show proper error when using a sysroot without MIR

src/diagnostics.rs

@@ -132,8 +132,6 @@ pub fn report_error<'tcx, 'mir>(
             };
             #[rustfmt::skip]
             let helps = match e.kind() {
-                Unsupported(UnsupportedOpInfo::NoMirFor(..)) =>
-                    vec![(None, format!("make sure to use a Miri sysroot, which you can prepare with `cargo miri setup`"))],
                 Unsupported(UnsupportedOpInfo::ThreadLocalStatic(_) | UnsupportedOpInfo::ReadExternStatic(_)) =>
                     panic!("Error should never be raised by Miri: {:?}", e.kind()),
                 Unsupported(_) =>

src/eval.rs

@@ -138,6 +138,12 @@ pub fn create_ecx<'mir, 'tcx: 'mir>(
     EnvVars::init(&mut ecx, config.excluded_env_vars)?;
     MemoryExtra::init_extern_statics(&mut ecx)?;
 
+    // Make sure we have MIR. We check MIR for some stable monomorphic function in libcore.
+    let sentinel = ecx.resolve_path(&["core", "ascii", "escape_default"]);
+    if !tcx.is_mir_available(sentinel.def.def_id()) {
+        tcx.sess.fatal("the current sysroot was built without `-Zalways-encode-mir`. Use `cargo miri setup` to prepare a sysroot that is suitable for Miri.");
+    }
+
     // Setup first stack-frame
     let main_instance = ty::Instance::mono(tcx, main_id);
     let main_mir = ecx.load_mir(main_instance.def, None)?;