Add a "by reference" adaptor for AllocRef

[TimDiekmann]

Fixes rust-lang/wg-allocators#53

r? @Amanieu

[Amanieu]

I've been thinking about how we're going to integrate AllocRef and GlobalAlloc and come to the conclusion that we may want to have all AllocRef methods take &self instead of &mut self.

This would allow us to have a blanket AllocRef impl for all types that implement GlobalAlloc, after which we could change #[global_allocator] to require AllocRef instead of GlobalAlloc.

I don't think it will cause any issues in practice since all currently existing allocators use &self in their methods, including bumpalo.

[TimDiekmann]

While it would be possible to simply implement AllocRef for A: GlobalAlloc this should not be the only reason to change this. Quoting the initial RFC:

The justification for &mut self is this:

• It does not restrict allocator implementors from making sharable allocators: to do so, just do impl<'a> Allocator for &'a MySharedAlloc, as illustrated in the DumbBumpPool example.
• &mut self is better than &self for simple allocators that are not sharable. &mut self ensures that the allocation methods have exclusive access to the underlying allocator state, without resorting to a lock. (Another way of looking at it: It moves the onus of using a lock outward, to the allocator clients.)

I think this should be discussed in deepth in the WG's repository. Even if we decide to change the signature to &self, then by_ref would be changed to return &Self instead of &mut Self.

[Amanieu]

@bors r+ rollup

[bors]

📌 Commit e602680 has been approved by Amanieu