Add error module with Error and FromError traits

As per [RFC 70](https://github.com/rust-lang/rfcs/blob/master/active/0070-error-chaining.md)

Closes #17747

Note that the `error` module must live in `std` in order to refer to `String`.

Note that, until multidispatch lands, the `FromError` trait cannot be
usefully implemented outside of the blanket impl given here.

1 files changed, 111 insertions, 0 deletions

diff --git a/src/libstd/error.rs b/src/libstd/error.rs
new file mode 100644
index 00000000000..6bb9f4b473b
--- /dev/null
+++ b/src/libstd/error.rs
@@ -0,0 +1,111 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Traits for working with Errors.
+//!
+//! # The `Error` trait
+//!
+//! `Error` is a trait representing the basic expectations for error values,
+//! i.e. values of type `E` in `Result<T, E>`. At a minimum, errors must provide
+//! a description, but they may optionally provide additional detail and cause
+//! chain information:
+//!
+//! ```
+//! trait Error: Send {
+//!     fn description(&self) -> &str;
+//!
+//!     fn detail(&self) -> Option<String> { None }
+//!     fn cause(&self) -> Option<&Error> { None }
+//! }
+//! ```
+//!
+//! The `cause` method is generally used when errors cross "abstraction
+//! boundaries", i.e.  when a one module must report an error that is "caused"
+//! by an error from a lower-level module. This setup makes it possible for the
+//! high-level module to provide its own errors that do not commit to any
+//! particular implementation, but also reveal some of its implementation for
+//! debugging via `cause` chains.
+//!
+//! The trait inherits from `Any` to allow *downcasting*: converting from a
+//! trait object to a specific concrete type when applicable.
+//!
+//! # The `FromError` trait
+//!
+//! `FromError` is a simple trait that expresses conversions between different
+//! error types. To provide maximum flexibility, it does not require either of
+//! the types to actually implement the `Error` trait, although this will be the
+//! common case.
+//!
+//! The main use of this trait is in the `try!` macro, which uses it to
+//! automatically convert a given error to the error specified in a function's
+//! return type.
+//!
+//! For example,
+//!
+//! ```
+//! use std::error::FromError;
+//! use std::io::{File, IoError};
+//! use std::os::{MemoryMap, MapError};
+//! use std::path::Path;
+//!
+//! enum MyError {
+//!     Io(IoError),
+//!     Map(MapError)
+//! }
+//!
+//! impl FromError<IoError> for MyError {
+//!     fn from_error(err: IoError) -> MyError {
+//!         Io(err)
+//!     }
+//! }
+//!
+//! impl FromError<MapError> for MyError {
+//!     fn from_error(err: MapError) -> MyError {
+//!         Map(err)
+//!     }
+//! }
+//!
+//! #[allow(unused_variables)]
+//! fn open_and_map() -> Result<(), MyError> {
+//!     let f = try!(File::open(&Path::new("foo.txt")));
+//!     let m = try!(MemoryMap::new(0, &[]));
+//!     // do something interesting here...
+//!     Ok(())
+//! }
+//! ```
+
+use option::{Option, None};
+use kinds::Send;
+use string::String;
+
+/// Base functionality for all errors in Rust.
+pub trait Error: Send {
+    /// A short description of the error; usually a static string.
+    fn description(&self) -> &str;
+
+    /// A detailed description of the error, usually including dynamic information.
+    fn detail(&self) -> Option<String> { None }
+
+    /// The lower-level cause of this error, if any.
+    fn cause(&self) -> Option<&Error> { None }
+}
+
+/// A trait for types that can be converted from a given error type `E`.
+pub trait FromError<E> {
+    /// Perform the conversion.
+    fn from_error(err: E) -> Self;
+}
+
+// Any type is convertable from itself
+impl<E> FromError<E> for E {
+    fn from_error(err: E) -> E {
+        err
+    }
+}