Arena::alloc_copy_inner (at least) should be renamed and made public

[comex]

I think this is small enough not to need a RFC.

Just as we have both box and heap::allocate, Arena has a safe method to construct an object of a fixed-size type, but should also have an unsafe method to allocate an arbitrary number of bytes. This is what alloc_copy_inner currently does (used for types without a destructor), but it is private. It should be made public (and unsafe?) and given a more descriptive name.

It would be nice to have functionality such as custom destructors and a safe way to allocate DSTs in an Arena, but those would require, respectively, some refactoring and placement new in the language. What this issue is asking for, however, is a simple change that doesn't require design work.

[arielb1]

You don't "allocate DST-s" in Rust: for trait objects, you allocate a regular struct and then cast the &Struct to an &Trait (or Box<Struct> to Box<Trait>), and for arrays you use something like Vec, which malloc-s a block of memory and creates a &mut [T] with the right length that points to it.

Trait objects already work with arenas, and teaching Vec about them requires allocator traits.

[pnkfelix]

cc me