mozilla-central/third_party/rust/thiserror/src/lib.rs (file symbol)

Source code

Revision control

Copy as Markdown

Other Tools

//!
//!
//! <br>
//!
//! This library provides a convenient derive macro for the standard library's
//! [`std::error::Error`] trait.
//!
//!
//! <br>
//!
//! # 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,
//! }
//! ```
//!
//! <br>
//!
//! # 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}")]`&ensp;⟶&ensp;`write!("{}", self.var)`
//! - `#[error("{0}")]`&ensp;⟶&ensp;`write!("{}", self.0)`
//! - `#[error("{var:?}")]`&ensp;⟶&ensp;`write!("{:?}", self.var)`
//! - `#[error("{0:?}")]`&ensp;⟶&ensp;`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 < {})", 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: anyhow::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 with Rust version 1.73 or newer.
//!
//! ```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 with Rust version 1.73 or newer.
//!
//! ```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] 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 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 [`anyhow`] library for a convenient single error type to use
//! in application code.
//!
#![doc(html_root_url = "https://docs.rs/thiserror/1.0.64")]
#![allow(
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.");
mod aserror;
mod display;
#[cfg(error_generic_member_access)]
mod provide;
pub use thiserror_impl::*;
// Not public API.
#[doc(hidden)]
pub mod __private {
#[doc(hidden)]
pub use crate::aserror::AsDynError;
#[doc(hidden)]
pub use crate::display::AsDisplay;
#[cfg(error_generic_member_access)]
#[doc(hidden)]
pub use crate::provide::ThiserrorProvide;
}