In Rust, are there any functional differences between
Box<Pin<T>>? I think that they should behave the same, but I’m not sure.
Pin<Box<T>> is what you want.
Box<Pin<T>> will not work at all.
Pin requires its type to be a pointer of some kind. It then prevents you from moving out of this pointer (if the pointee isn’t
Unpin), by requiring
unsafe to access it mutably. In
Box<T> is the pointer. It is common because you can create it safely (as opposed to
Pin<&mut T> that without macros can only be created unsafely) because you give the ownership of the
Box to it, and thus you cannot access the inner
T not through the
Box<Pin<T>>, on the other hand, is useless: it is impossible to create if
T does not implement
Pin‘s constructors require that, because they are meant to use with pointers) and even if
T does, the
Box is redundant: you already have a pointer, there is no need to wrap it in
Box. In addition, you cannot create an instance of
Box<Pin<T>> if the
<T as Deref>::Target does not implement
Unpin without unsafe code, and there is little benefit in
Unpin types (it can be passed to APIs that require it, such as
Future::poll(), but in that case you don’t need the