Return a non_null::dangling() for 0-sized allocations.

PiperOrigin-RevId: 606980834

rust/cpp_kernel/cpp_api.h

@@ -42,6 +42,9 @@ extern "C" void* __pb_rust_alloc(size_t size, size_t align);
 inline SerializedData SerializeMsg(const google::protobuf::Message* msg) {
   size_t len = msg->ByteSizeLong();
   void* bytes = __pb_rust_alloc(len, alignof(char));
+  if (bytes == nullptr) {
+    ABSL_LOG(FATAL) << "Rust allocator failed to allocate memory.";
+  }
   if (!msg->SerializeToArray(bytes, static_cast<int>(len))) {
     ABSL_LOG(FATAL) << "Couldn't serialize the message.";
   }

rust/cpp_kernel/rust_alloc_for_cpp_api.rs

@@ -9,6 +9,12 @@ use std::alloc::{alloc, Layout};
 
 #[no_mangle]
 extern "C" fn __pb_rust_alloc(size: usize, align: usize) -> *mut u8 {
-    let layout = Layout::from_size_align(size, align).unwrap();
-    unsafe { alloc(layout) }
+    if size == 0 {
+        // A 0-sized layout is legal but the global allocator isn't required to support
+        // it so return a dangling pointer instead.
+        std::ptr::null_mut::<u8>().wrapping_add(align)
+    } else {
+        let layout = Layout::from_size_align(size, 1).unwrap();
+        unsafe { alloc(layout) }
+    }
 }