Replace `objc` with `objc2`?

[madsmtm]

Hey, macOS maintainer of winit here!

I've been working on a replacement for objc for around a year now, and am finally getting close to something I'm happy with, see repo, docs and reasons for fork.

The selling point for your crates (cocoa and cocoa-foundation) is... Honestly not as strong as I'd like, my improvements mostly benefit higher-level crates and users that try to make a safer API, and your crates don't really do safe. That said, there are reasons, and I think they are enough to outweigh the downsides. I'm opening this PR to get some thoughts on what it would take for you to make the switch, would love to get some feedback!

Concrete improvements that directly benefit you:

• Using the "verify_message" feature (planning to enable it with debug_assertions) now works, which is why we implement Encode for a lot of types - this is great for catching bugs, see the first commit in this PR for a few issues I found using this.
• Fixed soundness issues concerning message sending, namely:
  • Stop dereferencing the receiver - This means that sending messages to NULL is no longer immediate UB.
  • Make Object store an UnsafeCell - This enables interior mutability of Object, meaning that if user code had a &Object, converted that to *mut Object and sent a mutating message, this would not be immediate UB.
  • Don't try to access the receiver behind & - Might not be sound to access id behind &id even if Object contains UnsafeCell - A bit more theoretical, but every bit helps!
• Easier way of making methods take and return bool instead of BOOL (done in follow-up PR)
• NSInteger/NSUInteger is now isize/usize (done in follow-up PR)
• Greatly improved docs for msg_send!, hopefully new contributors will be less confused.
• Static selectors (users can opt-in using "unstable-static-sel" feature flag for now).

Future improvements that could benefit you (if one of these would be the tipping point for wanting to make the switch, please state so):

• A way to make a retainable object that doesn't guarantee the mutability of the pointer, this would allow your crates to easily follow Cocoa's memory model using msg_send_id!. Examples: You have to manually call release on the object gotten from +[NSWindow alloc] + init and -[NSMenuItem separatorItem] returns an autoreleased object that really needs to be retain'ed before use.
• derive(Encode, RefEncode) macro so we don't need to do it ourselves, which is error-prone. Not sure what you would think regarding the compile-time cost, so I went for defining a local helper macro impl_Encode instead.
• objc2 bindgen support so that the cocoa crate doesn't have to be written manually any more.

Downsides:

• Ecosystem split. I intend to submit PRs to a lot of the popular crates using objc to help this along!
• Breaking changes. I've tried to keep it as simple a replacement as possible though, so not as big an issue as one would expect.
• New, while it builds upon objc it has still seen less real-world usage.
• Will probably see a bit more frequent releases (I may have to do a breaking release or two until I'm satisfied with the API). Trying to prevent this by opening PRs and gathering feedback first!
• No MSRV policy (please provide input here!)

Breaking changes (apart from the obvious "new crate, new types"):

• Object is no longer Send or Sync, since it may be used to represent objects that aren't (like NSAutoreleasePool and NSLock).
• Methods now take and return bool instead of BOOL (follow-up PR)
• NSInteger/NSUInteger is now isize/usize (follow-up PR)

A lot of this is written assuming intimate knowledge of the existing objc crate, happy to answer any questions you might have if something is unclear!

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EDIT: Have added static selectors now.

[madsmtm]

Regarding compile-times, did a quick check, a clean build seems to be about the same (~14s) for cocoa - objc2 uses objc-sys, which has a build script, but on the other hand it doesn't depend on libc, which allows for more parallelism.

A clean build of winit regressed slightly (from ~16s to ~18s on my machine).

[bors-servo]

:umbrella: The latest upstream changes (presumably #508) made this pull request unmergeable. Please resolve the merge conflicts.

[jrmuizel]

I looked a little at objc2 last night and think it's overall the right direction to be heading in. My opinion here is based on reviewing https://github.com/gfx-rs/wgpu/pull/2826/files and the need for https://github.com/gfx-rs/wgpu/pull/2976.

objc2 is definitely a lot more complex so I haven't had a chance to look at any details yet.

[madsmtm]

I think there's a lot of potential in making things even more ergonomic, but I'll be experimenting with that in cacao first (especially msg_send_id! for imitating Objective-C ARC is promising).

I'll try to write a bit more introductory text at some point, to make the differences between objc and objc2 clearer for newcomers.

Also, at some point I'll probably submit a similar PR to this one to metal and wgpu to improve the situation there.