Add rustc_on_unimplemented message to std::ops::Try

Author: estebank

src/libcore/ops/try.rs

@@ -15,6 +15,8 @@
 /// extracting those success or failure values from an existing instance and
 /// creating a new instance from a success or failure value.
 #[unstable(feature = "try_trait", issue = "42327")]
+#[rustc_on_unimplemented = "the `?` operator can only be used in a function that returns `Result` \
+                            (or another type that implements `std::ops::Try`)"]
 pub trait Try {
     /// The type of this value when viewed as successful.
     #[unstable(feature = "try_trait", issue = "42327")]

src/test/ui/suggestions/try-operator-on-main.rs

@@ -0,0 +1,13 @@
+// Copyright 2017 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() {
+    std::fs::File::open("foo")?;
+}

src/test/ui/suggestions/try-operator-on-main.stderr

@@ -0,0 +1,14 @@
+error[E0277]: the trait bound `(): std::ops::Try` is not satisfied
+  --> $DIR/try-operator-on-main.rs:12:5
+   |
+12 |     std::fs::File::open("foo")?;
+   |     ---------------------------
+   |     |
+   |     the `?` operator can only be used in a function that returns `Result` (or another type that implements `std::ops::Try`)
+   |     in this macro invocation
+   |
+   = help: the trait `std::ops::Try` is not implemented for `()`
+   = note: required by `std::ops::Try::from_error`
+
+error: aborting due to previous error
+