Check for broken full reference links in Markdown

[norswap]

Currently lychee does not check for bad "link references" (not sure if this is the proper name), e.g.

This is a [link text][link-ref].

[link-ref-with-typo]: https://nyan.cat

It would be great if that was caught!

[lebensterben]

First, this is not the correct syntax for link reference. See https://github.github.com/gfm/#link-reference-definitions

It should be

[foo]: /url "title"

[foo]

Second, we relies on pulldown-cmark to parse hyperlinks in markdown documents. Thankfully, it's able to find broken links via https://docs.rs/pulldown-cmark/latest/pulldown_cmark/struct.Parser.html#method.new_with_broken_link_callback

But if you want to identify possible 'typo's, the job would be harder as we may need to calculate some text distances.

[mre]

Oh, I did not know about new_with_broken_link_callback! Thanks for mentioning it. We could start by printing a warning for broken links? That would already be a step into the right direction.

[norswap]

@lebensterben These types of links have been in Markdown since the very start (https://daringfireball.net/projects/markdown/syntax#link) and have always been supported by Github. They are in fact specified in the document you linked (https://github.github.com/gfm/#example-535). What you linked is the reference for the link definition (the [ref]: link part).

@mre A warning meaning it does not cause the program to return a non-zero code?

[lebensterben]

@norswap Thanks for pointing that out. It's a valid full reference link. https://github.github.com/gfm/#full-reference-link

I suggest to change the title of this issue accordingly.

[mre]

@mre A warning meaning it does not cause the program to return a non-zero code?

Good point. We can actually treat it as an error. After all, the link is broken.