Link no-deps documentation with no html_root_url to docs.rs by default

[dtolnay]

As of Rust 1.19.0-nightly, documenting a crate with cargo doc --no-deps will link to the html_root_url of any dependencies that have one set, and other dependencies will not be linked.

In the screenshot, the crate providing HasHtmlRootUrl has an html_root_url so the type is linked. The crate providing NoHtmlRootUrl does not have an html_root_url so the type is not linked.

Linking NoHtmlRootUrl to docs.rs would be a reasonable fallback.

[lnicola]

rustdoc can't check (presumably) that the crate really exists on docs.rs. Would it be reasonable to simply link to a page that might 404?

[lnicola]

I tried to look into this, but I couldn't figure out how to get the crate versions. The best I managed to do was linking to https://docs.rs/crate/*/crate, which loads the latest version (I assume). Of course, it won't work if the crate is not published, or if the versions are different.

Any other ideas?

[natemara]

It seems like possibly, rustdoc could get the version information from the Cargo.toml of the package, and make a request to https://docs.rs/crate/version/crate, and if it comes back with a 200, include a docs.rs link.