add test for main thread thread-local destructors

tests/ui/thread-local/main-thread-dtor.rs

@@ -0,0 +1,33 @@
+//! Ensure that TLS destructors run on the main thread.
+//@ run-pass
+//@ check-run-results
+// targets without threads tend to implement thread-locals as `static`s so no dtors are running
+//@ needs-threads
+// some targets do not run dtors on the main thread (issue #126858)
+//@ ignore-musl
+//@ ignore-android
+
+struct Bar;
+
+impl Drop for Bar {
+    fn drop(&mut self) {
+        println!("Bar dtor");
+    }
+}
+
+struct Foo;
+
+impl Drop for Foo {
+    fn drop(&mut self) {
+        println!("Foo dtor");
+        // We initialize another thread-local inside the dtor, which is an interesting corner case.
+        thread_local!(static BAR: Bar = Bar);
+        BAR.with(|_| {});
+    }
+}
+
+thread_local!(static FOO: Foo = Foo);
+
+fn main() {
+    FOO.with(|_| {});
+}

tests/ui/thread-local/main-thread-dtor.run.stdout

@@ -0,0 +1,2 @@
+Foo dtor
+Bar dtor