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/*! This crate provides macros for deriving some useful methods and traits for Error enums. All of these macros are designed to be used with the [`custom_derive`](https://crates.io/crates/custom_derive) crate, though they can be used independent of it. # Example Derive implementation of the standard `Error` trait. This also provides the requisite `Display` implementation. ```rust #[macro_use] extern crate custom_derive; #[macro_use] extern crate error_derive; # use std::error::Error as StdError; # use std::io; # use std::str::{from_utf8, Utf8Error}; custom_derive! { #[derive(Debug, ErrorFrom, ErrorDisplay, Error("very bad error"))] pub enum Error { Io(io::Error), Utf8(Utf8Error), } } # fn main() { let io_error = Error::Io(io::Error::last_os_error()); let utf8_error = Error::Utf8(from_utf8(&[0, 142]).unwrap_err()); assert_eq!("very bad error", io_error.description()); assert_eq!("very bad error", utf8_error.description()); assert!(io_error.cause().is_some()); assert!(utf8_error.cause().is_some()); # } ``` # Overview This crate allows to derive the following traits: - `ErrorFrom`, which creates `From` trait implementations from each enum variants that wraps an inner `Error` - `ErrorDisplay`, which creates a `Display` trait implementation showing the entire causal chain for an error - `Error`, which implements the standard `Error` trait with a given description `Error` and `ErrorDisplay` are typically derived together, though either can of course be implemented separately. # Usage without `custom_derive!` Although designed to be used with `custom_derive!`, all of the macros in this crate can be used without it. The following: ```rust # #[macro_use] extern crate custom_derive; # #[macro_use] extern crate error_derive; # use std::io; custom_derive! { #[derive(Debug, ErrorDisplay, Error("just I/O error"))] pub enum JustIoError { ThisOne(io::Error) } } # fn main() {} ``` Can also be writtten as: ```rust # #[macro_use] extern crate custom_derive; # #[macro_use] extern crate error_derive; # use std::io; #[derive(Debug)] pub enum JustIoError { ThisOne(io::Error) } ErrorDisplay! { () pub enum JustIoError { ThisOne(io::Error) } } Error! { ("just I/O error") pub enum JustIoError { ThisOne(io::Error) } } # fn main() {} ``` */ #![doc(html_root_uri = "https://xion.github.io/rust-error-derive")] #[macro_use] mod util; #[macro_export] macro_rules! ErrorFrom { (@expand $name:ident ($($var_names:ident($var_tys:ty),)*)) => { $( impl ::std::convert::From<$var_tys> for $name { fn from(v: $var_tys) -> $name { $name::$var_names(v) } } )* }; (() $(pub)* enum $name:ident { $($body:tt)* }) => { enum_derive_util! { @collect_unary_variants (ErrorFrom { @expand $name }), ($($body)*,) -> () } }; } #[macro_export] macro_rules! ErrorDisplay { (@expand $name:ident ($($var_names:ident($var_tys:ty),)*)) => { impl ::std::fmt::Display for $name { fn fmt(&self, fmt: &mut ::std::fmt::Formatter) -> ::std::fmt::Result { try!(write!(fmt, "{}", ::std::error::Error::description(self))); if let Some(cause) = ::std::error::Error::cause(self) { try!(write!(fmt, "\n-- caused by {}", cause)); } Ok(()) } } }; (() $(pub)* enum $name:ident { $($body:tt)* }) => { enum_derive_util! { @collect_unary_variants (ErrorDisplay { @expand $name }), ($($body)*,) -> () } }; } #[macro_export] macro_rules! Error { (@expand $desc:expr, $name:ident ($($var_names:ident($var_tys:ty),)*)) => { impl ::std::error::Error for $name { fn description(&self) -> &str { $desc } fn cause(&self) -> Option<&::std::error::Error> { match *self { $( $name::$var_names(ref c) => Some(c), )* } } } }; (($desc:expr) $(pub)* enum $name:ident { $($body:tt)* }) => { enum_derive_util! { @collect_unary_variants (Error { @expand $desc, $name }), ($($body)*,) -> () } }; }