Add Sync/Send wrapper for main-thread-only data

src/threads.rs

@@ -4,6 +4,8 @@
 use crate::flog::{FloggableDebug, FLOG};
 use crate::reader::ReaderData;
 use once_cell::race::OnceBox;
+use std::cell::RefCell;
+use std::marker::PhantomData;
 use std::num::NonZeroU64;
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::{Arc, Mutex};
@@ -358,6 +360,44 @@ impl ThreadPool {
     }
 }
 
+/// A `Sync` and `Send` wrapper for non-`Sync`/`Send` types.
+/// Only allows access from the main thread.
+pub struct MainThread<T> {
+    data: RefCell<T>,
+    // Make type !Send and !Sync by default
+    _marker: PhantomData<*const ()>,
+}
+
+// Manually implement Send and Sync for MainThread<T> to ensure it can be shared across threads
+// as long as T is 'static.
+unsafe impl<T: 'static> Send for MainThread<T> {}
+unsafe impl<T: 'static> Sync for MainThread<T> {}
+
+impl<T> MainThread<T> {
+    pub const fn new(value: T) -> Self {
+        Self {
+            data: RefCell::new(value),
+            _marker: PhantomData,
+        }
+    }
+
+    pub fn with<F, R>(&self, f: F) -> R
+    where
+        F: FnOnce(&T) -> R,
+    {
+        assert_is_main_thread();
+        f(&self.data.borrow())
+    }
+
+    pub fn with_mut<F, R>(&self, f: F) -> R
+    where
+        F: FnOnce(&mut T) -> R,
+    {
+        assert_is_main_thread();
+        f(&mut self.data.borrow_mut())
+    }
+}
+
 pub struct WorkerThread {
     /// The data shared with the [`ThreadPool`].
     shared: Arc<ThreadPoolShared>,