std: Relax UnwindSafe impl for Unique

Add the `?Sized` bound as we don't require the type to be sized.

Closes #40011

Author: alexcrichton

src/libstd/panic.rs

@@ -198,7 +198,7 @@ impl<T: RefUnwindSafe + ?Sized> UnwindSafe for *const T {}
 #[stable(feature = "catch_unwind", since = "1.9.0")]
 impl<T: RefUnwindSafe + ?Sized> UnwindSafe for *mut T {}
 #[unstable(feature = "unique", issue = "27730")]
-impl<T: UnwindSafe> UnwindSafe for Unique<T> {}
+impl<T: UnwindSafe + ?Sized> UnwindSafe for Unique<T> {}
 #[unstable(feature = "shared", issue = "27730")]
 impl<T: RefUnwindSafe + ?Sized> UnwindSafe for Shared<T> {}
 #[stable(feature = "catch_unwind", since = "1.9.0")]

src/test/run-pass/panic-safe.rs

@@ -9,7 +9,6 @@
 // except according to those terms.
 
 #![allow(dead_code)]
-#![feature(recover)]
 
 use std::panic::{UnwindSafe, AssertUnwindSafe};
 use std::cell::RefCell;
@@ -40,6 +39,10 @@ fn main() {
     assert::<&RwLock<i32>>();
     assert::<Rc<i32>>();
     assert::<Arc<i32>>();
+    assert::<Box<[u8]>>();
+
+    trait Trait: UnwindSafe {}
+    assert::<Box<Trait>>();
 
     fn bar<T>() {
         assert::<Mutex<T>>();