Tracking Issue for Swift function call ABI

This is a tracking issue for the lang experiment for Swift function call ABI.
The feature gate for the issue is #![feature(abi_swift)].

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

• Find a lang champion willing to sponsor this lang experiment.
  https://rust-lang.zulipchat.com/#narrow/channel/213817-t-lang/topic/Swift.20call.20ABI/with/593327052
• Initial impl + feature gate: #155815
• Figure out the stability story (see Unresolved Questions).
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Discussions

• #t-lang > Swift call ABI

Unresolved Questions

• How useful is this without repr(Swift)?
  • How to soundly implement repr(Swift) given that it breaks the stride=size assumption that is true for all current memory layouts? comment
• Does the Swift abi needs its own classify_arg? comment
• Stability story: Hypothetically, if 12 years from now the Swift ABI was no longer a thing, would we have any reasonable way to remove this, and would we want to?
  • Some kind of ABI Tiering (akin to Target Tiers) that allows promotion/demotion (including potential removal)?
• While Swift call ABI is currently (as of May 2026) only stable on Darwin-like targets, do we expand that support if Swift call ABI becomes stable on other non-Darwin-like targets?
  • (Related to stabiliyt story) will Darwin-like vs non-Darwin-like targets receive different ABI Tiers?

Implementation history

• Initial exploration: https://github.com/rust-lang/rust/pull/64582
• Initial impl based on #64582: https://github.com/rust-lang/rust/pull/155815

Start with the initial implementation in PR #155815 and the unresolved questions about stability, repr(Swift), argument classification, and target support. Read the rustc-dev-guide stabilization and documentation-pr guidance. Done means the stability story is resolved, documentation is adjusted, and a stabilization PR is prepared.