rustdoc: Pre-calculate traits that are in scope for doc links

This eliminates one more late use of resolver

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -108,7 +108,7 @@ impl<'a> Resolver<'a> {
     /// Reachable macros with block module parents exist due to `#[macro_export] macro_rules!`,
     /// but they cannot use def-site hygiene, so the assumption holds
     /// (<https://github.com/rust-lang/rust/pull/77984#issuecomment-712445508>).
-    crate fn get_nearest_non_block_module(&mut self, mut def_id: DefId) -> Module<'a> {
+    pub fn get_nearest_non_block_module(&mut self, mut def_id: DefId) -> Module<'a> {
         loop {
             match self.get_module(def_id) {
                 Some(module) => return module,

compiler/rustc_resolve/src/lib.rs

@@ -614,7 +614,8 @@ impl<'a> ModuleData<'a> {
         }
     }
 
-    fn def_id(&self) -> DefId {
+    // Public for rustdoc.
+    pub fn def_id(&self) -> DefId {
         self.opt_def_id().expect("`ModuleData::def_id` is called on a block module")
     }
 
@@ -3407,6 +3408,16 @@ impl<'a> Resolver<'a> {
         &self.all_macros
     }
 
+    /// For rustdoc.
+    /// For local modules returns only reexports, for external modules returns all children.
+    pub fn module_children_or_reexports(&self, def_id: DefId) -> Vec<ModChild> {
+        if let Some(def_id) = def_id.as_local() {
+            self.reexport_map.get(&def_id).cloned().unwrap_or_default()
+        } else {
+            self.cstore().module_children_untracked(def_id, self.session)
+        }
+    }
+
     /// Retrieves the span of the given `DefId` if `DefId` is in the local crate.
     #[inline]
     pub fn opt_span(&self, def_id: DefId) -> Option<Span> {