From 7949a62bf88e1d4e97c93b3a7b8a8cfb201b713a Mon Sep 17 00:00:00 2001
From: Jakob Buchgraber <buchgr@google.com>
Date: Fri, 16 Feb 2024 00:54:55 -0800
Subject: [PATCH] #rust #protobuf #concurrency Make messages Send

PiperOrigin-RevId: 607600614
---
 src/google/protobuf/compiler/rust/message.cc | 21 +++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/src/google/protobuf/compiler/rust/message.cc b/src/google/protobuf/compiler/rust/message.cc
index a44a4b94f1..65c176bc3d 100644
--- a/src/google/protobuf/compiler/rust/message.cc
+++ b/src/google/protobuf/compiler/rust/message.cc
@@ -875,12 +875,15 @@ void GenerateRs(Context& ctx, const Descriptor& msg) {
         }
 
         // SAFETY:
-        // - `$Msg$` does not provide shared mutation with its arena.
-        // - `$Msg$Mut` is not `Send`, and so even in the presence of mutator
-        //   splitting, synchronous access of an arena that would conflict with
-        //   field access is impossible.
+        // - `$Msg$` is `Sync` because it does not implement interior mutability.
+        //    Neither does `$Msg$Mut`.
         unsafe impl Sync for $Msg$ {}
 
+        // SAFETY:
+        // - `$Msg$` is `Send` because it uniquely owns its arena and does
+        //   not use thread-local data.
+        unsafe impl Send for $Msg$ {}
+
         impl $pb$::Proxied for $Msg$ {
           type View<'msg> = $Msg$View<'msg>;
           type Mut<'msg> = $Msg$Mut<'msg>;
@@ -912,12 +915,12 @@ void GenerateRs(Context& ctx, const Descriptor& msg) {
         }
 
         // SAFETY:
-        // - `$Msg$View` does not perform any mutation.
-        // - While a `$Msg$View` exists, a `$Msg$Mut` can't exist to mutate
-        //   the arena that would conflict with field access.
-        // - `$Msg$Mut` is not `Send`, and so even in the presence of mutator
-        //   splitting, synchronous access of an arena is impossible.
+        // - `$Msg$View` is `Sync` because it does not support mutation.
         unsafe impl Sync for $Msg$View<'_> {}
+
+        // SAFETY:
+        // - `$Msg$View` is `Send` because while its alive a `$Msg$Mut` cannot.
+        // - `$Msg$View` does not use thread-local data.
         unsafe impl Send for $Msg$View<'_> {}
 
         impl<'msg> $pb$::ViewProxy<'msg> for $Msg$View<'msg> {