Only assume Stacked Borrows if -Zunsound-mir-opts is given

7 files changed, 137 insertions, 86 deletions

diff --git a/compiler/rustc_middle/src/mir/visit.rs b/compiler/rustc_middle/src/mir/visit.rs
index ddcf3711bfc..d87eb28970e 100644
--- a/compiler/rustc_middle/src/mir/visit.rs
+++ b/compiler/rustc_middle/src/mir/visit.rs
@@ -1320,6 +1320,15 @@ impl PlaceContext {
         )
     }
 
+    /// Returns `true` if this place context represents an address-of.
+    pub fn is_address_of(&self) -> bool {
+        matches!(
+            self,
+            PlaceContext::NonMutatingUse(NonMutatingUseContext::AddressOf)
+                | PlaceContext::MutatingUse(MutatingUseContext::AddressOf)
+        )
+    }
+
     /// Returns `true` if this place context represents a storage live or storage dead marker.
     #[inline]
     pub fn is_storage_marker(&self) -> bool {
diff --git a/compiler/rustc_mir_dataflow/src/value_analysis.rs b/compiler/rustc_mir_dataflow/src/value_analysis.rs
index fbaaee48148..9ae87418bc8 100644
--- a/compiler/rustc_mir_dataflow/src/value_analysis.rs
+++ b/compiler/rustc_mir_dataflow/src/value_analysis.rs
@@ -70,6 +70,7 @@ use std::fmt::{Debug, Formatter};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_index::vec::IndexVec;
 use rustc_middle::mir::tcx::PlaceTy;
+use rustc_middle::mir::visit::{PlaceContext, Visitor};
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::DUMMY_SP;
@@ -567,7 +568,17 @@ impl Map {
         filter: impl FnMut(Ty<'tcx>) -> bool,
     ) -> Self {
         let mut map = Self::new();
-        map.register_with_filter(tcx, body, 3, filter);
+
+        // If `-Zunsound-mir-opts` is given, tracking through references, and tracking of places
+        // that have their reference taken is allowed. This would be "unsound" in the sense that
+        // the correctness relies on an aliasing model similar to Stacked Borrows (which is
+        // not yet guaranteed).
+        if tcx.sess.opts.unstable_opts.unsound_mir_opts {
+            map.register_with_filter(tcx, body, 3, filter, &[]);
+        } else {
+            map.register_with_filter(tcx, body, 0, filter, &escaped_places(body));
+        }
+
         map
     }
 
@@ -577,6 +588,7 @@ impl Map {
         body: &Body<'tcx>,
         max_derefs: u32,
         mut filter: impl FnMut(Ty<'tcx>) -> bool,
+        exclude: &[Place<'tcx>],
     ) {
         let param_env = tcx.param_env_reveal_all_normalized(body.source.def_id());
         let mut projection = Vec::new();
@@ -589,6 +601,7 @@ impl Map {
                 decl.ty,
                 &mut filter,
                 param_env,
+                exclude,
             );
         }
     }
@@ -602,11 +615,18 @@ impl Map {
         ty: Ty<'tcx>,
         filter: &mut impl FnMut(Ty<'tcx>) -> bool,
         param_env: ty::ParamEnv<'tcx>,
+        exclude: &[Place<'tcx>],
     ) {
+        // This check could be improved.
+        if exclude.contains(&Place { local, projection: tcx.intern_place_elems(projection) }) {
+            return;
+        }
+
         if filter(ty) {
             // This might fail if `ty` is not scalar.
             let _ = self.register_with_ty(local, projection, ty);
         }
+
         if max_derefs > 0 {
             if let Some(ty::TypeAndMut { ty: deref_ty, .. }) = ty.builtin_deref(false) {
                 // References can only be tracked if the target is `!Freeze`.
@@ -620,6 +640,7 @@ impl Map {
                         deref_ty,
                         filter,
                         param_env,
+                        exclude,
                     );
                     projection.pop();
                 }
@@ -632,7 +653,7 @@ impl Map {
             }
             projection.push(PlaceElem::Field(field, ty));
             self.register_with_filter_rec(
-                tcx, max_derefs, local, projection, ty, filter, param_env,
+                tcx, max_derefs, local, projection, ty, filter, param_env, exclude,
             );
             projection.pop();
         });
@@ -751,6 +772,25 @@ impl PlaceInfo {
     }
 }
 
+/// Returns all places, that have their reference or address taken.
+fn escaped_places<'tcx>(body: &Body<'tcx>) -> Vec<Place<'tcx>> {
+    struct Collector<'tcx> {
+        result: Vec<Place<'tcx>>,
+    }
+
+    impl<'tcx> Visitor<'tcx> for Collector<'tcx> {
+        fn visit_place(&mut self, place: &Place<'tcx>, context: PlaceContext, _location: Location) {
+            if context.is_borrow() || context.is_address_of() {
+                self.result.push(*place);
+            }
+        }
+    }
+
+    let mut collector = Collector { result: Vec::new() };
+    collector.visit_body(body);
+    collector.result
+}
+
 struct Children<'a> {
     map: &'a Map,
     next: Option<PlaceIndex>,
diff --git a/src/test/mir-opt/dataflow-const-prop/cast.rs b/src/test/mir-opt/dataflow-const-prop/cast.rs
index bf5838cb89d..23c360ed874 100644
--- a/src/test/mir-opt/dataflow-const-prop/cast.rs
+++ b/src/test/mir-opt/dataflow-const-prop/cast.rs
@@ -1,4 +1,5 @@
 // unit-test: DataflowConstProp
+// compile-flags: -Zunsound-mir-opts
 
 // EMIT_MIR cast.main.DataflowConstProp.diff
 fn main() {
diff --git a/src/test/mir-opt/dataflow-const-prop/if.main.DataflowConstProp.diff b/src/test/mir-opt/dataflow-const-prop/if.main.DataflowConstProp.diff
index b2c2ba6fa5c..1a5ded8cc0d 100644
--- a/src/test/mir-opt/dataflow-const-prop/if.main.DataflowConstProp.diff
+++ b/src/test/mir-opt/dataflow-const-prop/if.main.DataflowConstProp.diff
@@ -3,30 +3,30 @@
   
   fn main() -> () {
       let mut _0: ();                      // return place in scope 0 at $DIR/if.rs:+0:11: +0:11
-      let _1: i32;                         // in scope 0 at $DIR/if.rs:+2:9: +2:10
-      let mut _3: bool;                    // in scope 0 at $DIR/if.rs:+3:16: +3:24
-      let mut _4: i32;                     // in scope 0 at $DIR/if.rs:+3:16: +3:19
-      let mut _5: &i32;                    // in scope 0 at $DIR/if.rs:+3:17: +3:19
-      let mut _7: i32;                     // in scope 0 at $DIR/if.rs:+4:13: +4:14
-      let mut _9: bool;                    // in scope 0 at $DIR/if.rs:+6:16: +6:24
-      let mut _10: i32;                    // in scope 0 at $DIR/if.rs:+6:16: +6:19
-      let mut _11: &i32;                   // in scope 0 at $DIR/if.rs:+6:17: +6:19
-      let mut _12: i32;                    // in scope 0 at $DIR/if.rs:+6:38: +6:39
-      let mut _14: i32;                    // in scope 0 at $DIR/if.rs:+7:13: +7:14
+      let _1: i32;                         // in scope 0 at $DIR/if.rs:+1:9: +1:10
+      let mut _3: bool;                    // in scope 0 at $DIR/if.rs:+2:16: +2:24
+      let mut _4: i32;                     // in scope 0 at $DIR/if.rs:+2:16: +2:19
+      let mut _5: &i32;                    // in scope 0 at $DIR/if.rs:+2:17: +2:19
+      let mut _7: i32;                     // in scope 0 at $DIR/if.rs:+3:13: +3:14
+      let mut _9: bool;                    // in scope 0 at $DIR/if.rs:+5:16: +5:24
+      let mut _10: i32;                    // in scope 0 at $DIR/if.rs:+5:16: +5:19
+      let mut _11: &i32;                   // in scope 0 at $DIR/if.rs:+5:17: +5:19
+      let mut _12: i32;                    // in scope 0 at $DIR/if.rs:+5:38: +5:39
+      let mut _14: i32;                    // in scope 0 at $DIR/if.rs:+6:13: +6:14
       scope 1 {
-          debug a => _1;                   // in scope 1 at $DIR/if.rs:+2:9: +2:10
-          let _2: i32;                     // in scope 1 at $DIR/if.rs:+3:9: +3:10
+          debug a => _1;                   // in scope 1 at $DIR/if.rs:+1:9: +1:10
+          let _2: i32;                     // in scope 1 at $DIR/if.rs:+2:9: +2:10
           scope 2 {
-              debug b => _2;               // in scope 2 at $DIR/if.rs:+3:9: +3:10
-              let _6: i32;                 // in scope 2 at $DIR/if.rs:+4:9: +4:10
+              debug b => _2;               // in scope 2 at $DIR/if.rs:+2:9: +2:10
+              let _6: i32;                 // in scope 2 at $DIR/if.rs:+3:9: +3:10
               scope 3 {
-                  debug c => _6;           // in scope 3 at $DIR/if.rs:+4:9: +4:10
-                  let _8: i32;             // in scope 3 at $DIR/if.rs:+6:9: +6:10
+                  debug c => _6;           // in scope 3 at $DIR/if.rs:+3:9: +3:10
+                  let _8: i32;             // in scope 3 at $DIR/if.rs:+5:9: +5:10
                   scope 4 {
-                      debug d => _8;       // in scope 4 at $DIR/if.rs:+6:9: +6:10
-                      let _13: i32;        // in scope 4 at $DIR/if.rs:+7:9: +7:10
+                      debug d => _8;       // in scope 4 at $DIR/if.rs:+5:9: +5:10
+                      let _13: i32;        // in scope 4 at $DIR/if.rs:+6:9: +6:10
                       scope 5 {
-                          debug e => _13;  // in scope 5 at $DIR/if.rs:+7:9: +7:10
+                          debug e => _13;  // in scope 5 at $DIR/if.rs:+6:9: +6:10
                       }
                   }
               }
@@ -34,87 +34,87 @@
       }
   
       bb0: {
-          StorageLive(_1);                 // scope 0 at $DIR/if.rs:+2:9: +2:10
-          _1 = const 1_i32;                // scope 0 at $DIR/if.rs:+2:13: +2:14
-          StorageLive(_2);                 // scope 1 at $DIR/if.rs:+3:9: +3:10
-          StorageLive(_3);                 // scope 1 at $DIR/if.rs:+3:16: +3:24
-          StorageLive(_4);                 // scope 1 at $DIR/if.rs:+3:16: +3:19
-          StorageLive(_5);                 // scope 1 at $DIR/if.rs:+3:17: +3:19
-          _5 = &_1;                        // scope 1 at $DIR/if.rs:+3:17: +3:19
--         _4 = (*_5);                      // scope 1 at $DIR/if.rs:+3:16: +3:19
--         _3 = Eq(move _4, const 1_i32);   // scope 1 at $DIR/if.rs:+3:16: +3:24
-+         _4 = const 1_i32;                // scope 1 at $DIR/if.rs:+3:16: +3:19
-+         _3 = const true;                 // scope 1 at $DIR/if.rs:+3:16: +3:24
-          StorageDead(_5);                 // scope 1 at $DIR/if.rs:+3:23: +3:24
-          StorageDead(_4);                 // scope 1 at $DIR/if.rs:+3:23: +3:24
--         switchInt(move _3) -> [false: bb2, otherwise: bb1]; // scope 1 at $DIR/if.rs:+3:16: +3:24
-+         switchInt(const true) -> [false: bb2, otherwise: bb1]; // scope 1 at $DIR/if.rs:+3:16: +3:24
+          StorageLive(_1);                 // scope 0 at $DIR/if.rs:+1:9: +1:10
+          _1 = const 1_i32;                // scope 0 at $DIR/if.rs:+1:13: +1:14
+          StorageLive(_2);                 // scope 1 at $DIR/if.rs:+2:9: +2:10
+          StorageLive(_3);                 // scope 1 at $DIR/if.rs:+2:16: +2:24
+          StorageLive(_4);                 // scope 1 at $DIR/if.rs:+2:16: +2:19
+          StorageLive(_5);                 // scope 1 at $DIR/if.rs:+2:17: +2:19
+          _5 = &_1;                        // scope 1 at $DIR/if.rs:+2:17: +2:19
+-         _4 = (*_5);                      // scope 1 at $DIR/if.rs:+2:16: +2:19
+-         _3 = Eq(move _4, const 1_i32);   // scope 1 at $DIR/if.rs:+2:16: +2:24
++         _4 = const 1_i32;                // scope 1 at $DIR/if.rs:+2:16: +2:19
++         _3 = const true;                 // scope 1 at $DIR/if.rs:+2:16: +2:24
+          StorageDead(_5);                 // scope 1 at $DIR/if.rs:+2:23: +2:24
+          StorageDead(_4);                 // scope 1 at $DIR/if.rs:+2:23: +2:24
+-         switchInt(move _3) -> [false: bb2, otherwise: bb1]; // scope 1 at $DIR/if.rs:+2:16: +2:24
++         switchInt(const true) -> [false: bb2, otherwise: bb1]; // scope 1 at $DIR/if.rs:+2:16: +2:24
       }
   
       bb1: {
-          _2 = const 2_i32;                // scope 1 at $DIR/if.rs:+3:27: +3:28
-          goto -> bb3;                     // scope 1 at $DIR/if.rs:+3:13: +3:41
+          _2 = const 2_i32;                // scope 1 at $DIR/if.rs:+2:27: +2:28
+          goto -> bb3;                     // scope 1 at $DIR/if.rs:+2:13: +2:41
       }
   
       bb2: {
-          _2 = const 3_i32;                // scope 1 at $DIR/if.rs:+3:38: +3:39
-          goto -> bb3;                     // scope 1 at $DIR/if.rs:+3:13: +3:41
+          _2 = const 3_i32;                // scope 1 at $DIR/if.rs:+2:38: +2:39
+          goto -> bb3;                     // scope 1 at $DIR/if.rs:+2:13: +2:41
       }
   
       bb3: {
-          StorageDead(_3);                 // scope 1 at $DIR/if.rs:+3:40: +3:41
-          StorageLive(_6);                 // scope 2 at $DIR/if.rs:+4:9: +4:10
-          StorageLive(_7);                 // scope 2 at $DIR/if.rs:+4:13: +4:14
--         _7 = _2;                         // scope 2 at $DIR/if.rs:+4:13: +4:14
--         _6 = Add(move _7, const 1_i32);  // scope 2 at $DIR/if.rs:+4:13: +4:18
-+         _7 = const 2_i32;                // scope 2 at $DIR/if.rs:+4:13: +4:14
-+         _6 = const 3_i32;                // scope 2 at $DIR/if.rs:+4:13: +4:18
-          StorageDead(_7);                 // scope 2 at $DIR/if.rs:+4:17: +4:18
-          StorageLive(_8);                 // scope 3 at $DIR/if.rs:+6:9: +6:10
-          StorageLive(_9);                 // scope 3 at $DIR/if.rs:+6:16: +6:24
-          StorageLive(_10);                // scope 3 at $DIR/if.rs:+6:16: +6:19
-          StorageLive(_11);                // scope 3 at $DIR/if.rs:+6:17: +6:19
-          _11 = &_1;                       // scope 3 at $DIR/if.rs:+6:17: +6:19
--         _10 = (*_11);                    // scope 3 at $DIR/if.rs:+6:16: +6:19
--         _9 = Eq(move _10, const 1_i32);  // scope 3 at $DIR/if.rs:+6:16: +6:24
-+         _10 = const 1_i32;               // scope 3 at $DIR/if.rs:+6:16: +6:19
-+         _9 = const true;                 // scope 3 at $DIR/if.rs:+6:16: +6:24
-          StorageDead(_11);                // scope 3 at $DIR/if.rs:+6:23: +6:24
-          StorageDead(_10);                // scope 3 at $DIR/if.rs:+6:23: +6:24
--         switchInt(move _9) -> [false: bb5, otherwise: bb4]; // scope 3 at $DIR/if.rs:+6:16: +6:24
-+         switchInt(const true) -> [false: bb5, otherwise: bb4]; // scope 3 at $DIR/if.rs:+6:16: +6:24
+          StorageDead(_3);                 // scope 1 at $DIR/if.rs:+2:40: +2:41
+          StorageLive(_6);                 // scope 2 at $DIR/if.rs:+3:9: +3:10
+          StorageLive(_7);                 // scope 2 at $DIR/if.rs:+3:13: +3:14
+-         _7 = _2;                         // scope 2 at $DIR/if.rs:+3:13: +3:14
+-         _6 = Add(move _7, const 1_i32);  // scope 2 at $DIR/if.rs:+3:13: +3:18
++         _7 = const 2_i32;                // scope 2 at $DIR/if.rs:+3:13: +3:14
++         _6 = const 3_i32;                // scope 2 at $DIR/if.rs:+3:13: +3:18
+          StorageDead(_7);                 // scope 2 at $DIR/if.rs:+3:17: +3:18
+          StorageLive(_8);                 // scope 3 at $DIR/if.rs:+5:9: +5:10
+          StorageLive(_9);                 // scope 3 at $DIR/if.rs:+5:16: +5:24
+          StorageLive(_10);                // scope 3 at $DIR/if.rs:+5:16: +5:19
+          StorageLive(_11);                // scope 3 at $DIR/if.rs:+5:17: +5:19
+          _11 = &_1;                       // scope 3 at $DIR/if.rs:+5:17: +5:19
+-         _10 = (*_11);                    // scope 3 at $DIR/if.rs:+5:16: +5:19
+-         _9 = Eq(move _10, const 1_i32);  // scope 3 at $DIR/if.rs:+5:16: +5:24
++         _10 = const 1_i32;               // scope 3 at $DIR/if.rs:+5:16: +5:19
++         _9 = const true;                 // scope 3 at $DIR/if.rs:+5:16: +5:24
+          StorageDead(_11);                // scope 3 at $DIR/if.rs:+5:23: +5:24
+          StorageDead(_10);                // scope 3 at $DIR/if.rs:+5:23: +5:24
+-         switchInt(move _9) -> [false: bb5, otherwise: bb4]; // scope 3 at $DIR/if.rs:+5:16: +5:24
++         switchInt(const true) -> [false: bb5, otherwise: bb4]; // scope 3 at $DIR/if.rs:+5:16: +5:24
       }
   
       bb4: {
--         _8 = _1;                         // scope 3 at $DIR/if.rs:+6:27: +6:28
-+         _8 = const 1_i32;                // scope 3 at $DIR/if.rs:+6:27: +6:28
-          goto -> bb6;                     // scope 3 at $DIR/if.rs:+6:13: +6:45
+-         _8 = _1;                         // scope 3 at $DIR/if.rs:+5:27: +5:28
++         _8 = const 1_i32;                // scope 3 at $DIR/if.rs:+5:27: +5:28
+          goto -> bb6;                     // scope 3 at $DIR/if.rs:+5:13: +5:45
       }
   
       bb5: {
-          StorageLive(_12);                // scope 3 at $DIR/if.rs:+6:38: +6:39
-          _12 = _1;                        // scope 3 at $DIR/if.rs:+6:38: +6:39
-          _8 = Add(move _12, const 1_i32); // scope 3 at $DIR/if.rs:+6:38: +6:43
-          StorageDead(_12);                // scope 3 at $DIR/if.rs:+6:42: +6:43
-          goto -> bb6;                     // scope 3 at $DIR/if.rs:+6:13: +6:45
+          StorageLive(_12);                // scope 3 at $DIR/if.rs:+5:38: +5:39
+          _12 = _1;                        // scope 3 at $DIR/if.rs:+5:38: +5:39
+          _8 = Add(move _12, const 1_i32); // scope 3 at $DIR/if.rs:+5:38: +5:43
+          StorageDead(_12);                // scope 3 at $DIR/if.rs:+5:42: +5:43
+          goto -> bb6;                     // scope 3 at $DIR/if.rs:+5:13: +5:45
       }
   
       bb6: {
-          StorageDead(_9);                 // scope 3 at $DIR/if.rs:+6:44: +6:45
-          StorageLive(_13);                // scope 4 at $DIR/if.rs:+7:9: +7:10
-          StorageLive(_14);                // scope 4 at $DIR/if.rs:+7:13: +7:14
--         _14 = _8;                        // scope 4 at $DIR/if.rs:+7:13: +7:14
--         _13 = Add(move _14, const 1_i32); // scope 4 at $DIR/if.rs:+7:13: +7:18
-+         _14 = const 1_i32;               // scope 4 at $DIR/if.rs:+7:13: +7:14
-+         _13 = const 2_i32;               // scope 4 at $DIR/if.rs:+7:13: +7:18
-          StorageDead(_14);                // scope 4 at $DIR/if.rs:+7:17: +7:18
-          _0 = const ();                   // scope 0 at $DIR/if.rs:+0:11: +8:2
-          StorageDead(_13);                // scope 4 at $DIR/if.rs:+8:1: +8:2
-          StorageDead(_8);                 // scope 3 at $DIR/if.rs:+8:1: +8:2
-          StorageDead(_6);                 // scope 2 at $DIR/if.rs:+8:1: +8:2
-          StorageDead(_2);                 // scope 1 at $DIR/if.rs:+8:1: +8:2
-          StorageDead(_1);                 // scope 0 at $DIR/if.rs:+8:1: +8:2
-          return;                          // scope 0 at $DIR/if.rs:+8:2: +8:2
+          StorageDead(_9);                 // scope 3 at $DIR/if.rs:+5:44: +5:45
+          StorageLive(_13);                // scope 4 at $DIR/if.rs:+6:9: +6:10
+          StorageLive(_14);                // scope 4 at $DIR/if.rs:+6:13: +6:14
+-         _14 = _8;                        // scope 4 at $DIR/if.rs:+6:13: +6:14
+-         _13 = Add(move _14, const 1_i32); // scope 4 at $DIR/if.rs:+6:13: +6:18
++         _14 = const 1_i32;               // scope 4 at $DIR/if.rs:+6:13: +6:14
++         _13 = const 2_i32;               // scope 4 at $DIR/if.rs:+6:13: +6:18
+          StorageDead(_14);                // scope 4 at $DIR/if.rs:+6:17: +6:18
+          _0 = const ();                   // scope 0 at $DIR/if.rs:+0:11: +7:2
+          StorageDead(_13);                // scope 4 at $DIR/if.rs:+7:1: +7:2
+          StorageDead(_8);                 // scope 3 at $DIR/if.rs:+7:1: +7:2
+          StorageDead(_6);                 // scope 2 at $DIR/if.rs:+7:1: +7:2
+          StorageDead(_2);                 // scope 1 at $DIR/if.rs:+7:1: +7:2
+          StorageDead(_1);                 // scope 0 at $DIR/if.rs:+7:1: +7:2
+          return;                          // scope 0 at $DIR/if.rs:+7:2: +7:2
       }
   }
   
diff --git a/src/test/mir-opt/dataflow-const-prop/if.rs b/src/test/mir-opt/dataflow-const-prop/if.rs
index 8df89080477..cc41da2dc9a 100644
--- a/src/test/mir-opt/dataflow-const-prop/if.rs
+++ b/src/test/mir-opt/dataflow-const-prop/if.rs
@@ -1,8 +1,8 @@
 // unit-test: DataflowConstProp
+// compile-flags: -Zunsound-mir-opts
 
 // EMIT_MIR if.main.DataflowConstProp.diff
 fn main() {
-    // This does not work (yet). Needs perhaps additional state to track unreachability.
     let a = 1;
     let b = if *&a == 1 { 2 } else { 3 };
     let c = b + 1;
diff --git a/src/test/mir-opt/dataflow-const-prop/ref.main.DataflowConstProp.diff b/src/test/mir-opt/dataflow-const-prop/ref.main.DataflowConstProp.diff
index 6af381617f9..f500c827786 100644
--- a/src/test/mir-opt/dataflow-const-prop/ref.main.DataflowConstProp.diff
+++ b/src/test/mir-opt/dataflow-const-prop/ref.main.DataflowConstProp.diff
@@ -37,7 +37,7 @@
           StorageLive(_6);                 // scope 2 at $DIR/ref.rs:+3:16: +3:34
           _6 = id() -> bb1;                // scope 2 at $DIR/ref.rs:+3:16: +3:34
                                            // mir::Constant
-                                           // + span: $DIR/ref.rs:7:16: 7:32
+                                           // + span: $DIR/ref.rs:8:16: 8:32
                                            // + literal: Const { ty: fn() -> u32 {id}, val: Value(<ZST>) }
       }
   
diff --git a/src/test/mir-opt/dataflow-const-prop/ref.rs b/src/test/mir-opt/dataflow-const-prop/ref.rs
index 6da613fc516..6c0e3063b0a 100644
--- a/src/test/mir-opt/dataflow-const-prop/ref.rs
+++ b/src/test/mir-opt/dataflow-const-prop/ref.rs
@@ -1,4 +1,5 @@
 // unit-test: DataflowConstProp
+// compile-flags: -Zunsound-mir-opts
 
 // EMIT_MIR ref.main.DataflowConstProp.diff
 fn main() {