Make Global and System allocators unstably implement const Clone + const Default.

[kpreid]

Allocators implementing const Default is useful for generically constructing empty collections inside of constants:

struct Foo<A: Allocator>(Vec<i32, A>);
impl<A: Allocator + const Default> Foo<A> {
    pub const EMPTY: Self = Self(Vec::new_in(A::default()));
}

const Clone is not strictly necessary to permit such constants to exist, but may make more complex code easier to write (e.g. so a const constructor can accept one allocator and clone it).

@rustbot label +A-allocators

[rustbot]

r? @clarfonthey

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[clarfonthey]

Based upon the ruling in #155816 I'm tempted to just close this. You can already construct both of these in const code because they're just ZSTs with public constructors, and even though adding const traits is easy, similarly, this means we can do that later.

[kpreid]

You can already construct both of these in const code because they're just ZSTs with public constructors

You can’t construct them in generic const code.

Well, can’t without defining your own ConstAllocatorDefault trait to substitute for Default entirely. I suppose that would work. I do think, though, that it would be particularly useful to have this impl for people experimenting with Allocator before its stabilization, because even if it’s not strictly necessary it’s a speedbump.

[clarfonthey]

Right, so, I had missed the discussion on Zulip, and you're right these should be reasonable to add. Sorry for the confusion.

@bors r+ rollup