Rollup merge of #21312 - michaelsproul:remove-error-send-bound, r=aturon

As discussed with @aturon, this PR removes the `Send` bound from `std::error::Error`, allowing us to implement `Error` for error types containing non-`Send` types. Current examples include `PoisonError` and `TryLockError` from `std::sync` which contain a Guard that we don't want sent between tasks.

[breaking-change]

1 files changed, 3 insertions, 3 deletions

diff --git a/src/libstd/io/mod.rs b/src/libstd/io/mod.rs
index e2b71cd43af..dc21416df7b 100644
--- a/src/libstd/io/mod.rs
+++ b/src/libstd/io/mod.rs
@@ -234,7 +234,7 @@ use error::{FromError, Error};
 use fmt;
 use int;
 use iter::{Iterator, IteratorExt};
-use marker::Sized;
+use marker::{Sized, Send};
 use mem::transmute;
 use ops::FnOnce;
 use option::Option;
@@ -363,8 +363,8 @@ impl Error for IoError {
     }
 }
 
-impl FromError<IoError> for Box<Error> {
-    fn from_error(err: IoError) -> Box<Error> {
+impl FromError<IoError> for Box<Error + Send> {
+    fn from_error(err: IoError) -> Box<Error + Send> {
         box err
     }
 }