diff --git a/tests/compile-fail/dangling_pointers/dangling_zst_deref.rs b/tests/compile-fail/dangling_pointers/dangling_zst_deref.rs
index f1b5149dabb4..01e864213df7 100644
--- a/tests/compile-fail/dangling_pointers/dangling_zst_deref.rs
+++ b/tests/compile-fail/dangling_pointers/dangling_zst_deref.rs
@@ -1,5 +1,6 @@
 // Make sure we find these even with many checks disabled.
-// compile-flags: -Zmiri-disable-alignment-check -Zmiri-disable-stacked-borrows -Zmiri-disable-validation
+// Some optimizations remove ZST accesses, thus masking this UB.
+// compile-flags: -Zmir-opt-level=0 -Zmiri-disable-alignment-check -Zmiri-disable-stacked-borrows -Zmiri-disable-validation
 
 fn main() {
     let p = {
diff --git a/tests/compile-fail/dangling_pointers/maybe_null_pointer_deref_zst.rs b/tests/compile-fail/dangling_pointers/maybe_null_pointer_deref_zst.rs
index d9f5ad4c696e..9db8a7dcba98 100644
--- a/tests/compile-fail/dangling_pointers/maybe_null_pointer_deref_zst.rs
+++ b/tests/compile-fail/dangling_pointers/maybe_null_pointer_deref_zst.rs
@@ -1,3 +1,6 @@
+// Some optimizations remove ZST accesses, thus masking this UB.
+// compile-flags: -Zmir-opt-level=0
+
 fn main() {
     // This pointer *could* be NULL so we cannot load from it, not even at ZST
     let ptr = (&0u8 as *const u8).wrapping_sub(0x800) as *const ();
diff --git a/tests/compile-fail/dangling_pointers/maybe_null_pointer_write_zst.rs b/tests/compile-fail/dangling_pointers/maybe_null_pointer_write_zst.rs
index ef46a469c3ad..79eff2507ceb 100644
--- a/tests/compile-fail/dangling_pointers/maybe_null_pointer_write_zst.rs
+++ b/tests/compile-fail/dangling_pointers/maybe_null_pointer_write_zst.rs
@@ -1,3 +1,6 @@
+// Some optimizations remove ZST accesses, thus masking this UB.
+// compile-flags: -Zmir-opt-level=0
+
 fn main() {
     // This pointer *could* be NULL so we cannot load from it, not even at ZST.
     // Not using the () type here, as writes of that type do not even have MIR generated.
diff --git a/tests/compile-fail/null_pointer_deref_zst.rs b/tests/compile-fail/null_pointer_deref_zst.rs
index cfd3a75e3b8c..d0f21a04d364 100644
--- a/tests/compile-fail/null_pointer_deref_zst.rs
+++ b/tests/compile-fail/null_pointer_deref_zst.rs
@@ -1,3 +1,6 @@
+// Some optimizations remove ZST accesses, thus masking this UB.
+// compile-flags: -Zmir-opt-level=0
+
 fn main() {
     let x: () = unsafe { *std::ptr::null() }; //~ ERROR memory access failed: 0x0 is not a valid pointer
     panic!("this should never print: {:?}", x);
diff --git a/tests/compile-fail/null_pointer_write_zst.rs b/tests/compile-fail/null_pointer_write_zst.rs
index 0f7244ba25cc..1ee1a4b8a303 100644
--- a/tests/compile-fail/null_pointer_write_zst.rs
+++ b/tests/compile-fail/null_pointer_write_zst.rs
@@ -1,3 +1,6 @@
+// Some optimizations remove ZST accesses, thus masking this UB.
+// compile-flags: -Zmir-opt-level=0
+
 fn main() {
     // Not using the () type here, as writes of that type do not even have MIR generated.
     // Also not assigning directly as that's array initialization, not assignment.
diff --git a/tests/compile-fail/unaligned_pointers/unaligned_ptr_zst.rs b/tests/compile-fail/unaligned_pointers/unaligned_ptr_zst.rs
index a9db5ff7df39..27403c11abc7 100644
--- a/tests/compile-fail/unaligned_pointers/unaligned_ptr_zst.rs
+++ b/tests/compile-fail/unaligned_pointers/unaligned_ptr_zst.rs
@@ -1,5 +1,6 @@
 // This should fail even without validation
-// compile-flags: -Zmiri-disable-validation
+// Some optimizations remove ZST accesses, thus masking this UB.
+// compile-flags: -Zmir-opt-level=0 -Zmiri-disable-validation
 
 fn main() {
     for i in 0..10 { // Try many times as this might work by chance.
diff --git a/tests/compile-fail/zst2.rs b/tests/compile-fail/zst2.rs
index 907337dcdb2b..a602cb731e40 100644
--- a/tests/compile-fail/zst2.rs
+++ b/tests/compile-fail/zst2.rs
@@ -1,3 +1,6 @@
+// Some optimizations remove ZST accesses, thus masking this UB.
+// compile-flags: -Zmir-opt-level=0
+
 fn main() {
     // Not using the () type here, as writes of that type do not even have MIR generated.
     // Also not assigning directly as that's array initialization, not assignment.
diff --git a/tests/compile-fail/zst3.rs b/tests/compile-fail/zst3.rs
index c94591174ec5..734c4b8ac4b8 100644
--- a/tests/compile-fail/zst3.rs
+++ b/tests/compile-fail/zst3.rs
@@ -1,3 +1,6 @@
+// Some optimizations remove ZST accesses, thus masking this UB.
+// compile-flags: -Zmir-opt-level=0
+
 fn main() {
     // Not using the () type here, as writes of that type do not even have MIR generated.
     // Also not assigning directly as that's array initialization, not assignment.