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]

Conflicts:
	src/libcore/marker.rs

1 files changed, 8 insertions, 3 deletions

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index 7696abd659f..7a089d733cf 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -240,6 +240,7 @@ impl<T: ?Sized + Hash> Hash for Box<T> {
 impl Box<Any> {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
+    /// Attempt to downcast the box to a concrete type.
     pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
         if self.is::<T>() {
             unsafe {
@@ -257,11 +258,15 @@ impl Box<Any> {
     }
 }
 
-impl Box<Any+Send> {
+impl Box<Any + Send> {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
-        <Box<Any>>::downcast(self)
+    /// Attempt to downcast the box to a concrete type.
+    pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any + Send>> {
+        <Box<Any>>::downcast(self).map_err(|s| unsafe {
+            // reapply the Send marker
+            mem::transmute::<Box<Any>, Box<Any + Send>>(s)
+        })
     }
 }