start adding --extern-html-root-url to rustdoc flags

src/utils/build_doc.rs

@@ -10,6 +10,8 @@ use std::sync::Arc;
use cargo::core::{self, SourceId, Dependency, Source, Package, Workspace};
use cargo::core::compiler::{DefaultExecutor, CompileMode, MessageFormat, BuildConfig, Executor};
use cargo::core::package::PackageSet;
use cargo::core::registry::PackageRegistry;
use cargo::core::resolver;
use cargo::core::source::SourceMap;
use cargo::util::{CargoResult, Config, internal, Filesystem};
use cargo::sources::SourceConfigMap;
@@ -70,12 +72,20 @@ pub fn build_doc(name: &str, vers: Option<&str>, target: Option<&str>) -> Result

    // since https://github.com/rust-lang/rust/pull/48511 we can pass --resource-suffix to
    // add correct version numbers to css and javascript files
    // TODO: we can add --extern-html-root-url too, thanks to
    // https://github.com/rust-lang/rust/pull/51384
    let mut rustdoc_args: Vec<String> =
        vec!["-Z".to_string(), "unstable-options".to_string(),
             "--resource-suffix".to_string(),
             format!("-{}", parse_rustc_version(get_current_versions()?.0)?)];

    // since https://github.com/rust-lang/rust/pull/51384, we can pass --extern-html-root-url to
    // force rustdoc to link to other docs.rs docs for dependencies
    let source = try!(source_cfg_map.load(&source_id));
    for (name, dep) in try!(resolve_deps(&pkg, &config, source)) {
        rustdoc_args.push("--extern-html-root-url".to_string());
        rustdoc_args.push(format!("{}=https://docs.rs/{}/{}",
                                  name.replace("-", "_"), dep.name(), dep.version()));
    }

    if let Some(package_rustdoc_args) = metadata.rustdoc_args {
        rustdoc_args.append(&mut package_rustdoc_args.iter().map(|s| s.to_owned()).collect());
    }
@@ -114,7 +124,34 @@ pub fn build_doc(name: &str, vers: Option<&str>, target: Option<&str>) -> Result
    Ok(try!(ws.current()).clone())
}

fn resolve_deps<'cfg>(pkg: &Package, config: &'cfg Config, src: Box<Source + 'cfg>)
    -> CargoResult<Vec<(String, Package)>>
{
    let mut registry = try!(PackageRegistry::new(config));
    registry.add_preloaded(src);
    registry.lock_patches();

    let resolver = try!(resolver::resolve(
        &[(pkg.summary().clone(), resolver::Method::Everything)],
        pkg.manifest().replace(),
        &mut registry,
        &Default::default(),
        None,
        false,
    ));
    let dep_ids = resolver.deps(pkg.package_id()).map(|p| p.0).cloned().collect::<Vec<_>>();
    let pkg_set = try!(registry.get(&dep_ids));
    let deps = try!(pkg_set.get_many(&dep_ids));

    let mut ret = Vec::new();
    for dep in deps {
        if let Some(d) = pkg.dependencies().iter().find(|d| d.package_name() == dep.name()) {
            ret.push((d.name_in_toml().to_string(), dep.clone()));
        }
    }

    Ok(ret)
}

/// Downloads a crate and returns Cargo Package.
pub fn get_package(name: &str, vers: Option<&str>) -> CargoResult<Package> {