Adjust tests for newly uplifted cast_ref_to_mut lint

src/tools/miri/tests/fail/modifying_constants.rs

@@ -1,6 +1,8 @@
 // This should fail even without validation/SB
 //@compile-flags: -Zmiri-disable-validation -Zmiri-disable-stacked-borrows
 
+#![allow(cast_ref_to_mut)]
+
 fn main() {
     let x = &1; // the `&1` is promoted to a constant, but it used to be that only the pointer is marked static, not the pointee
     let y = unsafe { &mut *(x as *const i32 as *mut i32) };

src/tools/miri/tests/fail/stacked_borrows/shr_frozen_violation1.rs

@@ -1,3 +1,5 @@
+#![allow(cast_ref_to_mut)]
+
 fn foo(x: &mut i32) -> i32 {
     *x = 5;
     unknown_code(&*x);

tests/ui/const-generics/issues/issue-100313.rs

@@ -9,6 +9,7 @@ impl <const B: &'static bool> T<B> {
         unsafe {
             *(B as *const bool as *mut bool) = false;
             //~^ ERROR evaluation of constant value failed [E0080]
+            //~| ERROR casting `&T` to `&mut T` is undefined behavior
         }
     }
 }

tests/ui/const-generics/issues/issue-100313.stderr

@@ -1,3 +1,11 @@
+error: casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
+  --> $DIR/issue-100313.rs:10:13
+   |
+LL |             *(B as *const bool as *mut bool) = false;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(cast_ref_to_mut)]` on by default
+
 error[E0080]: evaluation of constant value failed
   --> $DIR/issue-100313.rs:10:13
    |
@@ -10,11 +18,11 @@ note: inside `T::<&true>::set_false`
 LL |             *(B as *const bool as *mut bool) = false;
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 note: inside `_`
-  --> $DIR/issue-100313.rs:18:5
+  --> $DIR/issue-100313.rs:19:5
    |
 LL |     x.set_false();
    |     ^^^^^^^^^^^^^
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0080`.