gecko-dev/third_party/rust/thiserror

derive(Error)

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

[dependencies]
thiserror = "1.0"

Compiler support: requires rustc 1.31+

Example

use thiserror::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 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)

  You may alternatively write out the full format args yourself, using arbitrary expressions.

  When providing your own format args, the shorthand does not kick in so you need to specify .var in the argument list to refer to named fields and .0 to refer to tuple fields.

  #[derive(Error, Debug)]
  pub enum Error {
      #[error("invalid rdo_lookahead_frames {} (expected < {})", .0, i32::max_value())]
      InvalidLookahead(i32),
  }
  

• 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 backtrace() method is implemented to return whichever field has a type named Backtrace, if any.

  use std::backtrace::Backtrace;
  
  #[derive(Error, Debug)]
  pub struct MyError {
      msg: String,
      backtrace: Backtrace, // automatically detected
  }
  

• See also the anyhow library for a convenient single error type to use in application code.

Comparison to anyhow

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.

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.}