transmute_ptr_to_ref suggestion doesn't with some outer expression types

[oli-obk]

clippy suggests the following replacement

-                mem::transmute::<*mut c_void, &mut Box<FnMut(&Button)>>(data)(&button)
+                &mut *(data as *mut Box<FnMut(&Button)>)(&button)

but we should suggest parens around the &mut *...

bonus points if the parens only appear if necessary

[Arc8ne]

@rustbot claim