Rollup merge of #76454 - poliorcetics:ui-to-unit-test-1, r=matklad

UI to unit test for those using Cell/RefCell/UnsafeCell

Helps with #76268.

I'm working on all files using `Cell` and moving them to unit tests when possible.

r? @matklad

library/std/src/panic.rs

@@ -411,3 +411,6 @@ pub fn catch_unwind<F: FnOnce() -> R + UnwindSafe, R>(f: F) -> Result<R> {
 pub fn resume_unwind(payload: Box<dyn Any + Send>) -> ! {
     panicking::rust_panic_without_hook(payload)
 }
+
+#[cfg(test)]
+mod tests;

library/std/src/panic/tests.rs

@@ -0,0 +1,56 @@
+#![allow(dead_code)]
+
+use crate::cell::RefCell;
+use crate::panic::{AssertUnwindSafe, UnwindSafe};
+use crate::rc::Rc;
+use crate::sync::{Arc, Mutex, RwLock};
+
+struct Foo {
+    a: i32,
+}
+
+fn assert<T: UnwindSafe + ?Sized>() {}
+
+#[test]
+fn panic_safety_traits() {
+    assert::<i32>();
+    assert::<&i32>();
+    assert::<*mut i32>();
+    assert::<*const i32>();
+    assert::<usize>();
+    assert::<str>();
+    assert::<&str>();
+    assert::<Foo>();
+    assert::<&Foo>();
+    assert::<Vec<i32>>();
+    assert::<String>();
+    assert::<RefCell<i32>>();
+    assert::<Box<i32>>();
+    assert::<Mutex<i32>>();
+    assert::<RwLock<i32>>();
+    assert::<&Mutex<i32>>();
+    assert::<&RwLock<i32>>();
+    assert::<Rc<i32>>();
+    assert::<Arc<i32>>();
+    assert::<Box<[u8]>>();
+
+    {
+        trait Trait: UnwindSafe {}
+        assert::<Box<dyn Trait>>();
+    }
+
+    fn bar<T>() {
+        assert::<Mutex<T>>();
+        assert::<RwLock<T>>();
+    }
+
+    fn baz<T: UnwindSafe>() {
+        assert::<Box<T>>();
+        assert::<Vec<T>>();
+        assert::<RefCell<T>>();
+        assert::<AssertUnwindSafe<T>>();
+        assert::<&AssertUnwindSafe<T>>();
+        assert::<Rc<AssertUnwindSafe<T>>>();
+        assert::<Arc<AssertUnwindSafe<T>>>();
+    }
+}