Rollup merge of #122018 - RalfJung:box-custom-alloc, r=oli-obk

only set noalias on Box with the global allocator

As discovered in https://github.com/rust-lang/miri/issues/3341, `noalias` and custom allocators don't go well together.

rustc can now check whether a Box uses the global allocator. This replaces the previous ad-hoc and rather unprincipled check for a zero-sized allocator.

This is the rustc part of fixing that; Miri will also need a patch.

compiler/rustc_const_eval/src/interpret/place.rs

@@ -437,6 +437,7 @@ where
         trace!("deref to {} on {:?}", val.layout.ty, *val);
 
         if val.layout.ty.is_box() {
+            // Derefer should have removed all Box derefs
             bug!("dereferencing {}", val.layout.ty);
         }
 

compiler/rustc_const_eval/src/interpret/terminator.rs

@@ -359,14 +359,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             Ok(Some(match ty.kind() {
                 ty::Ref(_, ty, _) => *ty,
                 ty::RawPtr(mt) => mt.ty,
-                // We should only accept `Box` with the default allocator.
-                // It's hard to test for that though so we accept every 1-ZST allocator.
-                ty::Adt(def, args)
-                    if def.is_box()
-                        && self.layout_of(args[1].expect_ty()).is_ok_and(|l| l.is_1zst()) =>
-                {
-                    args[0].expect_ty()
-                }
+                // We only accept `Box` with the default allocator.
+                _ if ty.is_box_global(*self.tcx) => ty.boxed_ty(),
                 _ => return Ok(None),
             }))
         };