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

src/libcore/error.rs

@@ -82,16 +82,21 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use prelude::*;
-use fmt::Display;
+use fmt::{Debug, Display};
 
 /// Base functionality for all errors in Rust.
-#[unstable(feature = "core",
-           reason = "the exact API of this trait may change")]
-pub trait Error: Display {
-    /// A short description of the error; usually a static string.
+#[stable(feature = "rust1", since = "1.0.0")]
+pub trait Error: Debug + Display + Send {
+    /// A short description of the error.
+    ///
+    /// The description should not contain newlines or sentence-ending
+    /// punctuation, to facilitate embedding in larger user-facing
+    /// strings.
+    #[stable(feature = "rust1", since = "1.0.0")]
     fn description(&self) -> &str;
 
     /// The lower-level cause of this error, if any.
+    #[stable(feature = "rust1", since = "1.0.0")]
     fn cause(&self) -> Option<&Error> { None }
 }
 

src/libstd/io/buffered.rs

@@ -258,7 +258,7 @@ impl<W> FromError<IntoInnerError<W>> for Error {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<W> error::Error for IntoInnerError<W> {
+impl<W: Send + fmt::Debug> error::Error for IntoInnerError<W> {
     fn description(&self) -> &str {
         error::Error::description(self.error())
     }

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 {

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(),

src/rustbook/error.rs

@@ -20,6 +20,7 @@ pub type CommandError = Box<Error + 'static>;
 pub type CommandResult<T> = Result<T, CommandError>;
 
 pub fn err(s: &str) -> CliError {
+    #[derive(Debug)]
     struct E(String);
 
     impl Error for E {