Rollup merge of #65366 - faern:source-on-intostringerror, r=bluss

Implement Error::source on IntoStringError + Remove superfluous cause impls

IntoStringError only implemented `Error::cause`, which is deprecated. This implemements `Error::source` instead.
`Error::cause` will still work as before, thanks to the default implementation.

I think this was the only/last `Error` impl in the standard library to have a cause, but not a source.

src/libstd/ffi/c_str.rs

@@ -919,7 +919,7 @@ impl Error for IntoStringError {
         "C string contained non-utf8 bytes"
     }
 
-    fn cause(&self) -> Option<&dyn Error> {
+    fn source(&self) -> Option<&(dyn Error + 'static)> {
         Some(&self.error)
     }
 }

src/libstd/sync/mpsc/mod.rs

@@ -1581,10 +1581,6 @@ impl<T: Send> error::Error for SendError<T> {
     fn description(&self) -> &str {
         "sending on a closed channel"
     }
-
-    fn cause(&self) -> Option<&dyn error::Error> {
-        None
-    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1624,10 +1620,6 @@ impl<T: Send> error::Error for TrySendError<T> {
             }
         }
     }
-
-    fn cause(&self) -> Option<&dyn error::Error> {
-        None
-    }
 }
 
 #[stable(feature = "mpsc_error_conversions", since = "1.24.0")]
@@ -1652,10 +1644,6 @@ impl error::Error for RecvError {
     fn description(&self) -> &str {
         "receiving on a closed channel"
     }
-
-    fn cause(&self) -> Option<&dyn error::Error> {
-        None
-    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1685,10 +1673,6 @@ impl error::Error for TryRecvError {
             }
         }
     }
-
-    fn cause(&self) -> Option<&dyn error::Error> {
-        None
-    }
 }
 
 #[stable(feature = "mpsc_error_conversions", since = "1.24.0")]
@@ -1726,10 +1710,6 @@ impl error::Error for RecvTimeoutError {
             }
         }
     }
-
-    fn cause(&self) -> Option<&dyn error::Error> {
-        None
-    }
 }
 
 #[stable(feature = "mpsc_error_conversions", since = "1.24.0")]