rollup merge of #23541: aturon/stab-error

This small commit stabilizes the `Error` trait as-is, except that `Send`
and `Debug` are added as constraints. The `Send` constraint is because
most uses of `Error` will be for trait objects, and by default we would
like these objects to be transferrable between threads. The `Debug`
constraint is to ensure that e.g. `Box<Error>` is `Debug`, and because
types that implement `Display` should certainly implement `Debug` in any case.

In the near future we expect to add `Any`-like downcasting features to
`Error`, but this is waiting on some additional
mechanisms (`Reflect`). It will be added before 1.0 via default methods.

[breaking-change]

r? @alexcrichton

Closes #21790

2 files changed, 6 insertions, 6 deletions

diff --git a/src/libstd/sync/mpsc/mod.rs b/src/libstd/sync/mpsc/mod.rs
index 123dad978f5..35125125d14 100644
--- a/src/libstd/sync/mpsc/mod.rs
+++ b/src/libstd/sync/mpsc/mod.rs
@@ -977,7 +977,7 @@ impl<T> fmt::Display for SendError<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> error::Error for SendError<T> {
+impl<T: Send> error::Error for SendError<T> {
 
     fn description(&self) -> &str {
         "sending on a closed channel"
@@ -1013,7 +1013,7 @@ impl<T> fmt::Display for TrySendError<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> error::Error for TrySendError<T> {
+impl<T: Send> error::Error for TrySendError<T> {
 
     fn description(&self) -> &str {
         match *self {
diff --git a/src/libstd/sync/poison.rs b/src/libstd/sync/poison.rs
index 2587ff5238e..c07c83d37f4 100644
--- a/src/libstd/sync/poison.rs
+++ b/src/libstd/sync/poison.rs
@@ -105,11 +105,11 @@ impl<T> fmt::Debug for PoisonError<T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> fmt::Display for PoisonError<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        self.description().fmt(f)
+        "poisoned lock: another task failed inside".fmt(f)
     }
 }
 
-impl<T> Error for PoisonError<T> {
+impl<T: Send> Error for PoisonError<T> {
     fn description(&self) -> &str {
         "poisoned lock: another task failed inside"
     }
@@ -161,13 +161,13 @@ impl<T> fmt::Debug for TryLockError<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> fmt::Display for TryLockError<T> {
+impl<T: Send> fmt::Display for TryLockError<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         self.description().fmt(f)
     }
 }
 
-impl<T> Error for TryLockError<T> {
+impl<T: Send> Error for TryLockError<T> {
     fn description(&self) -> &str {
         match *self {
             TryLockError::Poisoned(ref p) => p.description(),