223 lines
6.9 KiB
Markdown
223 lines
6.9 KiB
Markdown
derive(Error)
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=============
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[<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)
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[<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)
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[<img alt="docs.rs" src="https://img.shields.io/badge/docs.rs-thiserror-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs" height="20">](https://docs.rs/thiserror)
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[<img alt="build status" src="https://img.shields.io/github/actions/workflow/status/dtolnay/thiserror/ci.yml?branch=master&style=for-the-badge" height="20">](https://github.com/dtolnay/thiserror/actions?query=branch%3Amaster)
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This library provides a convenient derive macro for the standard library's
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[`std::error::Error`] trait.
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[`std::error::Error`]: https://doc.rust-lang.org/std/error/trait.Error.html
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```toml
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[dependencies]
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thiserror = "1.0"
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```
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*Compiler support: requires rustc 1.56+*
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<br>
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## Example
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```rust
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use thiserror::Error;
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#[derive(Error, Debug)]
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pub enum DataStoreError {
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#[error("data store disconnected")]
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Disconnect(#[from] io::Error),
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#[error("the data for key `{0}` is not available")]
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Redaction(String),
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#[error("invalid header (expected {expected:?}, found {found:?})")]
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InvalidHeader {
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expected: String,
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found: String,
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},
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#[error("unknown data store error")]
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Unknown,
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}
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```
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<br>
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## Details
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- Thiserror deliberately does not appear in your public API. You get the same
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thing as if you had written an implementation of `std::error::Error` by hand,
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and switching from handwritten impls to thiserror or vice versa is not a
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breaking change.
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- Errors may be enums, structs with named fields, tuple structs, or unit
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structs.
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- A `Display` impl is generated for your error if you provide `#[error("...")]`
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messages on the struct or each variant of your enum, as shown above in the
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example.
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The messages support a shorthand for interpolating fields from the error.
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- `#[error("{var}")]` ⟶ `write!("{}", self.var)`
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- `#[error("{0}")]` ⟶ `write!("{}", self.0)`
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- `#[error("{var:?}")]` ⟶ `write!("{:?}", self.var)`
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- `#[error("{0:?}")]` ⟶ `write!("{:?}", self.0)`
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These shorthands can be used together with any additional format args, which
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may be arbitrary expressions. For example:
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```rust
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#[derive(Error, Debug)]
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pub enum Error {
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#[error("invalid rdo_lookahead_frames {0} (expected < {})", i32::MAX)]
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InvalidLookahead(u32),
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}
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```
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If one of the additional expression arguments needs to refer to a field of the
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struct or enum, then refer to named fields as `.var` and tuple fields as `.0`.
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```rust
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#[derive(Error, Debug)]
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pub enum Error {
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#[error("first letter must be lowercase but was {:?}", first_char(.0))]
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WrongCase(String),
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#[error("invalid index {idx}, expected at least {} and at most {}", .limits.lo, .limits.hi)]
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OutOfBounds { idx: usize, limits: Limits },
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}
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```
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- A `From` impl is generated for each variant containing a `#[from]` attribute.
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Note that the variant must not contain any other fields beyond the source
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error and possibly a backtrace. A backtrace is captured from within the `From`
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impl if there is a field for it.
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```rust
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#[derive(Error, Debug)]
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pub enum MyError {
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Io {
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#[from]
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source: io::Error,
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backtrace: Backtrace,
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},
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}
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```
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- The Error trait's `source()` method is implemented to return whichever field
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has a `#[source]` attribute or is named `source`, if any. This is for
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identifying the underlying lower level error that caused your error.
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The `#[from]` attribute always implies that the same field is `#[source]`, so
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you don't ever need to specify both attributes.
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Any error type that implements `std::error::Error` or dereferences to `dyn
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std::error::Error` will work as a source.
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```rust
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#[derive(Error, Debug)]
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pub struct MyError {
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msg: String,
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#[source] // optional if field name is `source`
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source: anyhow::Error,
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}
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```
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- The Error trait's `provide()` method is implemented to provide whichever field
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has a type named `Backtrace`, if any, as a `std::backtrace::Backtrace`.
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```rust
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use std::backtrace::Backtrace;
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#[derive(Error, Debug)]
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pub struct MyError {
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msg: String,
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backtrace: Backtrace, // automatically detected
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}
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```
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- If a field is both a source (named `source`, or has `#[source]` or `#[from]`
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attribute) *and* is marked `#[backtrace]`, then the Error trait's `provide()`
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method is forwarded to the source's `provide` so that both layers of the error
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share the same backtrace.
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```rust
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#[derive(Error, Debug)]
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pub enum MyError {
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Io {
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#[backtrace]
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source: io::Error,
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},
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}
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```
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- Errors may use `error(transparent)` to forward the source and Display methods
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straight through to an underlying error without adding an additional message.
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This would be appropriate for enums that need an "anything else" variant.
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```rust
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#[derive(Error, Debug)]
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pub enum MyError {
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...
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#[error(transparent)]
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Other(#[from] anyhow::Error), // source and Display delegate to anyhow::Error
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}
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```
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Another use case is hiding implementation details of an error representation
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behind an opaque error type, so that the representation is able to evolve
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without breaking the crate's public API.
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```rust
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// PublicError is public, but opaque and easy to keep compatible.
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#[derive(Error, Debug)]
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#[error(transparent)]
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pub struct PublicError(#[from] ErrorRepr);
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impl PublicError {
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// Accessors for anything we do want to expose publicly.
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}
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// Private and free to change across minor version of the crate.
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#[derive(Error, Debug)]
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enum ErrorRepr {
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...
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}
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```
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- See also the [`anyhow`] library for a convenient single error type to use in
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application code.
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[`anyhow`]: https://github.com/dtolnay/anyhow
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<br>
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## Comparison to anyhow
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Use thiserror if you care about designing your own dedicated error type(s) so
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that the caller receives exactly the information that you choose in the event of
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failure. This most often applies to library-like code. Use [Anyhow] if you don't
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care what error type your functions return, you just want it to be easy. This is
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common in application-like code.
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[Anyhow]: https://github.com/dtolnay/anyhow
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<br>
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#### License
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<sup>
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Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
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2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
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</sup>
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<br>
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<sub>
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Unless you explicitly state otherwise, any contribution intentionally submitted
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for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
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be dual licensed as above, without any additional terms or conditions.
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</sub>
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