https://github.com/hello-rust/lychee/pull/15/files#diff-9bfca03419915f4d4c69bb43d30e158a84c63d0f68c35c4991fd659d8b158f1cR10-R12

pulldown-cmark library is splitting http://[::1] into four parts (http://,[,::1,]) when the URL is not in a URL section.

Looks great!
Only thing I'd add is a unit test for extract_links with a some base_url (on top of the integration test you already added). Other than that it's ready to merge from my perspective. Code is very readable.

Thanks for adding the unit tests and for your contribution in general @whizsid. LGTM. 😊