v2.0.0

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Breaking changes

• Referencing keyword-named fields by a raw identifier like {r#type} inside a format string is no longer accepted; simply use the unraw name like {type} (#​347)

  This aligns thiserror with the standard library's formatting macros, which gained support for implicit argument capture later than the release of this feature in thiserror 1.x.

  #[derive(Error, Debug)]
  #[error("... {type} ...")]  // Before: {r#type}
  pub struct Error {
      pub r#type: Type,
  }

• Trait bounds are no longer inferred on fields whose value is shadowed by an explicit named argument in a format message (#​345)

  // Before: impl<T: Octal> Display for Error<T>
  // After: impl<T> Display for Error<T>
  #[derive(Error, Debug)]
  #[error("{thing:o}", thing = "...")]
  pub struct Error<T> {
      thing: T,
  }

• Tuple structs and tuple variants can no longer use numerical {0} {1} access at the same time as supplying extra positional arguments for a format message, as this makes it ambiguous whether the number refers to a tuple field vs a different positional arg (#​354)

  #[derive(Error, Debug)]
  #[error("ambiguous: {0} {}", $N)]
  //                  ^^^ Not allowed, use #[error("... {0} {n}", n = $N)]
  pub struct TupleError(i32);

• Code containing invocations of thiserror's derive(Error) must now have a direct dependency on the thiserror crate regardless of the error data structure's contents (#​368, #​369, #​370, #​372)

Features

• Support disabling thiserror's standard library dependency by disabling the default "std" Cargo feature: thiserror = { version = "2", default-features = false } (#​373)

• Support using r#source as field name to opt out of a field named "source" being treated as an error's Error::source() (#​350)

  #[derive(Error, Debug)]
  #[error("{source} ==> {destination}")]
  pub struct Error {
      r#source: char,
      destination: char,
  }
  
  let error = Error { source: 'S', destination: 'D' };

• Infinite recursion in a generated Display impl now produces an unconditional_recursion warning (#​359)

  #[derive(Error, Debug)]
  #[error("??? {self}")]
  pub struct Error;

• A new attribute #[error(fmt = path::to::myfmt)] can be used to write formatting logic for an enum variant out-of-line (#​367)

  #[derive(Error, Debug)]
  pub enum Error {
      #[error(fmt = demo_fmt)]
      Demo { code: u16, message: Option<String> },
  }
  
  fn demo_fmt(code: &u16, message: &Option<String>, formatter: &mut fmt::Formatter) -> fmt::Result {
      write!(formatter, "{code}")?;
      if let Some(msg) = message {
          write!(formatter, " - {msg}")?;
      }
      Ok(())
  }

• Enums with an enum-level format message are now able to have individual variants that are transparent to supersede the enum-level message (#​366)

  #[derive(Error, Debug)]
  #[error("my error {0}")]
  pub enum Error {
      Json(#[from] serde_json::Error),
      Yaml(#[from] serde_yaml::Error),
      #[error(transparent)]
      Other(#[from] anyhow::Error),
  }