Rollup merge of rust-lang#107955 - RalfJung:ancient-ub, r=jyn514

fix UB in ancient test

This seems to go back all the way to the [original version of this test](https://github.com/rust-lang/rust/blob/b9aa9def858cfc66d411972b10ce3d98479acd78/src/test/run-pass/regions-mock-trans.rs) from ten years ago... ``@nikomatsakis`` trip down memory lane? ;)

Clearly deallocation is a form of mutation so doing it to a (pointer derived from a) shared reference cannot be legal. Let's use mutable references instead.

Author: matthiaskrgr

tests/ui/regions/regions-mock-codegen.rs

@@ -22,23 +22,23 @@ struct Ccx {
     x: isize,
 }
 
-fn allocate(_bcx: &arena) -> &Bcx<'_> {
+fn allocate(_bcx: &arena) -> &mut Bcx<'_> {
     unsafe {
         let layout = Layout::new::<Bcx>();
         let ptr = Global.allocate(layout).unwrap_or_else(|_| handle_alloc_error(layout));
-        &*(ptr.as_ptr() as *const _)
+        &mut *ptr.as_ptr().cast()
     }
 }
 
-fn h<'a>(bcx: &'a Bcx<'a>) -> &'a Bcx<'a> {
+fn h<'a>(bcx: &'a Bcx<'a>) -> &'a mut Bcx<'a> {
     return allocate(bcx.fcx.arena);
 }
 
 fn g(fcx: &Fcx) {
     let bcx = Bcx { fcx };
     let bcx2 = h(&bcx);
     unsafe {
-        Global.deallocate(NonNull::new_unchecked(bcx2 as *const _ as *mut _), Layout::new::<Bcx>());
+        Global.deallocate(NonNull::new_unchecked(bcx2 as *mut _ as *mut _), Layout::new::<Bcx>());
     }
 }