thiserror/

lib.rs

//! A convenient derive macro for the standard library's [`std::error::Error`]
//! and [`core::error::Error`] traits.
//!
//! # Example
//!
//! ```rust
//! # use std::io;
//! 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)`
//!
//!   These shorthands can be used together with any additional format args,
//!   which may be arbitrary expressions. For example:
//!
//!   ```rust
//!   # use core::i32;
//!   # use thiserror::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`.
//!
//!   ```rust
//!   # use thiserror::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 backtrace &mdash; see below). Usually
//!   `#[from]` fields are unnamed, but `#[from]` is allowed on a named field
//!   too.
//!
//!   ```rust
//!   # use core::fmt::{self, Display};
//!   # use std::io;
//!   # use thiserror::Error;
//!   #
//!   # mod globset {
//!   #     #[derive(thiserror::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!()
//!   #     }
//!   # }
//!   ```
//!
//! - 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.
//!
//!   ```rust
//!   # use core::fmt::{self, Display};
//!   # use thiserror::Error;
//!   #
//!   #[derive(Error, Debug)]
//!   pub struct MyError {
//!       msg: String,
//!       #[source]  // optional if field name is `source`
//!       source: anyerr::Error,
//!   }
//!   #
//!   # impl Display for MyError {
//!   #     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
//!   #         unimplemented!()
//!   #     }
//!   # }
//!   ```
//!
//! - 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.
//!
//!   ```rust
//!   # const IGNORE: &str = stringify! {
//!   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. The `#[backtrace]`
//!   attribute requires a nightly compiler.
//!
//!   ```rust
//!   # const IGNORE: &str = stringify! {
//!   #[derive(Error, Debug)]
//!   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.
//!
//!   ```rust
//!   # const IGNORE: &str = stringify! {
//!   #[derive(Error, Debug)]
//!   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 thiserror::Error;
//!   #
//!   #[derive(Error, Debug)]
//!   pub enum MyError {
//!       # /*
//!       ...
//!       # */
//!
//!       #[error(transparent)]
//!       Other(#[from] anyerr::Error),  // source and Display delegate to anyerr::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 thiserror::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 [`anyerr`] library for a convenient single error type to use
//!   in application code.
//!
//! [`anyerr`]: https://github.com/rust-stdx/stdx
//! [`source()`]: std::error::Error::source
//! [`provide()`]: std::error::Error::provide
//! [`Display`]: std::fmt::Display

#![no_std]
#![allow(
    clippy::elidable_lifetime_names,
    clippy::module_name_repetitions,
    clippy::needless_lifetimes,
    clippy::return_self_not_must_use,
    clippy::wildcard_imports
)]
#![cfg_attr(error_generic_member_access, feature(error_generic_member_access))]

#[cfg(all(thiserror_nightly_testing, not(error_generic_member_access)))]
compile_error!("Build script probe failed to compile.");

#[cfg(feature = "std")]
extern crate std;
#[cfg(feature = "std")]
extern crate std as core;

mod aserror;
mod display;
#[cfg(error_generic_member_access)]
mod provide;
mod var;

pub use thiserror_impl::*;

mod private;

include!(concat!(env!("OUT_DIR"), "/private.rs"));