyError { 121 msg: String, 122 #[source] // optional if field name is `source` 123 source: anyhow::Error, 124 } 125 ``` 126 127- The Error trait's `backtrace()` method is implemented to return whichever 128 field has a type named `Backtrace`, if any. 129 130 ```rust 131 use std::backtrace::Backtrace; 132 133 #[derive(Error, Debug)] 134 pub struct MyError { 135 msg: String, 136 backtrace: Backtrace, // automatically detected 137 } 138 ``` 139 140- Errors may use `error(transparent)` to forward the source and Display methods 141 straight through to an underlying error without adding an additional message. 142 This would be appropriate for enums that need an "anything else" variant. 143 144 ```rust 145 #[derive(Error, Debug)] 146 pub enum MyError { 147 ... 148 149 #[error(transparent)] 150 Other(#[from] anyhow::Error), // source and Display delegate to anyhow::Error 151 } 152 ``` 153 154- See also the [`anyhow`] library for a convenient single error type to use in 155 application code. 156 157 [`anyhow`]: https://github.com/dtolnay/anyhow 158 159<br> 160 161## Comparison to anyhow 162 163Use thiserror if you care about designing your own dedicated error type(s) so 164that the caller receives exactly the information that you choose in the event of 165failure. This most often applies to library-like code. Use [Anyhow] if you don't 166care what error type your functions return, you just want it to be easy. This is 167common in application-like code. 168 169[Anyhow]: https://github.com/dtolnay/anyhow 170 171<br> 172 173#### License 174 175<sup> 176Licensed under either of <a href="LICENSE-APACHE">Apache License, Version 1772.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option. 178</sup> 179 180<br> 181 182<sub> 183Unless you explicitly state otherwise, any contribution intentionally submitted 184for inclusion in this crate by you, as defined in the Apache-2.0 license, shall 185be dual licensed as above, without any additional terms or conditions. 186</sub> 187