rustdoc: add --extern-html-root-url flag

src/doc/rustdoc/src/unstable-features.md

@@ -361,6 +361,21 @@ This flag allows rustdoc to treat your rust code as the given edition. It will c
 the given edition as well. As with `rustc`, the default edition that `rustdoc` will use is `2015`
 (the first edition).
 
+### `--extern-html-root-url`: control how rustdoc links to non-local crates
+
+Using this flag looks like this:
+
+```bash
+$ rustdoc src/lib.rs -Z unstable-options --extern-html-root-url some-crate=https://example.com/some-crate/1.0.1
+```
+
+Ordinarily, when rustdoc wants to link to a type from a different crate, it looks in two places:
+docs that already exist in the output directory, or the `#![doc(doc_html_root)]` set in the other
+crate. However, if you want to link to docs that exist in neither of those places, you can use these
+flags to control that behavior. When the `--extern-html-root-url` flag is given with a name matching
+one of your dependencies, rustdoc use that URL for those docs. Keep in mind that if those docs exist
+in the output directory, those local docs will still override this flag.
+
 ### `-Z force-unstable-if-unmarked`
 
 Using this flag looks like this:

src/librustdoc/html/render.rs

@@ -507,6 +507,7 @@ pub fn derive_id(candidate: String) -> String {
 
 /// Generates the documentation for `crate` into the directory `dst`
 pub fn run(mut krate: clean::Crate,
+           extern_urls: BTreeMap<String, String>,
            external_html: &ExternalHtml,
            playground_url: Option<String>,
            dst: PathBuf,
@@ -636,8 +637,9 @@ pub fn run(mut krate: clean::Crate,
             },
             _ => PathBuf::new(),
         };
+        let extern_url = extern_urls.get(&e.name).map(|u| &**u);
         cache.extern_locations.insert(n, (e.name.clone(), src_root,
-                                          extern_location(e, &cx.dst)));
+                                          extern_location(e, extern_url, &cx.dst)));
 
         let did = DefId { krate: n, index: CRATE_DEF_INDEX };
         cache.external_paths.insert(did, (vec![e.name.to_string()], ItemType::Module));
@@ -1088,13 +1090,23 @@ fn clean_srcpath<F>(src_root: &Path, p: &Path, keep_filename: bool, mut f: F) wh
 
 /// Attempts to find where an external crate is located, given that we're
 /// rendering in to the specified source destination.
-fn extern_location(e: &clean::ExternalCrate, dst: &Path) -> ExternalLocation {
+fn extern_location(e: &clean::ExternalCrate, extern_url: Option<&str>, dst: &Path)
+    -> ExternalLocation
+{
     // See if there's documentation generated into the local directory
     let local_location = dst.join(&e.name);
     if local_location.is_dir() {
         return Local;
     }
 
+    if let Some(url) = extern_url {
+        let mut url = url.to_string();
+        if !url.ends_with("/") {
+            url.push('/');
+        }
+        return Remote(url);
+    }
+
     // Failing that, see if there's an attribute specifying where to find this
     // external crate
     e.attrs.lists("doc")

src/librustdoc/lib.rs

@@ -158,6 +158,10 @@ pub fn opts() -> Vec<RustcOptGroup> {
         stable("extern", |o| {
             o.optmulti("", "extern", "pass an --extern to rustc", "NAME=PATH")
         }),
+        unstable("extern-html-root-url", |o| {
+            o.optmulti("", "extern-html-root-url",
+                       "base URL to use for dependencies", "NAME=URL")
+        }),
         stable("plugin-path", |o| {
             o.optmulti("", "plugin-path", "directory to load plugins from", "DIR")
         }),
@@ -423,6 +427,13 @@ pub fn main_args(args: &[String]) -> isize {
             return 1;
         }
     };
+    let extern_urls = match parse_extern_html_roots(&matches) {
+        Ok(ex) => ex,
+        Err(err) => {
+            diag.struct_err(err).emit();
+            return 1;
+        }
+    };
 
     let test_args = matches.opt_strs("test-args");
     let test_args: Vec<String> = test_args.iter()
@@ -521,7 +532,7 @@ pub fn main_args(args: &[String]) -> isize {
         info!("going to format");
         match output_format.as_ref().map(|s| &**s) {
             Some("html") | None => {
-                html::render::run(krate, &external_html, playground_url,
+                html::render::run(krate, extern_urls, &external_html, playground_url,
                                   output.unwrap_or(PathBuf::from("doc")),
                                   resource_suffix.unwrap_or(String::new()),
                                   passes.into_iter().collect(),
@@ -580,6 +591,23 @@ fn parse_externs(matches: &getopts::Matches) -> Result<Externs, String> {
     Ok(Externs::new(externs))
 }
 
+/// Extracts `--extern-html-root-url` arguments from `matches` and returns a map of crate names to
+/// the given URLs. If an `--extern-html-root-url` argument was ill-formed, returns an error
+/// describing the issue.
+fn parse_extern_html_roots(matches: &getopts::Matches)
+    -> Result<BTreeMap<String, String>, &'static str>
+{
+    let mut externs = BTreeMap::new();
+    for arg in &matches.opt_strs("extern-html-root-url") {
+        let mut parts = arg.splitn(2, '=');
+        let name = parts.next().ok_or("--extern-html-root-url must not be empty")?;
+        let url = parts.next().ok_or("--extern-html-root-url must be of the form name=url")?;
+        externs.insert(name.to_string(), url.to_string());
+    }
+
+    Ok(externs)
+}
+
 /// Interprets the input file as a rust source file, passing it through the
 /// compiler all the way through the analysis passes. The rustdoc output is then
 /// generated from the cleaned AST of the crate.