resolve: Rename some cstore methods to match queries and add comments

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -1041,13 +1041,6 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         self.root.tables.optimized_mir.get(self, id).is_some()
     }
 
-    fn module_expansion(self, id: DefIndex, sess: &Session) -> ExpnId {
-        match self.def_kind(id) {
-            DefKind::Mod | DefKind::Enum | DefKind::Trait => self.get_expn_that_defined(id, sess),
-            _ => panic!("Expected module, found {:?}", self.local_def_id(id)),
-        }
-    }
-
     fn get_fn_has_self_parameter(self, id: DefIndex, sess: &'a Session) -> bool {
         self.root
             .tables

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

@@ -490,6 +490,9 @@ pub(in crate::rmeta) fn provide(providers: &mut Providers) {
                 .alloc_slice(&CStore::from_tcx(tcx).crate_dependencies_in_postorder(LOCAL_CRATE))
         },
         crates: |tcx, ()| {
+            // The list of loaded crates is now frozen in query cache,
+            // so make sure cstore is not mutably accessed from here on.
+            tcx.untracked().cstore.leak();
             tcx.arena.alloc_from_iter(CStore::from_tcx(tcx).iter_crate_data().map(|(cnum, _)| cnum))
         },
         ..*providers
@@ -537,16 +540,16 @@ impl CStore {
         )
     }
 
-    pub fn get_span_untracked(&self, def_id: DefId, sess: &Session) -> Span {
+    pub fn def_span_untracked(&self, def_id: DefId, sess: &Session) -> Span {
         self.get_crate_data(def_id.krate).get_span(def_id.index, sess)
     }
 
-    pub fn def_kind(&self, def: DefId) -> DefKind {
+    pub fn def_kind_untracked(&self, def: DefId) -> DefKind {
         self.get_crate_data(def.krate).def_kind(def.index)
     }
 
-    pub fn module_expansion_untracked(&self, def_id: DefId, sess: &Session) -> ExpnId {
-        self.get_crate_data(def_id.krate).module_expansion(def_id.index, sess)
+    pub fn expn_that_defined_untracked(&self, def_id: DefId, sess: &Session) -> ExpnId {
+        self.get_crate_data(def_id.krate).get_expn_that_defined(def_id.index, sess)
     }
 
     /// Only public-facing way to traverse all the definitions in a non-local crate.

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -114,13 +114,16 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         }
 
         if !def_id.is_local() {
-            let def_kind = self.cstore().def_kind(def_id);
+            // Query `def_kind` is not used because query system overhead is too expensive here.
+            let def_kind = self.cstore().def_kind_untracked(def_id);
             if let DefKind::Mod | DefKind::Enum | DefKind::Trait = def_kind {
                 let parent = self
                     .tcx
                     .opt_parent(def_id)
                     .map(|parent_id| self.get_nearest_non_block_module(parent_id));
-                let expn_id = self.cstore().module_expansion_untracked(def_id, &self.tcx.sess);
+                // Query `expn_that_defined` is not used because
+                // hashing spans in its result is expensive.
+                let expn_id = self.cstore().expn_that_defined_untracked(def_id, &self.tcx.sess);
                 return Some(self.new_module(
                     parent,
                     ModuleKind::Def(def_kind, def_id, self.tcx.item_name(def_id)),
@@ -194,6 +197,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
     }
 
     pub(crate) fn build_reduced_graph_external(&mut self, module: Module<'a>) {
+        // Query `module_children` is not used because hashing spans in its result is expensive.
         let children =
             Vec::from_iter(self.cstore().module_children_untracked(module.def_id(), self.tcx.sess));
         for child in children {

compiler/rustc_resolve/src/lib.rs

@@ -1871,7 +1871,8 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
     fn def_span(&self, def_id: DefId) -> Span {
         match def_id.as_local() {
             Some(def_id) => self.tcx.source_span(def_id),
-            None => self.cstore().get_span_untracked(def_id, self.tcx.sess),
+            // Query `def_span` is not used because hashing its result span is expensive.
+            None => self.cstore().def_span_untracked(def_id, self.tcx.sess),
         }
     }