From dedb31e76df69ac8be5d7ff62d7dd47d98decb5b Mon Sep 17 00:00:00 2001
From: "Dylan M. Taylor" <dylan@dylanmtaylor.com>
Date: Wed, 4 Nov 2026 01:34:12 +0000
Subject: [PATCH] address review: make ScriptSandbox !Send so it drops on the
 thread that entered it

---
 crates/engine/src/io.rs       |  4 ++--
 crates/engine/src/io/tests.rs | 13 +++++++++++++
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/crates/engine/src/io.rs b/crates/engine/src/io.rs
index 8c56cccd6..e25c2373c 100644
--- a/crates/engine/src/io.rs
+++ b/crates/engine/src/io.rs
@@ -755,12 +755,12 @@ thread_local! {
 /// While one is alive, [`read_file`] and [`write_file`] on this thread refuse every path: a
 /// stored script's steps run inside one (#2178), so no command reaches the file system from a
 /// script whatever its parameters are called. Dropping it (also on unwind) lifts the refusal.
-pub struct ScriptSandbox(());
+pub struct ScriptSandbox(std::marker::PhantomData<*const ()>);
 
 impl ScriptSandbox {
     pub fn enter() -> Self {
         let _ = SCRIPT_SANDBOX.try_with(|c| c.set(c.get().saturating_add(1)));
-        ScriptSandbox(())
+        ScriptSandbox(std::marker::PhantomData)
     }
 }
 
diff --git a/crates/engine/src/io/tests.rs b/crates/engine/src/io/tests.rs
index 9051c5c09..4b7fe2023 100644
--- a/crates/engine/src/io/tests.rs
+++ b/crates/engine/src/io/tests.rs
@@ -351,6 +351,19 @@ fn script_sandbox_refuses_file_access_until_dropped() {
     let _ = std::fs::remove_dir_all(&dir);
 }
 
+/// The guard's drop lowers the counter of the thread it runs on, so it must not move between threads:
+/// naming the trait's method is ambiguous (a compile error) if `ScriptSandbox` is `Send`.
+#[test]
+fn script_sandbox_is_not_send() {
+    trait AmbiguousIfSend<A> {
+        fn check() {}
+    }
+    impl<T: ?Sized> AmbiguousIfSend<()> for T {}
+    struct IsSend;
+    impl<T: ?Sized + Send> AmbiguousIfSend<IsSend> for T {}
+    <ScriptSandbox as AmbiguousIfSend<_>>::check();
+}
+
 /// A minimal compound (CFB/OLE2) file — FAT in sector 0, directory in sector 1 — holding a
 /// root entry and the given stream names.
 fn cfb(streams: &[&str]) -> Vec<u8> {