for support by default

for support via a flag

Rustdoc lets you write documentation comments in Markdown. At Rust 1.0, we were using the hoedown markdown implementation, written in C. Markdown is more of a family of implementations of an idea, and so hoedown had its own dialect, like many parsers. The CommonMark project has attempted to define a more strict version of Markdown, and so now, Rustdoc uses it by default.

As of Rust 1.23, we still defaulted to hoedown, but you could enable Commonmark via a flag, --enable-commonmark. Today, we only support CommonMark.