diff --git a/src/bootstrap/test.rs b/src/bootstrap/test.rs
index 854b7f5bd3a..374b6cafcac 100644
--- a/src/bootstrap/test.rs
+++ b/src/bootstrap/test.rs
@@ -613,6 +613,21 @@ impl Step for Miri {
             builder.run(&mut cargo);
         }
 
+        // Run it again for mir-opt-level 4 to catch some miscompilations.
+        if builder.config.test_args().is_empty() {
+            cargo.env("MIRIFLAGS", "-O -Zmir-opt-level=4 -Cdebug-assertions=yes");
+            // Optimizations can change backtraces
+            cargo.env("MIRI_SKIP_UI_CHECKS", "1");
+            // Optimizations can change error locations and remove UB so don't run `fail` tests.
+            cargo.args(&["tests/pass", "tests/panic"]);
+
+            let mut cargo = prepare_cargo_test(cargo, &[], &[], "miri", compiler, target, builder);
+            {
+                let _time = util::timeit(&builder);
+                builder.run(&mut cargo);
+            }
+        }
+
         // # Run `cargo miri test`.
         // This is just a smoke test (Miri's own CI invokes this in a bunch of different ways and ensures
         // that we get the desired output), but that is sufficient to make sure that the libtest harness
@@ -644,8 +659,10 @@ impl Step for Miri {
         cargo.env("RUST_BACKTRACE", "1");
 
         let mut cargo = Command::from(cargo);
-        let _time = util::timeit(&builder);
-        builder.run(&mut cargo);
+        {
+            let _time = util::timeit(&builder);
+            builder.run(&mut cargo);
+        }
     }
 }
 
diff --git a/src/tools/miri/ci.sh b/src/tools/miri/ci.sh
index 76cc2e94927..b5b3b211b05 100755
--- a/src/tools/miri/ci.sh
+++ b/src/tools/miri/ci.sh
@@ -45,8 +45,7 @@ function run_tests {
     # them. Also error locations change so we don't run the failing tests.
     # We explicitly enable debug-assertions here, they are disabled by -O but we have tests
     # which exist to check that we panic on debug assertion failures.
-    #FIXME: Disabled due to <https://github.com/rust-lang/rust/issues/111422>.
-    #MIRIFLAGS="${MIRIFLAGS:-} -O -Zmir-opt-level=4 -Cdebug-assertions=yes" MIRI_SKIP_UI_CHECKS=1 ./miri test -- tests/{pass,panic}
+    MIRIFLAGS="${MIRIFLAGS:-} -O -Zmir-opt-level=4 -Cdebug-assertions=yes" MIRI_SKIP_UI_CHECKS=1 ./miri test -- tests/{pass,panic}
 
     # Also run some many-seeds tests. 64 seeds means this takes around a minute per test.
     for FILE in tests/many-seeds/*.rs; do
diff --git a/src/tools/miri/rust-version b/src/tools/miri/rust-version
index 6ff63155f71..42687826b02 100644
--- a/src/tools/miri/rust-version
+++ b/src/tools/miri/rust-version
@@ -1 +1 @@
-699a862a3d4e4a2d5603c93297c0c44021ea72f5
+0b795044c6f0854445f1f2bb6443e87848e150d1