Description of `sending` in Flow-Sensitive Isolation Analysis needs more elaboration

[bjo0]

https://www.swift.org/migration/documentation/swift-6-concurrency-migration-guide/dataracesafety#Flow-Sensitive-Isolation-Analysis

The text on it is as follows:

Region-based isolation works without any code changes. But a function’s parameters and return values can also explicitly state that they support crossing domains using this mechanism.

func populate(island: Island, with chicken: sending Chicken) async {
    await island.adopt(chicken)
}

The compiler can now provide the guarantee that at all call sites, the chicken parameter will never be subject to unsafe access. This is a relaxing of an otherwise significant constraint. Without sending, this function would only be possible to implement by requiring that Chicken first conform to Sendable.

Based on this, I don't feel like I understand how to reason about using sending. Is the compiler still going to guarantee safety, or is this a foot gun? If it still guarantees safety, how is it any different than omitting sending? Another sentence or two, or a reference to a more detailed explanation, would be helpful.

[mattmassicotte]

Yes, I agree. There is no unsafety here. But, this example in isolation doesn't make it clear enough that without sending, this code will not compile because the compiler cannot guarantee that chicken can be transferred across domains.