Add downcasting to std::error::Error

This commit brings the `Error` trait in line with the [Error interoperation
RFC](https://github.com/rust-lang/rfcs/pull/201) by adding downcasting,
which has long been intended. This change means that for any `Error`
trait objects that are `'static`, you can downcast to concrete error
types.

To make this work, it is necessary for `Error` to inherit from
`Reflect` (which is currently used to mark concrete types as "permitted
for reflection, aka downcasting"). This is a breaking change: it means
that impls like

```rust
impl<T> Error for MyErrorType<T> { ... }
```

must change to something like

```rust
impl<T: Reflect> Error for MyErrorType<T> { ... }
```

except that `Reflect` is currently unstable (and should remain so for
the time being). For now, code can instead bound by `Any`:

```rust
impl<T: Any> Error for MyErrorType<T> { ... }
```

which *is* stable and has `Reflect` as a super trait. The downside is
that this imposes a `'static` constraint, but that only
constrains *when* `Error` is implemented -- it does not actually
constrain the types that can implement `Error`.

[breaking-change]

src/libstd/error.rs

@@ -47,19 +47,22 @@
 // coherence challenge (e.g., specialization, neg impls, etc) we can
 // reconsider what crate these items belong in.
 
-use boxed::Box;
+use any::TypeId;
+use boxed::{self, Box};
 use convert::From;
 use fmt::{self, Debug, Display};
-use marker::{Send, Sync};
+use marker::{Send, Sync, Reflect};
+use mem::transmute;
 use num;
-use option::Option;
-use option::Option::None;
+use option::Option::{self, Some, None};
+use result::Result::{self, Ok, Err};
+use raw::TraitObject;
 use str;
 use string::{self, String};
 
 /// Base functionality for all errors in Rust.
 #[stable(feature = "rust1", since = "1.0.0")]
-pub trait Error: Debug + Display {
+pub trait Error: Debug + Display + Reflect {
     /// A short description of the error.
     ///
     /// The description should not contain newlines or sentence-ending
@@ -71,6 +74,14 @@ pub trait Error: Debug + Display {
     /// The lower-level cause of this error, if any.
     #[stable(feature = "rust1", since = "1.0.0")]
     fn cause(&self) -> Option<&Error> { None }
+
+    /// Get the `TypeId` of `self`
+    #[doc(hidden)]
+    #[unstable(feature = "core",
+               reason = "unclear whether to commit to this public implementation detail")]
+    fn type_id(&self) -> TypeId where Self: 'static {
+        TypeId::of::<Self>()
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -154,3 +165,112 @@ impl Error for string::FromUtf16Error {
     }
 }
 
+// copied from any.rs
+impl Error + 'static {
+    /// Returns true if the boxed type is the same as `T`
+    #[unstable(feature = "error_downcast", reason = "recently added")]
+    #[inline]
+    pub fn is<T: Error + '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.type_id();
+
+        // Compare both TypeIds on equality
+        t == boxed
+    }
+
+    /// Returns some reference to the boxed value if it is of type `T`, or
+    /// `None` if it isn't.
+    #[unstable(feature = "error_downcast", reason = "recently added")]
+    #[inline]
+    pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
+        if self.is::<T>() {
+            unsafe {
+                // Get the raw representation of the trait object
+                let to: TraitObject = transmute(self);
+
+                // Extract the data pointer
+                Some(transmute(to.data))
+            }
+        } else {
+            None
+        }
+    }
+
+    /// Returns some mutable reference to the boxed value if it is of type `T`, or
+    /// `None` if it isn't.
+    #[unstable(feature = "error_downcast", reason = "recently added")]
+    #[inline]
+    pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
+        if self.is::<T>() {
+            unsafe {
+                // Get the raw representation of the trait object
+                let to: TraitObject = transmute(self);
+
+                // Extract the data pointer
+                Some(transmute(to.data))
+            }
+        } else {
+            None
+        }
+    }
+}
+
+impl Error + 'static + Send {
+    /// Forwards to the method defined on the type `Any`.
+    #[unstable(feature = "error_downcast", reason = "recently added")]
+    #[inline]
+    pub fn is<T: Error + 'static>(&self) -> bool {
+        <Error + 'static>::is::<T>(self)
+    }
+
+    /// Forwards to the method defined on the type `Any`.
+    #[unstable(feature = "error_downcast", reason = "recently added")]
+    #[inline]
+    pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
+        <Error + 'static>::downcast_ref::<T>(self)
+    }
+
+    /// Forwards to the method defined on the type `Any`.
+    #[unstable(feature = "error_downcast", reason = "recently added")]
+    #[inline]
+    pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
+        <Error + 'static>::downcast_mut::<T>(self)
+    }
+}
+
+impl Error {
+    #[inline]
+    #[unstable(feature = "error_downcast", reason = "recently added")]
+    /// Attempt to downcast the box to a concrete type.
+    pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<Error>> {
+        if self.is::<T>() {
+            unsafe {
+                // Get the raw representation of the trait object
+                let raw = boxed::into_raw(self);
+                let to: TraitObject =
+                    transmute::<*mut Error, TraitObject>(raw);
+
+                // Extract the data pointer
+                Ok(Box::from_raw(to.data as *mut T))
+            }
+        } else {
+            Err(self)
+        }
+    }
+}
+
+impl Error + Send {
+    #[inline]
+    #[unstable(feature = "error_downcast", reason = "recently added")]
+    /// Attempt to downcast the box to a concrete type.
+    pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<Error + Send>> {
+        let err: Box<Error> = self;
+        <Error>::downcast(err).map_err(|s| unsafe {
+            // reapply the Send marker
+            transmute::<Box<Error>, Box<Error + Send>>(s)
+        })
+    }
+}