Replace MaybeUninit in allocator arrays with Option

[caizixian]

I don't think the changes here are performance sensitive but I can run some benchmarks if needed

[wks]

Not sure if it is easy to test, but if it is not performance critical, we may just use Vec<XxxAllocator> instead of [Option<XxxAllocator>; MAX_XXX_ALLOCATORS].

[k-sareen]

Vec is not FFI-safe, technically. And currently the entire mutator struct is embedded inside the OpenJDK mutator.

[caizixian]

Option is not technically FFI safe either though

[k-sareen]

Right. Option<NonNullReference> is FFI-safe from memory. Do we want to Box the allocators then?

[qinsoon]

Does Option<XxxAllocator> have the same layout as XxxAllocator?

The other option to remove MaybeUninit is to actually init all the allocators, but for the ones that are not used, we initialize it with a special fake space. See #1486.

[qinsoon]

This test could check the layout and ffi compatibility:

// GITHUB-CI: MMTK_PLAN=NoGC
// Just test this onece.

use lazy_static::lazy_static;

use super::mock_test_prelude::*;
use crate::util::alloc::*;
use std::mem::size_of;

#[test]
pub fn test_option_allocator_layout() {
    assert_eq!(size_of::<Option<BumpAllocator<MockVM>>>(), size_of::<BumpAllocator<MockVM>>());
    assert_eq!(size_of::<Option<LargeObjectAllocator<MockVM>>>(), size_of::<LargeObjectAllocator<MockVM>>());
    assert_eq!(size_of::<Option<MallocAllocator<MockVM>>>(), size_of::<MallocAllocator<MockVM>>());
    assert_eq!(size_of::<Option<ImmixAllocator<MockVM>>>(), size_of::<ImmixAllocator<MockVM>>());
    assert_eq!(size_of::<Option<FreeListAllocator<MockVM>>>(), size_of::<FreeListAllocator<MockVM>>());
    assert_eq!(size_of::<Option<MarkCompactAllocator<MockVM>>>(), size_of::<MarkCompactAllocator<MockVM>>());
}

#[test]
pub fn test_0_transmute_as_option_none() {
    const BUMP_ALLOCATOR_SIZE: usize = size_of::<Option<BumpAllocator<MockVM>>>();
    let zero_bump = unsafe { std::mem::transmute::<_, Option<BumpAllocator<MockVM>>>([0u8; BUMP_ALLOCATOR_SIZE]) };
    assert!(zero_bump.is_none());
}

[qinsoon]

Can you check the performance of this PR? @caizixian

[caizixian]

Unit tests added and checked with cargo test mock_test_option_allocator_layout --features mock_test