alloc: Apply unsafe_attr_outside_unsafe

Author: ehuss

alloc/src/boxed.rs

@@ -97,12 +97,12 @@
 //! #[repr(C)]
 //! pub struct Foo;
 //!
-//! #[no_mangle]
+//! #[unsafe(no_mangle)]
 //! pub extern "C" fn foo_new() -> Box<Foo> {
 //!     Box::new(Foo)
 //! }
 //!
-//! #[no_mangle]
+//! #[unsafe(no_mangle)]
 //! pub extern "C" fn foo_delete(_: Option<Box<Foo>>) {}
 //! ```
 //!