# wherror
[![github]](https://github.com/dra11y/wherror) [![crates-io]](https://crates.io/crates/wherror) [![docs-rs]](https://docs.rs/wherror)
[github]: https://img.shields.io/badge/github-dra11y/wherror-8da0cb?style=for-the-badge&labelColor=555555&logo=github
[crates-io]: https://img.shields.io/crates/v/wherror.svg?style=for-the-badge&color=fc8d62&logo=rust
[docs-rs]: https://img.shields.io/badge/docs.rs-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs
<br>
Fork of [thiserror] `derive(Error)` with [`std::panic::Location`]
support. This library provides a convenient derive macro for the standard library's
[`std::error::Error`] trait.
This fork was created to add location support ([thiserror#291]) and additional convenience features.
[`std::error::Error`]: https://doc.rust-lang.org/std/error/trait.Error.html
[`std::panic::Location`]: https://doc.rust-lang.org/std/panic/struct.Location.html
[thiserror]: https://github.com/dtolnay/thiserror
[thiserror#291]: https://github.com/dtolnay/thiserror/pull/291
```toml
[dependencies]
wherror = "2"
```
### Location Support
Add a field of type `&'static std::panic::Location<'static>` to automatically capture where errors are created:
```rust
use wherror::Error;
#[derive(Error, Debug)]
#[error("Failed at {location}: {source}")]
pub struct MyError {
#[from]
source: std::io::Error,
location: &'static std::panic::Location<'static>, // Auto-populated
}
// Location automatically captured when using `?`
fn read_file() -> Result<String, MyError> {
let content = std::fs::read_to_string("file.txt")?;
Ok(content)
}
```
<br>
## Example
```rust
use wherror::Error;
#[derive(Error, Debug)]
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,
}
```
<br>
## Details
Wherror 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
# use core::i32;
# use wherror::Error;
#
#[derive(Error, Debug)]
pub enum Error {
#[error("invalid rdo_lookahead_frames {0} (expected < {max})", max = 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
# use wherror::Error;
#
# fn first_char(s: &String) -> char {
# s.chars().next().unwrap()
# }
#
# #[derive(Debug)]
# struct Limits {
# lo: usize,
# hi: usize,
# }
#
#[derive(Error, Debug)]
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 that contains a `#[from]`
attribute.
The variant using `#[from]` must not contain any other fields beyond the
source error (and possibly a location or backtrace — see below). Usually `#[from]`
fields are unnamed, but `#[from]` is allowed on a named field too.
```rust
# use core::fmt::{self, Display};
# use std::io;
# use wherror::Error;
#
# mod globset {
# #[derive(wherror::Error, Debug)]
# #[error("...")]
# pub struct Error;
# }
#
#[derive(Error, Debug)]
pub enum MyError {
Io(#[from] io::Error),
Glob(#[from] globset::Error),
}
#
# impl Display for MyError {
# fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
# unimplemented!()
# }
# }
```
For `Box<T>` fields with `#[from]`, both `From<Box<T>>` and `From<T>`
implementations are automatically generated for enhanced ergonomics:
```rust
# use std::io;
# use wherror::Error;
#
#[derive(Error, Debug)]
#[error("Error occurred")]
pub struct MyError {
#[from]
source: Box<io::Error>,
}
#
# fn example() -> Result<(), Box<dyn std::error::Error>> {
# let io_error = io::Error::new(io::ErrorKind::Other, "test");
#
# # let err1: MyError = Box::new(io_error).into();
# let err2: MyError = io::Error::new(io::ErrorKind::Other, "test").into(); # Ok(())
# }
```
- Use `#[error(debug)]` as a fallback to automatically generate Display
implementations using the Debug format. This is useful for rapid prototyping
or when you want to use the standard Debug representation.
For enums, you can apply `#[error(debug)]` at the type level to automatically
generate Display for all variants that don't have explicit `#[error("...")]`
messages:
```rust
# use wherror::Error;
#
#[derive(Error, Debug)]
#[error(debug)] pub enum MyError {
#[error("Custom message: {0}")]
WithMessage(String),
Simple,
Complex { code: u32, message: String },
WithData(i32, String),
}
```
You can also apply `#[error(debug)]` to individual variants or struct types:
```rust
# use wherror::Error;
#
#[derive(Error, Debug)]
pub enum MyError {
#[error("IO error: {0}")]
Io(std::io::Error),
#[error(debug)] Other { details: String, code: i32 },
}
#[derive(Error, Debug)]
#[error(debug)] pub struct DebugError {
message: String,
code: u32,
}
```
- 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
# use core::fmt::{self, Display};
# use wherror::Error;
#
#[derive(Error, Debug)]
pub struct MyError {
msg: String,
#[source] source: anyhow::Error,
}
#
# impl Display for MyError {
# fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
# unimplemented!()
# }
# }
```
- Fields of type `&'static std::panic::Location<'static>` are automatically
populated with the call site location when errors are created via `From` trait
conversion. This works seamlessly with the `?` operator for precise error tracking.
For errors with location fields, a `location()` method is generated that returns
`Option<&'static std::panic::Location<'static>>`.
```rust
# use wherror::Error;
#
#[derive(Error, Debug)]
#[error("Parse error at {location}: {source}")]
pub struct ParseError {
#[from]
source: std::num::ParseIntError,
location: &'static std::panic::Location<'static>, }
#
# fn example() -> Result<(), ParseError> {
# let result: Result<i32, _> = "not_a_number".parse();
# let error: ParseError = result.unwrap_err().into();
# if let Some(location) = error.location() {
# eprintln!("Error at {}:{}", location.file(), location.line());
# }
# Ok(())
# }
```
- The Error trait's `provide()` method is implemented to provide whichever field
has a type named `Backtrace`, if any, as a `std::backtrace::Backtrace`. Using
`Backtrace` in errors requires a nightly compiler with Rust version 1.73 or
newer.
```rust,ignore
# use std::backtrace::Backtrace;
# use wherror::Error;
#
#[derive(Error, Debug)]
#[error("Something went wrong: {msg}")]
pub struct MyError {
msg: String,
backtrace: Backtrace, // automatically detected
}
```
- If a field is both a source (named `source`, or has `#[source]` or `#[from]`
attribute) *and* is marked `#[backtrace]`, then the Error trait's `provide()`
method is forwarded to the source's `provide` so that both layers of the error
share the same backtrace. The `#[backtrace]` attribute requires a nightly
compiler with Rust version 1.73 or newer.
```rust,ignore
# use std::io;
# use wherror::Error;
#
#[derive(Error, Debug)]
#[error("IO error occurred")]
pub enum MyError {
Io {
#[backtrace]
source: io::Error,
},
}
```
- For variants that use `#[from]` and also contain a `Backtrace` field, a
backtrace is captured from within the `From` impl.
```rust,ignore
# use std::backtrace::Backtrace;
# use std::io;
# use wherror::Error;
#
#[derive(Error, Debug)]
#[error("IO error occurred")]
pub enum MyError {
Io {
#[from]
source: io::Error,
backtrace: Backtrace,
},
}
```
- 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.
```
# use wherror::Error;
#
#[derive(Error, Debug)]
pub enum MyError {
# /*
...
# */
#[error(transparent)]
Other(#[from] anyhow::Error), // source and Display delegate to anyhow::Error
}
```
Another use case is hiding implementation details of an error representation
behind an opaque error type, so that the representation is able to evolve
without breaking the crate's public API.
```
# use wherror::Error;
#
// PublicError is public, but opaque and easy to keep compatible.
#[derive(Error, Debug)]
#[error(transparent)]
pub struct PublicError(#[from] ErrorRepr);
impl PublicError {
// Accessors for anything we do want to expose publicly.
}
// Private and free to change across minor version of the crate.
#[derive(Error, Debug)]
enum ErrorRepr {
# /*
...
# */
}
```
- See also the [`anyhow`] library for a convenient single error type to use in
application code.
[`anyhow`]: https://github.com/dtolnay/anyhow
<br>
### Comparison to anyhow
Use wherror 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.
[Anyhow]: https://github.com/dtolnay/anyhow
<br>
#### License
<sup>
Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
</sup>
<br>
<sub>
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.
</sub>
<br>
#### Attribution
<sup>
Fork of <a href="https://github.com/dtolnay/thiserror">thiserror</a> by David Tolnay,
with location support by <a href="https://github.com/onlycs">Angad Tendulkar</a>
from <a href="https://github.com/dtolnay/thiserror/pull/291">thiserror#291</a>.
</sup>
License: MIT OR Apache-2.0