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]

1 files changed, 3 insertions, 3 deletions

diff --git a/src/libstd/sync/mpsc/mod.rs b/src/libstd/sync/mpsc/mod.rs
index 61932225d79..965ad74fb60 100644
--- a/src/libstd/sync/mpsc/mod.rs
+++ b/src/libstd/sync/mpsc/mod.rs
@@ -272,6 +272,7 @@ use error;
 use fmt;
 use mem;
 use cell::UnsafeCell;
+use marker::Reflect;
 
 pub use self::select::{Select, Handle};
 use self::select::StartResult;
@@ -955,8 +956,7 @@ impl<T> fmt::Display for SendError<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Send> error::Error for SendError<T> {
-
+impl<T: Send + Reflect> error::Error for SendError<T> {
     fn description(&self) -> &str {
         "sending on a closed channel"
     }
@@ -991,7 +991,7 @@ impl<T> fmt::Display for TrySendError<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Send> error::Error for TrySendError<T> {
+impl<T: Send + Reflect> error::Error for TrySendError<T> {
 
     fn description(&self) -> &str {
         match *self {