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: tokio-rs/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.

[Nadrieril]

If I'm reading this commit correctly, it appears that docs.rs does it now. This is not documented anywhere though, as far as I can see.

[Nadrieril]

Apparently that's not enough to not need html_root_url sadly; that seems to depend on #42301

[RodBurman]

I may not have a good understanding of the issue but I think what we would like is the ability to write (something like):

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

Where {version} refers to the value in Cargo.toml (which is available in macros c.f. the clap crate), so it might need to be (say) {Cargo.toml::version} to indicate the source without the compiler needing to just "know" it and thus Cargo.toml being somewhat "magical" (this assumes we may want/need other possible distinguishable sources). Of course #![doc("link")] is an attribute not a macro so this method maybe more easily suggested than coded. Once this substation mechanism is added it may well find a lot of uses elsewhere. Or I maybe talking tosh :(.

[TroyKomodo]

I think {Cargo.toml::version} is likely not a good idea since cargo isnt the only build system. I think just having a {version} and {name} templates for --crate-name and --crate-version would suffice for almost all cases. However #42300 (comment) would be the more general solution.