derive(Error)

This library provides a convenient derive macro for the standard library's std::error::Error trait.

[dependencies]
therror = "1.0"

Compiler support: requires rustc 1.56+

Example

use therror::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,
}

Details

• 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 therror 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:

  #[derive(Error, Debug)]
  pub enum Error {
      #[error("invalid rdo_lookahead_frames {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.

  #[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 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.

  #[derive(Error, Debug)]
  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.

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

• The Error trait's provide() method is implemented to provide whichever field has a type named Backtrace, if any, as a std::backtrace::Backtrace.

  use std::backtrace::Backtrace;
  
  #[derive(Error, Debug)]
  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.

  #[derive(Error, Debug)]
  pub enum MyError {
      Io {
          #[backtrace]
          source: io::Error,
      },
  }

• 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 therror 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.

A fork of therror

This fork was created with the idea of adding a killer-feature to therror which would definitely not make it into therror itself. We shall write more once it is more than an idea.

The idea is to start out at 0.X.Y where X is chosen to denote breaking changes, and Y is tracking the therror baseline while it matters. Once it becomes clear how therror differentiates itself, while retaining compatibility to therror for its baseline features, and once this differentiation becomes mature, we can continue increasing the major version to 2.0.0 and stand alone.

License