Provide a way to interpolate crate version in html_root_url

[dtolnay]

In Serde we work around this by having a reminder in Cargo.toml to update the html_root_url when releasing a new version. serde-rs/serde@dc7ab26

[package]
version = "1.0.8" # remember to update html_root_url

#![doc(html_root_url = "https://docs.rs/serde/1.0.8")]

This is annoying and easily falls out of sync. For example mio 0.6.8 docs point to version 0.6.1: carllerche/mio#613.

There should be a way to keep these synchronized automatically.

[durka]

Macros in attributes would neatly solve this, just sayin'.

[onur]

I recently implemented --extern-version argument to rustdoc and cargo for this purpose. I don't think this patches should be in rust or cargo but I am planning to solve all external crate links in docs.rs with this patches. Docs.rs is not using this compiler yet, building rustc for all tier one platforms requires a lot of effort.

Hopefully docs.rs will set html_root_url automatically for all crates soon and it will help maintainers to not care about html_root_url.