wherror 2.4.0

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

wherror

github crates-io docs-rs changelog

wherror is a fork of thiserror by David Tolnay. It periodically merges in thiserror's fixes and improvements.

wherror = thiserror + WHERE your errors occurred 🎯 + zero-boilerplate Debug fallback 🚀

What wherror Adds

wherror implements features requested by the community:

  • Automatically use Debug as Display with #[error(debug)] — thiserror does not plan to add this
  • Call-site location tracking — requested since 2021
  • #[from(no_source)] T where T: !Error + Debug + Display
  • Automatic Box<T> unwrapping
  • .location() method

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:

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.

  • 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 wherror 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:

    # 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.

    # 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.

    # 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:

    # 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");
#
#     // Both work:
#     let err1: MyError = Box::new(io_error).into();
#     let err2: MyError = io::Error::new(io::ErrorKind::Other, "test").into();  // automatically boxed
#     Ok(())
# }

    For non-Error types, use #[from(no_source)]:

    # use wherror::Error;
#
#[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:

    # use wherror::Error;
#
#[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:

    # use wherror::Error;
#
#[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.

    # use core::fmt::{self, Display};
# use wherror::Error;
#
#[derive(Error, Debug)]
pub struct MyError {
    msg: String,
    #[source]  // optional if field name is `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.

    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👍).

    # 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>,  // automatically detected
}

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

# fn demonstrate_usage() {
#     if let Err(error) = example() {
#         // Access the location where the error occurred
#         if let Some(location) = error.location() {
#             eprintln!("Error occurred at {}:{}", location.file(), location.line());
#         }
#     }
# }

  • 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.

    # 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.

    # 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.

    # 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.

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

Attribution

License: MIT OR Apache-2.0