Crate wherror

Expand description

The same derive(Error) macro thiserror provides + the features you want!

wherror = thiserror + WHERE your errors occurred 🎯

§Why Choose wherror Over thiserror?

wherror implements the most requested community features:

Feature	wherror	thiserror	Community Interest
Drop-in replacement for existing code	✅	✅	Zero migration effort
Automatically use `Debug` as `Display` with `#[error(debug)]`	✅	❌	#172 - not planned!
Call-site location tracking	✅	❌	#142 - 17👍 since 2021
`#[from(no_source)] T where T: !Error + Debug + Display`	✅	❌	wherror enhancements
Automatic `Box<T>` unwrapping	✅	❌	wherror enhancements
`.location()` method	✅	❌	wherror enhancements

Use wherror when you need these features today, with the same reliable API you know and love.

See the CHANGELOG

[dependencies]
wherror = "2"

§🎯 Instant Error Location Tracking

Know exactly where your errors originated with zero boilerplate:

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 `?` - no manual work required!
fn read_file() -> Result<String, MyError> {
    let content = std::fs::read_to_string("file.txt")?;  // 📍 Location captured here
    Ok(content)
}

§🚀 Debug Fallback - No More Boilerplate Messages

Sometimes your enum variant names are the error message. wherror lets you skip the redundant #[error("...")] attributes that thiserror forces you to write:

use wherror::Error;

#[derive(Error, Debug)]
#[error(debug)]  // 🎉 Fallback for variants without explicit messages
pub enum ValidationError {
    #[error("Email must contain @ symbol")]  // Custom message when needed
    InvalidEmail,

    // These use Debug formatting automatically - no boilerplate! ✨
    TooShort,
    TooLong,
    EmptyInput,
    InvalidCharacters { found: char, position: usize },
}

§Example: All Features in Action

use wherror::Error;

#[derive(Error, Debug)]
#[error(debug)]  // ✨ Fallback for variants without explicit messages
pub enum DataStoreError {
    #[error("data store disconnected at {location}")]  // 🎯 Location tracking
    Disconnect {
        #[from]
        source: io::Error,
        location: &'static std::panic::Location<'static>,  // Auto-captured
    },
    #[error("the data for key `{0}` is not available")]
    Redaction(String),
    // ✨ Use #[from(no_source)] for non-Error types, e.g. `String`!
    // Obviously, you can only have **one** variant with `#[from(no_source)] T` or `#[from] T`.
    String(#[from(no_source)] String),
    #[error("invalid header (expected {expected:?}, found {found:?})")]
    InvalidHeader { expected: String, found: String },
    // ✨ These use Debug formatting automatically - no #[error("...")] needed!
    Unknown,
    ConfigurationMissing,
    PermissionDenied { user_id: u64 },
}

§Quick Migration from thiserror

Step 1: Update your Cargo.toml:

[dependencies]
# thiserror = "2"  # Replace this
wherror = "2"       # With this

Step 2: Update imports:

// use thiserror::Error;  // Replace this
use wherror::Error;      // With this

Step 3: Your existing code works unchanged! Optionally add new features like location tracking.

§Detailed Features

wherror extends thiserror with community-requested features while maintaining thiserror API compatibility. All existing thiserror code works unchanged.

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:
```
#[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.
```
#[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.
```
#[derive(Error, Debug)]
pub enum MyError {
    Io(#[from] io::Error),
    Glob(#[from] globset::Error),
}
```
For Box<T> fields with #[from], both From<Box<T>> and From<T> implementations are automatically generated for enhanced ergonomics:
```
#[derive(Error, Debug)]
#[error("Error occurred")]
pub struct MyError {
    #[from]
    source: Box<io::Error>,
}
```
For non-Error types, use #[from(no_source)]:
```
#[derive(Error, Debug)]
pub enum MyError {
    #[error("HTTP {0}")]
    Http(#[from(no_source)] u16),

    #[error("IO: {0}")]
    Io(#[from] std::io::Error),
}
```
Fixing compile errors: If you see error[E0599]: the method as_dyn_error exists for reference &T, but its trait bounds were not satisfied, use #[from(no_source)] for non-Error types.

Use #[error(debug)] as a fallback to automatically generate Display implementations using the Debug format. This eliminates boilerplate when your enum variant names are already descriptive error messages.

This addresses the request in thiserror#172 for optional error messages, allowing you to skip redundant #[error("TooSmall")] when TooSmall is already a clear error name.

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:

#[derive(Error, Debug)]
#[error(debug)]  // fallback for variants without explicit messages
pub enum MyError {
    #[error("Custom message: {0}")]
    WithMessage(String),

    // These will use Debug formatting:
    Simple,
    Complex { code: u32, message: String },
    WithData(i32, String),
}

You can also apply #[error(debug)] to individual variants or struct types:

#[derive(Error, Debug)]
pub enum MyError {
    #[error("IO error: {0}")]
    Io(std::io::Error),

    #[error(debug)]  // This variant uses Debug formatting
    Other { details: String, code: i32 },
}

#[derive(Error, Debug)]
#[error(debug)]  // Entire struct uses Debug formatting
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.
```
#[derive(Error, Debug)]
pub struct MyError {
    msg: String,
    #[source]  // optional if field name is `source`
    source: anyhow::Error,
}
```
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.

wherror automatically generates a .location() method that returns Option<&'static std::panic::Location<'static>> for easy access to error origins.

This implements the feature requested in thiserror#142 (17👍).
```
#[derive(Error, Debug)]
#[error("Parse error at {location}: {source}")]
pub struct ParseError {
    #[from]
    source: std::num::ParseIntError,
    location: &'static std::panic::Location<'static>,  // automatically detected
}

fn example() -> Result<(), ParseError> {
    let _number: i32 = "not_a_number".parse()?;  // Location captured here automatically
    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.
ⓘ
```
#[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.
ⓘ
```
#[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.

#[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.

#[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.

// 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.

§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.

§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.}

§Attribution

^{Fork of thiserror by David Tolnay,
with location support by Angad Tendulkar
from thiserror#291.}

Derive Macros§

Error

Crate wherror

Crate wherror Copy item path

§Why Choose wherror Over thiserror?

§🎯 Instant Error Location Tracking

§🚀 Debug Fallback - No More Boilerplate Messages

§Example: All Features in Action

§Quick Migration from thiserror

§Detailed Features

§Comparison to anyhow

§License

§Attribution

Derive Macros§

Crate wherror