From 642278c45300445c547e2fb70bb4ab95c127e438 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Fri, 30 Aug 2024 13:06:56 -0700
Subject: [PATCH] Fix compatibility with compilers older than 1.72

Old compilers didn't used to consider `()` acceptable for FFI.

    error: `extern` block uses type `()`, which is not FFI-safe
      --> demo/src/main.rs:20:14
       |
    20 |           type BlobstoreClient;
       |  ______________^
    21 | |
    22 | |         fn new_blobstore_client() -> UniquePtr<BlobstoreClient>;
    23 | |         fn put(&self, parts: &mut MultiBuf) -> u64;
       | |________________^ not FFI-safe
       |
       = help: consider using a struct instead
       = note: tuples have unspecified layout
    note: the lint level is defined here
      --> demo/src/main.rs:1:1
       |
    1  | #[cxx::bridge(namespace = "org::blobstore")]
       | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       = note: this error originates in the attribute macro `cxx::bridge` (in Nightly builds, run with -Z macro-backtrace for more info)
---
 src/opaque.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/opaque.rs b/src/opaque.rs
index ccf74f415..2ca90af43 100644
--- a/src/opaque.rs
+++ b/src/opaque.rs
@@ -15,7 +15,7 @@ use core::mem;
 pub struct Opaque {
     _private: [*const void; 0],
     _pinned: PhantomData<PhantomPinned>,
-    _mutable: SyncUnsafeCell<()>,
+    _mutable: SyncUnsafeCell<PhantomData<()>>,
 }
 
 // TODO: https://github.com/rust-lang/rust/issues/95439