derive(Error)
=============
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This library provides a convenient derive macro for the standard library's
[`std::error::Error`] trait.
[`std::error::Error`]: https://doc.rust-lang.org/std/error/trait.Error.html
```toml
[dependencies]
thiserror = "1.0"
```
*Compiler support: requires rustc 1.31+*
## Example
```rust
use thiserror::Error;
#[derive(Error, Debug)]
pub enum DataStoreError {
#[error("data store disconnected")]
Disconnect(#[source] io::Error),
#[error("the data for key `{0}` is not available")]
Redaction(String),
#[error("invalid header (expected {expected:?}, found {found:?})")]
InvalidHeader {
expected: String,
found: String,
},
#[error("unknown data store error")]
Unknown,
}
```
## Details
- Thiserror deliberately does not appear in your public API. You get the same
thing as if you had written an implementation of `std::error::Error` by hand,
and switching from handwritten impls to thiserror or vice versa is not a
breaking change.
- Errors may be enums, structs with named fields, tuple structs, or unit
structs.
- A `Display` impl is generated for your error if you provide `#[error("...")]`
messages on the struct or each variant of your enum, as shown above in the
example.
The messages support a shorthand for interpolating fields from the error.
- `#[error("{var}")]` ⟶ `write!("{}", self.var)`
- `#[error("{0}")]` ⟶ `write!("{}", self.0)`
- `#[error("{var:?}")]` ⟶ `write!("{:?}", self.var)`
- `#[error("{0:?}")]` ⟶ `write!("{:?}", self.0)`
You may alternatively write out the full format args yourself, using arbitrary
expressions.
When providing your own format args, the shorthand does not kick in so you
need to specify `.var` in the argument list to refer to named fields and `.0`
to refer to tuple fields.
```rust
#[derive(Error, Debug)]
pub enum Error {
#[error("invalid rdo_lookahead_frames {} (expected < {})", .0, i32::max_value())]
InvalidLookahead(i32),
}
```
- The Error trait's `source()` method is implemented to return whichever field
has a `#[source]` attribute, if any. This is for identifying the underlying
lower level error that caused your error.
Any error type that implements `std::error::Error` or dereferences to `dyn
std::error::Error` will work as a source.
```rust
#[derive(Error, Debug)]
pub struct MyError {
msg: String,
#[source]
source: anyhow::Error,
}
```
- The Error trait's `backtrace()` method is implemented to return whichever
field has a type named `Backtrace`, if any.
```rust
use std::backtrace::Backtrace;
#[derive(Error, Debug)]
pub struct MyError {
msg: String,
backtrace: Backtrace, // automatically detected
}
```
- See also the [`anyhow`] library for a convenient single error type to use in
application code.
[`anyhow`]: https://github.com/dtolnay/anyhow
#### License
Licensed under either of Apache License, Version
2.0 or MIT license at your option.
Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
be dual licensed as above, without any additional terms or conditions.