From 46d31f92303c3dd8905aef1b71150acbd46e88c1 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sat, 24 Jul 2021 14:02:09 +0200
Subject: [PATCH] show proper error when using a sysroot without MIR

---
 src/diagnostics.rs | 2 --
 src/eval.rs        | 6 ++++++
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/diagnostics.rs b/src/diagnostics.rs
index b5b75a7fc31..cad08a2831a 100644
--- a/src/diagnostics.rs
+++ b/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(_) =>
diff --git a/src/eval.rs b/src/eval.rs
index ae9ff9c1f5a..02feae4a350 100644
--- a/src/eval.rs
+++ b/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)?;