better error on missing #[start]

Author: RalfJung

src/tools/miri/src/bin/miri.rs

@@ -67,7 +67,7 @@ impl rustc_driver::Callbacks for MiriCompilerCalls {
             if tcx.sess.compile_status().is_err() {
                 tcx.sess.fatal("miri cannot be run on programs that fail compilation");
             }
-;
+
             init_late_loggers(handler, tcx);
             if !tcx.sess.crate_types().contains(&CrateType::Executable) {
                 tcx.sess.fatal("miri only makes sense on bin crates");

src/tools/miri/src/eval.rs

@@ -365,7 +365,11 @@ pub fn create_ecx<'mir, 'tcx: 'mir>(
 
     match entry_type {
         EntryFnType::Main { .. } => {
-            let start_id = tcx.lang_items().start_fn().unwrap();
+            let start_id = tcx.lang_items().start_fn().unwrap_or_else(|| {
+                tcx.sess.fatal(
+                    "could not find start function. Make sure the entry point is marked with `#[start]`."
+                );
+            });
             let main_ret_ty = tcx.fn_sig(entry_id).no_bound_vars().unwrap().output();
             let main_ret_ty = main_ret_ty.no_bound_vars().unwrap();
             let start_instance = ty::Instance::resolve(