This library provides a convenient derive macro for the standard library's
[std::error::Error] trait.
toml
[dependencies]
thiserror = "1.0"
Compiler support: requires rustc 1.31+
```rust use thiserror::Error;
pub enum DataStoreError {
#[error("data store disconnected")]
Disconnect(#[from] 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,
}
```
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
pub enum Error { #[error("invalid rdolookaheadframes {} (expected < {})", .0, i32::max_value())] InvalidLookahead(i32), } ```
A From impl is generated for each variant containing a #[from] attribute.
Note that the variant must not contain any other fields beyond the source
error and possibly a backtrace. A backtrace is captured from within the From
impl if there is a field for it.
```rust
pub enum MyError { Io { #[from] source: io::Error, backtrace: Backtrace, }, } ```
The Error trait's source() method is implemented to return whichever field
has a #[source] attribute or is named source, if any. This is for
identifying the underlying lower level error that caused your error.
The #[from] attribute always implies that the same field is #[source], so
you don't ever need to specify both attributes.
Any error type that implements std::error::Error or dereferences to dyn
std::error::Error will work as a source.
```rust
pub struct MyError {
msg: String,
#[source] // optional if field name is 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;
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.
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.