add test for parsing comments in doctest headers

src/test/rustdoc/comment-in-doctest.rs

@@ -0,0 +1,30 @@
+// 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
+
+// comments, both doc comments and regular ones, used to trick rustdoc's doctest parser into
+// thinking that everything after it was part of the regular program. combined with the libsyntax
+// parser loop failing to detect the manual main function, it would wrap everything in `fn main`,
+// which would cause the doctest to fail as the "extern crate" declaration was no longer valid.
+// oddly enough, it would pass in 2018 if a crate was in the extern prelude. see
+// https://github.com/rust-lang/rust/issues/56727
+
+//! ```
+//! // crate: proc-macro-test
+//! //! this is a test
+//!
+//! // used to pull in proc-macro specific items
+//! extern crate proc_macro;
+//!
+//! use proc_macro::TokenStream;
+//!
+//! # fn main() {}
+//! ```