rustdoc: provide a way to override html_root_url

[SimonSapin]

Rustdoc generates external links for standard library items. For example, Result at http://servo.github.io/rust-url/url/type.ParseResult.html links to http://doc.rust-lang.org/nightly/core/result/enum.Result.html

This URL seems to be determined from the #[doc(html_root_url = "…")] attribute in the crate that defines the item. There is apparently no way to override it.

Servo uses a Rust version that is often behind nightly, so I’d like http://doc.servo.org/ to link to docs for the Rust version that’s actually being used. Currently, this is achieved by copying Rust docs from the snapshot to the same location as where Servo docs are being generated. As a result, the crate index side bar lists both Servo and Rust crates, and is hard to read since there are so many of them.

I’d like instead to have docs for our Rust version at a different location, maybe http://doc.servo.org/rust-snapshot/ , and have standard library items link to that. The latter requires rustdoc accepting an override to html_root_url of external crates.

[tomjakubowski]

A simple solution would be to recognize those attributes on extern crate view items.

[SimonSapin]

That would be hard to use with std (where the extern crate is usually implicit), and doesn't work for transitive dependencies (e.g. foo -> std -> core)

[steveklabnik]

Triage: I am not aware of any changes in this area.