add a way to reuse a `Box` allocation

Proposal

Problem statement

Add two associated functions for recovering the allocation from a Box<T, A> or Pin<Box<T, A>> while dropping the value in place.

Motivating examples or use cases

In my unpublished hobby kernel, I'm using intrusive data structures (wavltree to be precise) to store notification interest lists in the scheduler. As is common with intrusiveness, the nodes are !Unpin and need to be pinned in memory to be usable. At the same time, all memory has to be allocated ahead of time, I cannot just use Box::pin to create new nodes. While the creation of new nodes in an existing allocation is possible using Box::write, I also need a way to recover the allocation (expressed as Box<MaybeUninit<T>>) from a Pin<Box<T>> while dropping the T.

Solution sketch

Add drop as an associated function to Box that acts similar to Box::take, but only returns the allocation and drops the value in place. drop_pinned performs the same operation, but for pinned boxes. The requirements of Pin are upheld as the inner value is dropped before the allocation is reused.

// in alloc::boxed

impl<T, A: Allocator> Box<T, A> {
    pub fn drop(this: Box<T, A>) -> Box<MaybeUninit<T>, A>;
    pub fn drop_pinned(this: Pin<Box<T, A>>) -> Box<MaybeUninit<T>, A>;
}

Alternatives

It is possible to write helper functions that do this in user code.

Links and related work

Start in the alloc::boxed module and compare the proposed functions with the existing Box::take and Box::write APIs. Trace how Box and Pin<Box> ownership and dropping are handled, then review the related standard-library tests. Done means the API design is agreed and its allocation-reuse and pinned-drop behavior are covered by tests.