Add error message specific to \<carriage return>.

This can crop-up with a misconfigured editor or an unexpected
interaction between version control and certain operating systems.

Closes #11669.

src/libsyntax/parse/lexer/mod.rs

@@ -806,6 +806,13 @@ impl<'a> StringReader<'a> {
                                     if ascii_only { "unknown byte escape" }
                                     else { "unknown character escape" },
                                     c);
+                                if e == '\r' {
+                                    let sp = codemap::mk_sp(escaped_pos, last_pos);
+                                    self.span_diagnostic.span_help(
+                                        sp,
+                                        "this is an isolated carriage return; consider checking \
+                                         your editor and version control settings.")
+                                }
                                 false
                             }
                         }

src/test/compile-fail/.gitattributes

@@ -0,0 +1 @@
+trailing-carriage-return-in-string.rs -text

src/test/compile-fail/trailing-carriage-return-in-string.rs

@@ -0,0 +1,23 @@
+// Copyright 2014 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.
+
+// ignore-tidy-cr
+// Issue #11669
+
+fn main() {
+    // \r\n
+    let ok = "This is \
+ a test";
+    // \r only
+    let bad = "This is \
 a test";
+    //~^ ERROR unknown character escape: \r
+    //~^^ HELP this is an isolated carriage return
+
+}