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, 4 insertions, 3 deletions

diff --git a/src/libstd/sys/common/poison.rs b/src/libstd/sys/common/poison.rs
index 6deb4a48007..6c59231c23a 100644
--- a/src/libstd/sys/common/poison.rs
+++ b/src/libstd/sys/common/poison.rs
@@ -10,6 +10,7 @@
 
 use prelude::v1::*;
 
+use marker::Reflect;
 use cell::UnsafeCell;
 use error::{Error};
 use fmt;
@@ -109,7 +110,7 @@ impl<T> fmt::Display for PoisonError<T> {
     }
 }
 
-impl<T: Send> Error for PoisonError<T> {
+impl<T: Send + Reflect> Error for PoisonError<T> {
     fn description(&self) -> &str {
         "poisoned lock: another task failed inside"
     }
@@ -155,13 +156,13 @@ impl<T> fmt::Debug for TryLockError<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Send> fmt::Display for TryLockError<T> {
+impl<T: Send + Reflect> fmt::Display for TryLockError<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         self.description().fmt(f)
     }
 }
 
-impl<T: Send> Error for TryLockError<T> {
+impl<T: Send + Reflect> Error for TryLockError<T> {
     fn description(&self) -> &str {
         match *self {
             TryLockError::Poisoned(ref p) => p.description(),