Make sure formatter errors are emitted by the default Write::write_fmt

Previously, if an error was returned from the formatter that did not
originate in an underlying writer error, Write::write_fmt would return
successfully even if the formatting did not complete (was interrupted by
an `fmt::Error` return).

Now we choose to emit an io::Error with kind Other for formatter errors.

Since this may reveal error returns from `write!()` and similar that
previously passed silently, it's a kind of a [breaking-change].

src/libstd/io/mod.rs

@@ -1055,7 +1055,14 @@ pub trait Write {
         let mut output = Adaptor { inner: self, error: Ok(()) };
         match fmt::write(&mut output, fmt) {
             Ok(()) => Ok(()),
-            Err(..) => output.error
+            Err(..) => {
+                // check if the error came from the underlying `Write` or not
+                if output.error.is_err() {
+                    output.error
+                } else {
+                    Err(Error::new(ErrorKind::Other, "formatter error"))
+                }
+            }
         }
     }
 

src/test/run-pass/write-fmt-errors.rs

@@ -0,0 +1,54 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::fmt;
+use std::io::{self, Error, Write, sink};
+
+struct ErrorDisplay;
+
+impl fmt::Display for ErrorDisplay {
+    fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
+        Err(fmt::Error)
+    }
+}
+
+struct ErrorWriter;
+
+const FORMAT_ERROR: io::ErrorKind = io::ErrorKind::Other;
+const WRITER_ERROR: io::ErrorKind = io::ErrorKind::NotConnected;
+
+impl Write for ErrorWriter {
+    fn write(&mut self, _buf: &[u8]) -> io::Result<usize> {
+        Err(Error::new(WRITER_ERROR, "not connected"))
+    }
+
+    fn flush(&mut self) -> io::Result<()> { Ok(()) }
+}
+
+fn main() {
+    // Test that the error from the formatter is propagated.
+    let res = write!(sink(), "{} {} {}", 1, ErrorDisplay, "bar");
+    assert!(res.is_err(), "formatter error did not propagate");
+    assert_eq!(res.unwrap_err().kind(), FORMAT_ERROR);
+
+    // Test that an underlying error is propagated
+    let res = write!(ErrorWriter, "abc");
+    assert!(res.is_err(), "writer error did not propagate");
+
+    // Writer error
+    let res = write!(ErrorWriter, "abc {}", ErrorDisplay);
+    assert!(res.is_err(), "writer error did not propagate");
+    assert_eq!(res.unwrap_err().kind(), WRITER_ERROR);
+
+    // Formatter error
+    let res = write!(ErrorWriter, "{} abc", ErrorDisplay);
+    assert!(res.is_err(), "formatter error did not propagate");
+    assert_eq!(res.unwrap_err().kind(), FORMAT_ERROR);
+}