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)These shorthands can be used together with any additional format args, which may be arbitrary expressions. For example:
```rust
pub enum Error { #[error("invalid rdolookaheadframes {0} (expected < {})", i32::MAX)] InvalidLookahead(u32), } ```
If one of the additional expression arguments needs to refer to a field of the
struct or enum, then refer to named fields as .var and tuple fields as .0.
```rust
pub enum Error { #[error("first letter must be lowercase but was {:?}", first_char(.0))] WrongCase(String), #[error("invalid index {idx}, expected at least {} and at most {}", .limits.lo, .limits.hi)] OutOfBounds { idx: usize, limits: Limits }, } ```
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 } ```
Errors may use error(transparent) to forward the source and Display methods
straight through to an underlying error without adding an additional message.
This would be appropriate for enums that need an "anything else" variant.
```rust
pub enum MyError { ...
#[error(transparent)] Other(#[from] anyhow::Error), // source and Display delegate to anyhow::Error } ```
See also the [anyhow] library for a convenient single error type to use in
application code.
Use thiserror if you care about designing your own dedicated error type(s) so that the caller receives exactly the information that you choose in the event of failure. This most often applies to library-like code. Use [Anyhow] if you don't care what error type your functions return, you just want it to be easy. This is common in application-like 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.