On fmt string with unescaped `{` note how to escape

On cases of malformed format strings where a `{` hasn't been properly
escaped, like `println!("{");`, present a note explaining how to escape
the `{` char.

2 files changed, 36 insertions, 0 deletions

diff --git a/src/test/ui/fmt/format-string-error.rs b/src/test/ui/fmt/format-string-error.rs
new file mode 100644
index 00000000000..ec715b3f0ba
--- /dev/null
+++ b/src/test/ui/fmt/format-string-error.rs
@@ -0,0 +1,16 @@
+// 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.
+
+fn main() {
+    println!("{");
+    println!("{{}}");
+    println!("}");
+}
+
diff --git a/src/test/ui/fmt/format-string-error.stderr b/src/test/ui/fmt/format-string-error.stderr
new file mode 100644
index 00000000000..58b392f0b8d
--- /dev/null
+++ b/src/test/ui/fmt/format-string-error.stderr
@@ -0,0 +1,20 @@
+error: invalid format string: expected `'}'` but string was terminated
+  --> $DIR/format-string-error.rs:12:5
+   |
+12 |     println!("{");
+   |     ^^^^^^^^^^^^^^
+   |
+   = note: if you intended to print `{`, you can escape it using `{{`
+   = note: this error originates in a macro outside of the current crate
+
+error: invalid format string: unmatched `}` found
+  --> $DIR/format-string-error.rs:14:5
+   |
+14 |     println!("}");
+   |     ^^^^^^^^^^^^^^
+   |
+   = note: if you intended to print `}`, you can escape it using `}}`
+   = note: this error originates in a macro outside of the current crate
+
+error: aborting due to 2 previous errors
+