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.

Author: aturon

src/libcore/any.rs

@@ -114,6 +114,9 @@ pub trait AnyRefExt<'a> {
     fn downcast_ref<T: 'static>(self) -> Option<&'a T>;
 }
 
+// Note: this code is copied to std::error, and should be unified as
+// soon as possible.
+
 #[stable]
 impl<'a> AnyRefExt<'a> for &'a Any {
     #[inline]

src/libcore/prelude.rs

@@ -37,8 +37,6 @@ pub use ops::{Shl, Shr};
 pub use ops::{Index, IndexMut};
 pub use ops::{Slice, SliceMut};
 pub use ops::{Fn, FnMut, FnOnce};
-pub use option::{Option, Some, None};
-pub use result::{Result, Ok, Err};
 
 // Reexported functions
 pub use iter::range;
@@ -57,7 +55,9 @@ pub use iter::{OrdIterator, MutableDoubleEndedIterator, ExactSize};
 pub use num::{Num, NumCast, CheckedAdd, CheckedSub, CheckedMul};
 pub use num::{Signed, Unsigned, Float};
 pub use num::{Primitive, Int, ToPrimitive, FromPrimitive};
+pub use option::{Option, Some, None};
 pub use ptr::RawPtr;
+pub use result::{Result, Ok, Err};
 pub use str::{Str, StrSlice};
 pub use tuple::{Tuple1, Tuple2, Tuple3, Tuple4};
 pub use tuple::{Tuple5, Tuple6, Tuple7, Tuple8};

src/libstd/error.rs

@@ -0,0 +1,137 @@
+// 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:
+//!
+//! ```
+//! pub trait Error: Send + Any {
+//!     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.
+
+use any::{Any, AnyRefExt, AnyMutRefExt};
+use mem::{transmute, transmute_copy};
+use option::{Option, Some, None};
+use raw::TraitObject;
+use intrinsics::TypeId;
+use kinds::Send;
+use string::String;
+
+/// Base functionality for all errors in Rust.
+pub trait Error: Send + Any {
+    /// 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
+    }
+}
+
+// FIXME (#https://github.com/rust-lang/rust/pull/17669/): Add this once multidispatch lands
+// impl<E: Error> FromError<E> for Box<Error> {
+//     fn from_err(err: E) -> Box<Error> {
+//         box err as Box<Error>
+//     }
+// }
+
+// Note: the definitions below are copied from core::any, and should be unified
+// as soon as possible.
+
+impl<'a> AnyRefExt<'a> for &'a Error {
+    #[inline]
+    fn is<T: 'static>(self) -> bool {
+        // Get TypeId of the type this function is instantiated with
+        let t = TypeId::of::<T>();
+
+        // Get TypeId of the type in the trait object
+        let boxed = self.get_type_id();
+
+        // Compare both TypeIds on equality
+        t == boxed
+    }
+
+    #[inline]
+    fn downcast_ref<T: 'static>(self) -> Option<&'a T> {
+        if self.is::<T>() {
+            unsafe {
+                // Get the raw representation of the trait object
+                let to: TraitObject = transmute_copy(&self);
+
+                // Extract the data pointer
+                Some(transmute(to.data))
+            }
+        } else {
+            None
+        }
+    }
+}
+
+impl<'a> AnyMutRefExt<'a> for &'a mut Error {
+    #[inline]
+    fn downcast_mut<T: 'static>(self) -> Option<&'a mut T> {
+        if self.is::<T>() {
+            unsafe {
+                // Get the raw representation of the trait object
+                let to: TraitObject = transmute_copy(&self);
+
+                // Extract the data pointer
+                Some(transmute(to.data))
+            }
+        } else {
+            None
+        }
+    }
+}

src/libstd/lib.rs

@@ -218,6 +218,7 @@ pub mod time;
 
 /* Common traits */
 
+pub mod error;
 pub mod from_str;
 pub mod num;
 pub mod to_string;

src/libstd/prelude.rs

@@ -48,8 +48,6 @@
 #[doc(no_inline)] pub use ops::{Index, IndexMut};
 #[doc(no_inline)] pub use ops::{Slice, SliceMut};
 #[doc(no_inline)] pub use ops::{Fn, FnMut, FnOnce};
-#[doc(no_inline)] pub use option::{Option, Some, None};
-#[doc(no_inline)] pub use result::{Result, Ok, Err};
 
 // Reexported functions
 #[doc(no_inline)] pub use from_str::from_str;
@@ -75,8 +73,10 @@
 #[doc(no_inline)] pub use num::{Signed, Unsigned, Primitive, Int, Float};
 #[doc(no_inline)] pub use num::{FloatMath, ToPrimitive, FromPrimitive};
 #[doc(no_inline)] pub use boxed::Box;
+#[doc(no_inline)] pub use option::{Option, Some, None};
 #[doc(no_inline)] pub use path::{GenericPath, Path, PosixPath, WindowsPath};
 #[doc(no_inline)] pub use ptr::{RawPtr, RawMutPtr};
+#[doc(no_inline)] pub use result::{Result, Ok, Err};
 #[doc(no_inline)] pub use io::{Buffer, Writer, Reader, Seek};
 #[doc(no_inline)] pub use str::{Str, StrVector, StrSlice};
 #[doc(no_inline)] pub use str::{IntoMaybeOwned, StrAllocating, UnicodeStrSlice};