print stacktrace when miri can't find the MIR for something

src/error.rs

@@ -9,6 +9,7 @@ use syntax::codemap::Span;
 #[derive(Clone, Debug)]
 pub enum EvalError<'tcx> {
     FunctionPointerTyMismatch(&'tcx BareFnTy<'tcx>, &'tcx BareFnTy<'tcx>),
+    NoMirFor(String),
     DanglingPointerDeref,
     InvalidMemoryAccess,
     InvalidFunctionPointer,
@@ -82,6 +83,8 @@ impl<'tcx> Error for EvalError<'tcx> {
                 "array index out of bounds",
             EvalError::Math(..) =>
                 "mathematical operation failed",
+            EvalError::NoMirFor(..) =>
+                "mir not found",
             EvalError::InvalidChar(..) =>
                 "tried to interpret an invalid 32-bit value as a char",
             EvalError::OutOfMemory{..} =>
@@ -113,6 +116,7 @@ impl<'tcx> fmt::Display for EvalError<'tcx> {
                 write!(f, "memory access of {}..{} outside bounds of allocation {} which has size {}",
                        ptr.offset, ptr.offset + size, ptr.alloc_id, allocation_size)
             },
+            EvalError::NoMirFor(ref func) => write!(f, "no mir for `{}`", func),
             EvalError::FunctionPointerTyMismatch(expected, got) =>
                 write!(f, "tried to call a function of type {:?} through a function pointer of type {:?}", expected, got),
             EvalError::ArrayIndexOutOfBounds(span, len, index) =>

src/interpreter/mod.rs

@@ -267,22 +267,25 @@ impl<'a, 'tcx> EvalContext<'a, 'tcx> {
         ty.is_sized(self.tcx, &self.tcx.empty_parameter_environment(), DUMMY_SP)
     }
 
-    pub fn load_mir(&self, def_id: DefId) -> CachedMir<'a, 'tcx> {
+    pub fn load_mir(&self, def_id: DefId) -> EvalResult<'tcx, CachedMir<'a, 'tcx>> {
+        trace!("load mir {:?}", def_id);
         if def_id.is_local() {
-            CachedMir::Ref(self.mir_map.map.get(&def_id).unwrap())
+            Ok(CachedMir::Ref(self.mir_map.map.get(&def_id).unwrap()))
         } else {
             let mut mir_cache = self.mir_cache.borrow_mut();
             if let Some(mir) = mir_cache.get(&def_id) {
-                return CachedMir::Owned(mir.clone());
+                return Ok(CachedMir::Owned(mir.clone()));
             }
 
             let cs = &self.tcx.sess.cstore;
-            let mir = cs.maybe_get_item_mir(self.tcx, def_id).unwrap_or_else(|| {
-                panic!("no mir for `{}`", self.tcx.item_path_str(def_id));
-            });
+            match cs.maybe_get_item_mir(self.tcx, def_id) {
+                Some(mir) => {
                     let cached = Rc::new(mir);
                     mir_cache.insert(def_id, cached.clone());
-            CachedMir::Owned(cached)
+                    Ok(CachedMir::Owned(cached))
+                },
+                None => Err(EvalError::NoMirFor(self.tcx.item_path_str(def_id))),
+            }
         }
     }
 

src/interpreter/step.rs

@@ -125,8 +125,8 @@ impl<'a, 'b, 'tcx> ConstantExtractor<'a, 'b, 'tcx> {
         if self.ecx.statics.contains_key(&cid) {
             return;
         }
-        let mir = self.ecx.load_mir(def_id);
         self.try(|this| {
+            let mir = this.ecx.load_mir(def_id)?;
             let ptr = this.ecx.alloc_ret_ptr(mir.return_ty, substs)?;
             this.ecx.statics.insert(cid.clone(), ptr);
             let cleanup = if immutable && !mir.return_ty.type_contents(this.ecx.tcx).interior_unsafe() {

src/interpreter/terminator/mod.rs

@@ -184,7 +184,7 @@ impl<'a, 'tcx> EvalContext<'a, 'tcx> {
                         (def_id, substs)
                     };
 
-                let mir = self.load_mir(resolved_def_id);
+                let mir = self.load_mir(resolved_def_id)?;
                 let (return_ptr, return_to_block) = match destination {
                     Some((ptr, block)) => (Some(ptr), StackPopCleanup::Goto(block)),
                     None => (None, StackPopCleanup::None),