Tracking Issue for `darwin_objc`

[jbatez]

The feature gate for the issue is #![feature(darwin_objc)].

This feature makes it possible to reference Objective-C classes and selectors using the same ABI used by native Objective-C on Apple/Darwin platforms. Without it, Rust code interacting with Objective-C must resort to loading classes and selectors using costly string-based lookups at runtime. With it, these references can be loaded efficiently at dynamic load time.

• Summary: https://hackmd.io/@jubilee/B1jb8LXfgg
• Original issue: Incremental compilation fails when a generic function uses a private symbol #53929
• Zulip:
  • https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Low-overhead.20Objective-C.20interop
  • https://rust-lang.zulipchat.com/#narrow/channel/427678-t-lang.2Finterop/topic/Attribute.20Proposal.20for.20Low-Overhead.20Objective-C.20Interop
  • https://rust-lang.zulipchat.com/#narrow/channel/213817-t-lang/topic/Attribute.20Proposal.20for.20Low-Overhead.20Objective-C.20Interop

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Open and land implementation PRs
• Coordinate with lang/libs-api to decide on an API
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)
• Propose FCP

Unresolved Questions

• Is core::ffi::darwin::objc the module this should this live in?
• Should we remove no_dead_strip, to allow the selectors to be stripped if unused? CC macOS: Why are Objective-C sections marked with no_dead_strip? llvm/llvm-project#114111.
• How do we support more complex use-cases (weak classes, raw-dylib)?
• What about protocol references, and constant CFString? These need somewhat similar compiler support.
• Should this be a pure language feature instead? (e.g. remove the std parts, and only expose it as a built-in attribute on statics).

Implementation history

• Initial implementation PR: initial implementation of the darwin_objc unstable feature #145660

[tmandry]

I volunteer to champion this as an experiment. It will still need an RFC before stabilization.