Implement check mode

Fixes #973

src/driver/aot.rs

@@ -146,7 +146,13 @@ pub(super) fn run_aot(
 ) -> Box<(CodegenResults, FxHashMap<WorkProductId, WorkProduct>)> {
     let mut work_products = FxHashMap::default();
 
-    let (_, cgus) = tcx.collect_and_partition_mono_items(LOCAL_CRATE);
+    let cgus = if tcx.sess.opts.output_types.should_codegen() {
+        tcx.collect_and_partition_mono_items(LOCAL_CRATE).1
+    } else {
+        // If only `--emit metadata` is used, we shouldn't perform any codegen.
+        // Also `tcx.collect_and_partition_mono_items` may panic in that case.
+        &[]
+    };
 
     if tcx.dep_graph.is_fully_enabled() {
         for cgu in &*cgus {
@@ -239,6 +245,10 @@ pub(super) fn run_aot(
         None
     };
 
+    if tcx.sess.opts.output_types.should_codegen() {
+        rustc_incremental::assert_module_sources::assert_module_sources(tcx);
+    }
+
     Box::new((CodegenResults {
         crate_name: tcx.crate_name(LOCAL_CRATE),
         modules,

src/driver/jit.rs

@@ -39,6 +39,10 @@ pub(super) fn run_jit(tcx: TyCtxt<'_>) -> ! {
         .declare_function("main", Linkage::Import, &sig)
         .unwrap();
 
+    if !tcx.sess.opts.output_types.should_codegen() {
+        tcx.sess.fatal("JIT mode doesn't work with `cargo check`.");
+    }
+
     let (_, cgus) = tcx.collect_and_partition_mono_items(LOCAL_CRATE);
     let mono_items = cgus
         .iter()

src/lib.rs

@@ -200,7 +200,6 @@ impl CodegenBackend for CraneliftCodegenBackend {
     ) -> Box<dyn Any> {
         let res = driver::codegen_crate(tcx, metadata, need_metadata_module);
 
-        rustc_incremental::assert_module_sources::assert_module_sources(tcx);
         rustc_symbol_mangling::test::report_symbol_names(tcx);
 
         res