Allow the caller to allocate memory for opaque data

[Manishearth]

Currently opaque structs need to be allocated with the system allocator. It would be nice if C++ (at least) callers could handle their own memory management, perhaps using bump allocators for short-lived objects.

There are two ways I can see us doing this, which I'll write out in comments

[Manishearth]

MaybeUninit

The first way is supporting &mut MaybeUninit<T> as parameters.

E.g.

#[diplomat::bridge]
mod ffi {
    impl FixedDecimal {
        fn new_in_place(&mut MaybeUninit<Self>) {..}
    }
}

For this code, the following stuff happens:

• extern Rust / c:
  • FixedDecimal_new_in_place generated with a FixedDecimal* parameter
  • FixedDecimal_get_layout() generated, returns a size and alignment
  • FixedDecimal_drop_in_place() generated
• js: all these methods are ignored
• cpp:
  • FixedDecimal::new_in_place() generated with no parameters
  • FixedDecimal's unique-ptr is retooled to be freeable in different ways (or something else?)
  • ALLOCATE_FIXED_DECIMAL() macro generated to alloca() fixeddecimal

[Manishearth]

CustomAllocBox

#[diplomat::bridge] mod ffi {
impl PluralRules {
    fn new_alloc(Locale, alloc: CustomAllocFn) -> CustomAllocBox<PluralRules> {
       let pr = PluralRules::new(Locale);
       CustomAllocBox::new(pr, alloc);
    }
}
}

CustomAllocBox is a type with an aborting destructor, that is intended to be called with a new(alloc_fn) and paired with box.free(free_fn). This generates:

• extern Rust / c:
  • FixedDecimal_drop_in_place()
  • CustomAllocBox is internally a *mut T
• js: probably ignore these functions
• cpp: Perhaps have custom allocator parameters that can be passed in?