Support multiple crate versions in --extern-html-root-url

#76296

Author: kornelski

compiler/rustc_metadata/src/creader.rs

@@ -280,6 +280,14 @@ impl CStore {
         self.resolved_externs.insert(name, extern_crate);
     }
 
+    /// Crate resolved and loaded via the given extern name
+    /// (corresponds to names in `sess.opts.externs`)
+    ///
+    /// May be `None` if the crate wasn't used
+    pub fn resolved_extern_crate(&self, externs_name: Symbol) -> Option<CrateNum> {
+        self.resolved_externs.get(&externs_name).copied()
+    }
+
     pub(crate) fn iter_crate_data(&self) -> impl Iterator<Item = (CrateNum, &CrateMetadata)> {
         self.metas
             .iter_enumerated()

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

@@ -395,6 +395,11 @@ flags to control that behavior. When the `--extern-html-root-url` flag is given
 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.
 
+Crate names in this flag refer to the names in the [extern prelude], and support crates renamed
+via the `--extern` `rustc` flag.
+
+[extern prelude]: https://doc.rust-lang.org/reference/names/preludes.html#extern-prelude
+
 ## `-Z force-unstable-if-unmarked`
 
 Using this flag looks like this:

src/librustdoc/formats/cache.rs

@@ -3,6 +3,7 @@ use std::mem;
 use rustc_attr_data_structures::StabilityLevel;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap, FxIndexSet};
 use rustc_hir::def_id::{CrateNum, DefId, DefIdMap, DefIdSet};
+use rustc_metadata::creader::CStore;
 use rustc_middle::ty::{self, TyCtxt};
 use rustc_span::Symbol;
 use tracing::debug;
@@ -158,18 +159,31 @@ impl Cache {
         assert!(cx.external_traits.is_empty());
         cx.cache.traits = mem::take(&mut krate.external_traits);
 
+        let render_options = &cx.render_options;
+        let extern_url_takes_precedence = render_options.extern_html_root_takes_precedence;
+        let dst = &render_options.output;
+
+        // Make `--extern-html-root-url` support the same names as `--extern` whenever possible
+        let cstore = CStore::from_tcx(tcx);
+        for (name, extern_url) in &render_options.extern_html_root_urls {
+            if let Some(crate_num) = cstore.resolved_extern_crate(Symbol::intern(name)) {
+                let e = ExternalCrate { crate_num };
+                let location = e.location(Some(extern_url), extern_url_takes_precedence, dst, tcx);
+                cx.cache.extern_locations.insert(e.crate_num, location);
+            }
+        }
+
         // Cache where all our extern crates are located
         // FIXME: this part is specific to HTML so it'd be nice to remove it from the common code
         for &crate_num in tcx.crates(()) {
             let e = ExternalCrate { crate_num };
 
             let name = e.name(tcx);
-            let render_options = &cx.render_options;
-            let extern_url = render_options.extern_html_root_urls.get(name.as_str()).map(|u| &**u);
-            let extern_url_takes_precedence = render_options.extern_html_root_takes_precedence;
-            let dst = &render_options.output;
-            let location = e.location(extern_url, extern_url_takes_precedence, dst, tcx);
-            cx.cache.extern_locations.insert(e.crate_num, location);
+            cx.cache.extern_locations.entry(e.crate_num).or_insert_with(|| {
+                let extern_url =
+                    render_options.extern_html_root_urls.get(name.as_str()).map(|u| &**u);
+                e.location(extern_url, extern_url_takes_precedence, dst, tcx)
+            });
             cx.cache.external_paths.insert(e.def_id(), (vec![name], ItemType::Module));
         }