[Threading] Add the ability to use any C++ callable in swift::once().

This allows the use of C++ lambdas and functors, in addition to plain
old functions.

rdar://90776105

Author: al45tair

include/swift/Threading/Once.h

@@ -31,6 +31,17 @@ inline void once(once_t &predicate, void (*fn)(void *),
   threading_impl::once_impl(predicate, fn, context);
 }
 
+/// Executes the given callable exactly once.
+/// The predicate argument must refer to a global or static variable of static
+/// extent of type swift::once_t.
+template <typename Callable>
+inline void once(once_t &predicate, const Callable &callable) {
+  once(predicate, [](void *ctx) {
+    const Callable &callable = *(const Callable*)(ctx);
+    callable();
+  }, (void *)(&callable));
+}
+
 } // namespace swift
 
 #endif // SWIFT_THREADING_ONCE_H

unittests/Threading/Once.cpp

@@ -71,3 +71,18 @@ TEST(OnceTest, once_threaded) {
     ASSERT_EQ(1u, callCount);
   }
 }
+
+// Check that swift::once works with a C++ lambda
+TEST(OnceTest, once_lambda) {
+  static swift::once_t predicate;
+  unsigned callCount = 0;
+
+  auto fn = [&callCount]() {
+    ++callCount;
+  };
+
+  swift::once(predicate, fn);
+  swift::once(predicate, fn);
+
+  ASSERT_EQ(1u, callCount);
+}