Auto merge of #24793 - aturon:io-error-any, r=alexcrichton

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

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

This commit furthermore marks `Reflect` as stable, since we are already
essentially committed to it via `Any`. Note that in the future, if we
determine that the parametricity aspects of `Reflect` are not needed, we
can deprecate the trait and provide a blanket implementation for it
for *all* types (rather than by using OIBIT), which would allow all
mentions of `Reflect` to be dropped over time. So there is not a strong
commitment here.

[breaking-change]

r? @alexcrichton 

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 {