Only write metadata module when necessary

Author: bjorn3

src/driver.rs

@@ -17,7 +17,7 @@ use crate::prelude::*;
 pub fn codegen_crate<'a, 'tcx>(
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
     metadata: EncodedMetadata,
-    _need_metadata_module: bool,
+    need_metadata_module: bool,
 ) -> Box<dyn Any> {
     env_logger::init();
     if !tcx.sess.crate_types.get().contains(&CrateType::Executable)
@@ -146,7 +146,7 @@ pub fn codegen_crate<'a, 'tcx>(
         rustc_incremental::save_dep_graph(tcx);
         rustc_incremental::finalize_session_directory(tcx.sess, tcx.crate_hash(LOCAL_CRATE));
 
-        let metadata_module = {
+        let metadata_module = if need_metadata_module {
             use rustc::mir::mono::CodegenUnitNameBuilder;
 
             let cgu_name_builder = &mut CodegenUnitNameBuilder::new(tcx);
@@ -166,13 +166,15 @@ pub fn codegen_crate<'a, 'tcx>(
             let obj = metadata_artifact.emit().unwrap();
             std::fs::write(&tmp_file, obj).unwrap();
 
-            CompiledModule {
+            Some(CompiledModule {
                 name: metadata_cgu_name,
                 kind: ModuleKind::Metadata,
                 object: Some(tmp_file),
                 bytecode: None,
                 bytecode_compressed: None,
-            }
+            })
+        } else {
+            None
         };
 
         Box::new(CodegenResults {
@@ -193,7 +195,7 @@ pub fn codegen_crate<'a, 'tcx>(
             } else {
                 None
             },
-            metadata_module: Some(metadata_module),
+            metadata_module,
             crate_hash: tcx.crate_hash(LOCAL_CRATE),
             metadata,
             windows_subsystem: None, // Windows is not yet supported