rustc_metadata: Optimize and document module children decoding

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -1075,15 +1075,16 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         }
     }
 
-    /// Iterates over each child of the given item.
+    /// Iterates over all named children of the given module,
+    /// including both proper items and reexports.
+    /// Module here is understood in name resolution sense - it can be a `mod` item,
+    /// or a crate root, or an enum, or a trait.
     fn each_child_of_item(&self, id: DefIndex, mut callback: impl FnMut(Export), sess: &Session) {
         if let Some(data) = &self.root.proc_macro_data {
-            /* If we are loading as a proc macro, we want to return the view of this crate
-             * as a proc macro crate.
-             */
+            // If we are loading as a proc macro, we want to return
+            // the view of this crate as a proc macro crate.
             if id == CRATE_DEF_INDEX {
-                let macros = data.macros.decode(self);
-                for def_index in macros {
+                for def_index in data.macros.decode(self) {
                     let raw_macro = self.raw_proc_macro(def_index);
                     let res = Res::Def(
                         DefKind::Macro(macro_kind(raw_macro)),
@@ -1096,12 +1097,6 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
             return;
         }
 
-        // Find the item.
-        let kind = match self.maybe_kind(id) {
-            None => return,
-            Some(kind) => kind,
-        };
-
         // Iterate over all children.
         if let Some(children) = self.root.tables.children.get(self, id) {
             for child_index in children.decode((self, sess)) {
@@ -1162,10 +1157,14 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
             }
         }
 
-        if let EntryKind::Mod(exports) = kind {
-            for exp in exports.decode((self, sess)) {
-                callback(exp);
+        match self.kind(id) {
+            EntryKind::Mod(exports) => {
+                for exp in exports.decode((self, sess)) {
+                    callback(exp);
+                }
             }
+            EntryKind::Enum(..) | EntryKind::Trait(..) => {}
+            _ => bug!("`each_child_of_item` is called on a non-module: {:?}", self.def_kind(id)),
         }
     }
 

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -4,7 +4,7 @@ use crate::native_libs;
 
 use rustc_ast as ast;
 use rustc_data_structures::stable_map::FxHashMap;
-use rustc_hir::def::{CtorKind, DefKind};
+use rustc_hir::def::{CtorKind, DefKind, Res};
 use rustc_hir::def_id::{CrateNum, DefId, DefIdMap, CRATE_DEF_INDEX, LOCAL_CRATE};
 use rustc_hir::definitions::{DefKey, DefPath, DefPathHash};
 use rustc_middle::hir::exports::Export;
@@ -309,28 +309,33 @@ pub(in crate::rmeta) fn provide(providers: &mut Providers) {
                 bfs_queue.push_back(DefId { krate: cnum, index: CRATE_DEF_INDEX });
             }
 
-            let mut add_child = |bfs_queue: &mut VecDeque<_>, export: &Export, parent: DefId| {
-                if !export.vis.is_public() {
+            let mut add_child = |bfs_queue: &mut VecDeque<_>, child: &Export, parent: DefId| {
+                if !child.vis.is_public() {
                     return;
                 }
 
-                if let Some(child) = export.res.opt_def_id() {
-                    if export.ident.name == kw::Underscore {
-                        fallback_map.insert(child, parent);
+                if let Some(def_id) = child.res.opt_def_id() {
+                    if child.ident.name == kw::Underscore {
+                        fallback_map.insert(def_id, parent);
                         return;
                     }
 
-                    match visible_parent_map.entry(child) {
+                    match visible_parent_map.entry(def_id) {
                         Entry::Occupied(mut entry) => {
                             // If `child` is defined in crate `cnum`, ensure
                             // that it is mapped to a parent in `cnum`.
-                            if child.is_local() && entry.get().is_local() {
+                            if def_id.is_local() && entry.get().is_local() {
                                 entry.insert(parent);
                             }
                         }
                         Entry::Vacant(entry) => {
                             entry.insert(parent);
-                            bfs_queue.push_back(child);
+                            if matches!(
+                                child.res,
+                                Res::Def(DefKind::Mod | DefKind::Enum | DefKind::Trait, _)
+                            ) {
+                                bfs_queue.push_back(def_id);
+                            }
                         }
                     }
                 }

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1104,7 +1104,6 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
 
         record!(self.tables.kind[def_id] <- EntryKind::Mod(reexports));
         if self.is_proc_macro {
-            record!(self.tables.children[def_id] <- &[]);
             // Encode this here because we don't do it in encode_def_ids.
             record!(self.tables.expn_that_defined[def_id] <- tcx.expn_that_defined(local_def_id));
         } else {