Hoedown
Hoedown is a C library for processing and rendering Markdown as HTML. It also provides an API for hooking into the Markdown parsing.
Early on Rust lacked a fully-featured Markdown library and I needed one for the static site generator I was writing (diecast), so I wrote idiomatic bindings to hoedown for Rust, and in so doing, I exposed a few previously unknown edge cases in the C library through different combinations of feature flags.
See the PRs here.