Remove ModData from rustc_metadata

This avoids having to decode 2 `Lazy`s when decoding a modules exports.

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -1198,8 +1198,8 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
             }
         }
 
-        if let EntryKind::Mod(data) = kind {
-            for exp in data.decode((self, sess)).reexports.decode((self, sess)) {
+        if let EntryKind::Mod(exports) = kind {
+            for exp in exports.decode((self, sess)) {
                 match exp.res {
                     Res::Def(DefKind::Macro(..), _) => {}
                     _ if macros_only => continue,

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1086,9 +1086,7 @@ impl EncodeContext<'a, 'tcx> {
             Lazy::empty()
         };
 
-        let data = ModData { reexports };
-
-        record!(self.tables.kind[def_id] <- EntryKind::Mod(self.lazy(data)));
+        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.

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -346,7 +346,7 @@ enum EntryKind {
     Union(Lazy<VariantData>, ReprOptions),
     Fn(Lazy<FnData>),
     ForeignFn(Lazy<FnData>),
-    Mod(Lazy<ModData>),
+    Mod(Lazy<[Export]>),
     MacroDef(Lazy<MacroDef>),
     ProcMacro(MacroKind),
     Closure,
@@ -364,11 +364,6 @@ enum EntryKind {
 #[derive(Encodable, Decodable)]
 struct RenderedConst(String);
 
-#[derive(MetadataEncodable, MetadataDecodable)]
-struct ModData {
-    reexports: Lazy<[Export]>,
-}
-
 #[derive(MetadataEncodable, MetadataDecodable)]
 struct FnData {
     asyncness: hir::IsAsync,

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -145,12 +145,11 @@ impl<'a> Resolver<'a> {
                     } else {
                         def_key.disambiguated_data.data.get_opt_name().expect("module without name")
                     };
-                    let expn_id = self.cstore().module_expansion_untracked(def_id, &self.session);
 
                     Some(self.new_module(
                         parent,
                         ModuleKind::Def(def_kind, def_id, name),
-                        expn_id,
+                        self.cstore().module_expansion_untracked(def_id, &self.session),
                         self.cstore().get_span_untracked(def_id, &self.session),
                         // FIXME: Account for `#[no_implicit_prelude]` attributes.
                         parent.map_or(false, |module| module.no_implicit_prelude),