#isolation （だっけ？）

おそらくそれかと。

default argument expressions can now have the special form #isolation, which will be filled in with the actor isolation of the caller. If the default argument is for an isolated parameter, this allows isolation to be implicitly passed down.

https://github.com/apple/swift-evolution/blob/main/proposals/0420-inheritance-of-actor-isolation.md (edited)

swift-evolution/proposals/0420-inheritance-of-actor-isolation.md at...

This maintains proposals for changes and user-visible enhancements to the Swift Programming Language. - apple/swift-evolution