Add test for doctest edition support

Author: Munksgaard

src/doc/unstable-book/src/language-features/try-blocks.md

@@ -9,9 +9,7 @@ The tracking issue for this feature is: [#31436]
 The `try_blocks` feature adds support for `try` blocks. A `try`
 block creates a new scope one can use the `?` operator in.
 
-```rust,ignore
-// This code needs the 2018 edition
-
+```rust,edition2018
 #![feature(try_blocks)]
 
 use std::num::ParseIntError;

src/test/rustdoc/edition-doctest.rs

@@ -0,0 +1,54 @@
+// Copyright 2018 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.
+
+// compile-flags:--test
+
+/// ```rust,edition2018
+/// #![feature(try_blocks)]
+///
+/// use std::num::ParseIntError;
+///
+/// let result: Result<i32, ParseIntError> = try {
+///     "1".parse::<i32>()?
+///         + "2".parse::<i32>()?
+///         + "3".parse::<i32>()?
+/// };
+/// assert_eq!(result, Ok(6));
+///
+/// let result: Result<i32, ParseIntError> = try {
+///     "1".parse::<i32>()?
+///         + "foo".parse::<i32>()?
+///         + "3".parse::<i32>()?
+/// };
+/// assert!(result.is_err());
+/// ```
+
+
+/// ```rust,edition2015,compile_fail,E0574
+/// #![feature(try_blocks)]
+///
+/// use std::num::ParseIntError;
+///
+/// let result: Result<i32, ParseIntError> = try {
+///     "1".parse::<i32>()?
+///         + "2".parse::<i32>()?
+///         + "3".parse::<i32>()?
+/// };
+/// assert_eq!(result, Ok(6));
+///
+/// let result: Result<i32, ParseIntError> = try {
+///     "1".parse::<i32>()?
+///         + "foo".parse::<i32>()?
+///         + "3".parse::<i32>()?
+/// };
+/// assert!(result.is_err());
+/// ```
+
+pub fn foo() {}