Add support for doc(extern_html_root_url()) attribute

Author: GuillaumeGomez

compiler/rustc_passes/src/check_attr.rs

@@ -1020,6 +1020,7 @@ impl CheckAttrVisitor<'_> {
                         | sym::html_root_url
                         | sym::html_no_source
                         | sym::test
+                        | sym::extern_html_root_url
                             if !self.check_attr_crate_level(attr, meta, hir_id) =>
                         {
                             is_valid = false;
@@ -1052,6 +1053,7 @@ impl CheckAttrVisitor<'_> {
                         sym::alias
                         | sym::cfg
                         | sym::cfg_hide
+                        | sym::extern_html_root_url
                         | sym::hidden
                         | sym::html_favicon_url
                         | sym::html_logo_url

compiler/rustc_span/src/symbol.rs

@@ -653,6 +653,7 @@ symbols! {
         extern_absolute_paths,
         extern_crate_item_prelude,
         extern_crate_self,
+        extern_html_root_url,
         extern_in_paths,
         extern_prelude,
         extern_types,

src/librustdoc/config.rs

@@ -230,7 +230,8 @@ pub(crate) struct RenderOptions {
     pub(crate) extension_css: Option<PathBuf>,
     /// A map of crate names to the URL to use instead of querying the crate's `html_root_url`.
     pub(crate) extern_html_root_urls: BTreeMap<String, String>,
-    /// Whether to give precedence to `html_root_url` or `--exten-html-root-url`.
+    /// Whether to give precedence to `html_root_url` or `--extern-html-root-url`.
+    /// By default it is `html_root_url`.
     pub(crate) extern_html_root_takes_precedence: bool,
     /// A map of the default settings (values are as for DOM storage API). Keys should lack the
     /// `rustdoc-` prefix.

src/librustdoc/core.rs

@@ -21,6 +21,7 @@ use rustc_span::symbol::sym;
 use rustc_span::{source_map, Span, Symbol};
 
 use std::cell::RefCell;
+use std::collections::btree_map::Entry;
 use std::lazy::SyncLazy;
 use std::mem;
 use std::rc::Rc;
@@ -456,6 +457,44 @@ pub(crate) fn run_global_ctxt(
         if attr.is_word() && name == sym::document_private_items {
             ctxt.render_options.document_private = true;
         }
+
+        if name == sym::extern_html_root_url {
+            if let Some(attr) = attr.meta_item_list() {
+                for sub_attr in attr {
+                    let name = sub_attr.name_or_empty().as_str().to_owned();
+                    match sub_attr.value_str() {
+                        Some(value) => {
+                            let value = value.as_str().to_owned();
+                            if value.is_empty() {
+                                tcx.sess.span_err(sub_attr.span(), "URL cannot be empty");
+                                continue;
+                            }
+                            match ctxt.render_options.extern_html_root_urls.entry(name) {
+                                Entry::Occupied(_) => {
+                                    // do nothing since command line `--extern_html_root_url`
+                                    // takes precedence.
+                                }
+                                Entry::Vacant(v) => {
+                                    v.insert(value);
+                                }
+                            }
+                        }
+                        None => {
+                            tcx.sess.span_err(
+                                sub_attr.span(),
+                                "extern_html_root_url() only accepts `crate_name = \"url\"`",
+                            );
+                        }
+                    }
+                }
+            } else {
+                diag.span_err(
+                    attr.span(),
+                    "extern_html_root_url only accepts this form: \
+                     `extern_html_root_url(crate = \"url\")`",
+                );
+            }
+        }
     }
 
     info!("Executing passes");