disable MIR opts for ZST-related tests

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 = {

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 ();

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.

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);

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.

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.

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.

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.