Enable 'task list' markdown extension

- Add documentation about task lists

src/doc/rustdoc/src/how-to-write-documentation.md

@@ -221,6 +221,25 @@ This example will render similarly to this:
 See the specification for the [GitHub Tables extension][tables] for more
 details on the exact syntax supported.
 
+### Task lists
+
+Task lists can be used as a checklist of items that have been completed.
+Example:
+
+```md
+- [x] Complete task
+- [ ] IncComplete task
+```
+
+This will render as
+
+<ul>
+    <li><input type="checkbox"></li>
+    <li><input type="checkbox" checked></li>
+</ul>
+
+See the specification for the [task list extension] for more details.
+
 [`backtrace`]: https://docs.rs/backtrace/0.3.50/backtrace/
 [commonmark markdown specification]: https://commonmark.org/
 [commonmark quick reference]: https://commonmark.org/help/
@@ -234,3 +253,4 @@ details on the exact syntax supported.
 [`std::env`]: https://doc.rust-lang.org/stable/std/env/index.html#functions
 [strikethrough]: https://github.github.com/gfm/#strikethrough-extension-
 [tables]: https://github.github.com/gfm/#tables-extension-
+[task list extension]: https://github.github.com/gfm/#task-list-items-extension-

src/librustdoc/html/markdown.rs

@@ -48,7 +48,10 @@ mod tests;
 
 /// Options for rendering Markdown in the main body of documentation.
 pub(crate) fn opts() -> Options {
-    Options::ENABLE_TABLES | Options::ENABLE_FOOTNOTES | Options::ENABLE_STRIKETHROUGH
+    Options::ENABLE_TABLES
+        | Options::ENABLE_FOOTNOTES
+        | Options::ENABLE_STRIKETHROUGH
+        | Options::ENABLE_TASKLISTS
 }
 
 /// A subset of [`opts()`] used for rendering summaries.

src/test/rustdoc/task-lists.rs

@@ -0,0 +1,13 @@
+// ignore-tidy-linelength
+// FIXME: this doesn't test as much as I'd like; ideally it would have these query too:
+  // has task_lists/index.html '//li/input[@type="checkbox" and disabled]/following-sibling::text()' 'a'
+  // has task_lists/index.html '//li/input[@type="checkbox"]/following-sibling::text()' 'b'
+// Unfortunately that requires LXML, because the built-in xml module doesn't support all of xpath.
+
+// @has task_lists/index.html '//ul/li/input[@type="checkbox"]' ''
+// @has task_lists/index.html '//ul/li/input[@disabled]' ''
+// @has task_lists/index.html '//ul/li' 'a'
+// @has task_lists/index.html '//ul/li' 'b'
+//! This tests 'task list' support, a common markdown extension.
+//! - [ ] a
+//! - [x] b