The plan for Swift feature-parity

[madsmtm]

I've been quite focused on this project simply being "improve the safety of Objective-C in Rust", but @silvanshade recently proposed that we try to reach higher than just that, instead try going for "something that is (at least somewhat) competitive with Swift" (Swift interop is discussed in #524).

This required a bit of a mental shift for me, but ultimately, I think it is the right way to go about it. This change in mindset should help me realize that tooling, e.g. good rust-analyzer and debugger support, is really important, and we should spend effort on that not just in this project, but also in requesting changes to Rust to make it possible.

A few subtasks:

• Make ownership much simpler in the common case: Redo how ownership works #419
• Some way to allow defining your own Objective-C interfaces, see e.g. Merge header-translator into bindgen #729.
• All of Class definition improvements #428
• Better error/exception "user info" access (see discussion in Fix enums and typedefs in framework crates #310)
• Better main thread support #496
• Investigate and push forward Rust features that:
  • Add keyword-like parameters (framework functions are very long and ugly)
  • Improves library debugger support
  • Improves the ergonomics of using smart pointers Allocated and Id
  • Some way to better declare categories - for example allows us to represent them as a trait, possibly as "inherent traits?"
  • Some way for the compiler to automatically convert Id<NSButton> to Id<NSView>, since calling Id::into_super (potentially many times) is quite cumbersome. Maybe the coerce unsized feature will do it for us?
  • Some kind of #[distributed_slice], as suggested by epage in their blog post about testing would be really nice for making declare_class! cleaner.
    • Linking Zulip stream about this
    • Linking Zulip stream about linkme
    • Linking an older Zulip discussion about similar things
    • Note for future RFC: These statics would work similar to extern statics, in that you can't perform const operations on them.
    • Linking #[distributed_slice] aka a method to enumerate tests rust-lang/testing-devex-team#3.
    • Linking https://internals.rust-lang.org/t/global-registration-a-kind-of-pre-rfc/20813.
  • Improve our ability to name functions directly, meaning that we can associate e.g. a Sel with each function, very similar to what Swift does.
  • Make examples easier to run
  • Make MainThreadMarker, AutoReleasePool<'p> and availability easier to use (contexts & capabilities)
  • The ability to declare custom attributes as unsafe.
  • Allowing inherent associated types, just like Swift does.
  • Make it easier to link Rust-built static libraries with the system linker.
  • More ergonomic reference-counting, Simplify lightweight clones, including into closures and async blocks rust-lang/rfcs#3680.
  • Improving backtraces of msg_send!/msg_send_id!/extern_methods.
  • Improving macro expansion in extern_methods such that error spans point to the definition of the method, instead of the macro invocation itself.
  • Better const support to allow us to do Create blocks with BLOCK_HAS_SIGNATURE #442 without the user needing to write out the type themselves.
• Integrate header-translator work into bindgen.
• Add objc2 support in cbindgen.
• ...