Tracking Issue for `raw_dylib_macho`

This is a tracking issue for raw-dylib on Mach-O (no RFC, extending RFC 2627)
The feature gate for the issue is #![feature(raw_dylib_macho)].

raw-dylib on Mach-O links against a "Text-Based Stub" file (.tbd)[^tbd], allowing linking against libraries and frameworks that are not present on the host system (for example for cross-compilation scenarios).

See the unstable book (TODO) for example usage. See also https://github.com/rust-lang/rust/issues/58713 and https://github.com/rust-lang/rust/issues/135694.

[^tbd]: This is mostly an implementation detail that makes it nicer to inspect, it's possible to describe the same information in a generated stub .dylib.

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

• Initial implementation (https://github.com/rust-lang/rust/pull/146357)
• Wider testing
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• Should we allow omitting the verbatim modifier? How well is this supported by dyld?
• Add some way to specify current-version and compatibility-version.
• Should these overwrite the SDK, or augment the SDK? Especially relevant for the version fields above, could we make it so that the version is used from the SDK unless specified in Rust?
• Should we avoid linking an SDK if raw-dylib is the only linkage requested? How would that interact with https://github.com/rust-lang/rust/issues/121293?
• How do we make libc use this?

Implementation history

• First attempt at an implementation: https://github.com/rust-lang/rust/pull/100703
• Second implementation: https://github.com/rust-lang/rust/pull/146357