Fix wrong pointer type of static _dispatch_main_q binding

[MarijnS95]

While looking at a copy of this code (in another project that uses objc2, which does not yet define dispatch integration) this strange cast immediately stands out. _dispatch_main_q is defined here as a dispatch_queue_t, the queue argument in dispatch_data_create() is also defined as a dispatch_queue_t, yet there is a "borrow" (get pointer to) operation before passing it into the function.

Instead, this static field is supposed to be defined as the Object itself, so that any user could take a pointer to it if they wish or need it so. The same is seen in the dispatch crate: _dispatch_main_q is defined as a dispatch_object_s, and dispatch_queue_t is a typedef to a *mut dispatch_object_s to match the above convention.

Note that this is not a bugfix, but merely a readability improvement. Use of mut here is debatable.

[MarijnS95]

CI correctly suggests usage of the new &raw syntax, but that requires an MSRV bump to 1.82.

[madsmtm]

👍

Note that it might technically be a soundness fix, since we now go from telling the compiler that _dispatch_main_q has the size of a pointer (and asserting the validity of that by creating a reference in &_dispatch_main_q), to using Object which is zero-sized, and hence acts as-if the object may have an arbitrary size (in a similar way that extern types do).

But I doubt it matters at all in practice (esp. since LLVM migrated to opaque pointers).

Use of mut here is debatable.

Yeah, if we want to be really correct, I'd recommend something like _dispatch_main_q: SyncUnsafeCell<Object>.

MSRV

You can use std::ptr::addr_of[_mut]! instead (and #[allow(...)] the clippy warning for that).

[ErichDonGubler]

@cwfitzgerald: Now that Firefox has an MSRV of Rust 1.82, I wouldn't object to a bump to Rust 1.82 (as follow-up) so we can use &raw here later. Thoughts?

[cwfitzgerald]

Yeah that's fine.