Disable "optimization to avoid load of address" in InstCombine

Author: jonas-schievink

compiler/rustc_mir/src/transform/instcombine.rs

@@ -119,6 +119,11 @@ impl OptimizationFinder<'b, 'tcx> {
     }
 
     fn find_deref_of_address(&mut self, rvalue: &Rvalue<'tcx>, location: Location) -> Option<()> {
+        // FIXME(#78192): This optimization can result in unsoundness.
+        if !self.tcx.sess.opts.debugging_opts.unsound_mir_opts {
+            return None;
+        }
+
         // Look for the sequence
         //
         // _2 = &_1;

src/test/mir-opt/const_prop/ref_deref.main.ConstProp.diff

@@ -19,7 +19,7 @@
                                            // + span: $DIR/ref_deref.rs:5:6: 5:10
                                            // + literal: Const { ty: &i32, val: Unevaluated(WithOptConstParam { did: DefId(0:3 ~ ref_deref[317d]::main), const_param_did: None }, [], Some(promoted[0])) }
           _2 = _4;                         // scope 0 at $DIR/ref_deref.rs:5:6: 5:10
--         _1 = (*_4);                      // scope 0 at $DIR/ref_deref.rs:5:5: 5:10
+-         _1 = (*_2);                      // scope 0 at $DIR/ref_deref.rs:5:5: 5:10
 +         _1 = const 4_i32;                // scope 0 at $DIR/ref_deref.rs:5:5: 5:10
           StorageDead(_2);                 // scope 0 at $DIR/ref_deref.rs:5:10: 5:11
           StorageDead(_1);                 // scope 0 at $DIR/ref_deref.rs:5:10: 5:11

src/test/mir-opt/const_prop/ref_deref_project.main.ConstProp.diff

@@ -19,7 +19,7 @@
                                            // + span: $DIR/ref_deref_project.rs:5:6: 5:17
                                            // + literal: Const { ty: &(i32, i32), val: Unevaluated(WithOptConstParam { did: DefId(0:3 ~ ref_deref_project[317d]::main), const_param_did: None }, [], Some(promoted[0])) }
           _2 = &((*_4).1: i32);            // scope 0 at $DIR/ref_deref_project.rs:5:6: 5:17
-          _1 = ((*_4).1: i32);             // scope 0 at $DIR/ref_deref_project.rs:5:5: 5:17
+          _1 = (*_2);                      // scope 0 at $DIR/ref_deref_project.rs:5:5: 5:17
           StorageDead(_2);                 // scope 0 at $DIR/ref_deref_project.rs:5:17: 5:18
           StorageDead(_1);                 // scope 0 at $DIR/ref_deref_project.rs:5:17: 5:18
           _0 = const ();                   // scope 0 at $DIR/ref_deref_project.rs:4:11: 6:2

src/test/mir-opt/inst_combine_deref.rs

@@ -1,4 +1,4 @@
-// compile-flags: -O
+// compile-flags: -O -Zunsound-mir-opts
 // EMIT_MIR inst_combine_deref.simple_opt.InstCombine.diff
 fn simple_opt() -> u64 {
     let x = 5;

src/test/ui/issues/issue-78192.rs

@@ -0,0 +1,17 @@
+// run-pass
+
+#![allow(unused_assignments)]
+
+fn main() {
+    let a = 1u32;
+    let b = 2u32;
+
+    let mut c: *const u32 = &a;
+    let d: &u32 = &b;
+
+    let x = unsafe { &*c };
+    c = d;
+    let z = *x;
+
+    assert_eq!(1, z);
+}