tests: remove use of doc_comment crate

It relies on `cfg(doctest)`, which wasn't stabilized until Rust 1.43.
Interestingly, it compiled on Rust 1.28, but didn't compile on, e.g.,
Rust 1.39. This breaks our MSRV policy, so we unfortunately remove the
use of doc_comment for now. It's likely possible to conditionally
enable it, but the extra build script required to do version sniffing to
do it doesn't seem worth it.

Fixes rust-lang#684, Fixes rust-lang#685

CHANGELOG.md

@@ -1,3 +1,15 @@
+1.3.9 (2020-05-28)
+==================
+This release fixes a MSRV (Minimum Support Rust Version) regression in the
+1.3.8 release. Namely, while 1.3.8 compiles on Rust 1.28, it actually does not
+compile on other Rust versions, such as Rust 1.39.
+
+Bug fixes:
+
+* [BUG #685](https://github.com/rust-lang/regex/issue/685):
+  Remove use of `doc_comment` crate, which cannot be used before Rust 1.43.
+
+
 1.3.8 (2020-05-28)
 ==================
 This release contains a couple of important bug fixes driven

Cargo.toml

@@ -129,7 +129,10 @@ quickcheck = { version = "0.8", default-features = false }
 # For generating random test data.
 rand = "0.6.5"
 # To check README's example
-doc-comment = "0.3"
+# TODO: Re-enable this once the MSRV is 1.43 or greater.
+# See: https://github.com/rust-lang/regex/issues/684
+# See: https://github.com/rust-lang/regex/issues/685
+# doc-comment = "0.3"
 
 # Run the test suite on the default behavior of Regex::new.
 # This includes a mish mash of NFAs and DFAs, which are chosen automatically

src/lib.rs

@@ -622,8 +622,8 @@ compile_error!("`std` feature is currently required to build this crate");
 
 #[cfg(feature = "perf-literal")]
 extern crate aho_corasick;
-#[cfg(doctest)]
-extern crate doc_comment;
+// #[cfg(doctest)]
+// extern crate doc_comment;
 #[cfg(feature = "perf-literal")]
 extern crate memchr;
 #[cfg(test)]
@@ -633,8 +633,8 @@ extern crate regex_syntax as syntax;
 #[cfg(feature = "perf-cache")]
 extern crate thread_local;
 
-#[cfg(doctest)]
-doc_comment::doctest!("../README.md");
+// #[cfg(doctest)]
+// doc_comment::doctest!("../README.md");
 
 #[cfg(feature = "std")]
 pub use error::Error;