SE-0366 (second review): `take` operator to end the lifetime of a variable binding

[Karl]

It also seems strange because I can pass a borrow function parameter to another function which accepts its parameter by take (because the compiler will insert an implicit copy), but I can't actually perform a take manually.

func someFunction<T>(_ x: /* implicit: borrow */ T) {

  takesArg(x) // ok, compiler inserts a copy

  let _ = take x // error: 'x' is not 'take'
}

func takesArg<T>(_ x: take T) {

  let _ = take x // ok...
}

I wonder if that's going to feel confusing to people.

[jrose (Jordan Rose)]

Would it be better to make this a warning rather than an error? It has no semantic effect at run time, and (if the compiler takes it literally as “produce an owned value”) a negative performance effect, but it doesn’t otherwise make the program invalid. That would be in line with similar warnings like “cast always succeeds”.

(The take operator does have a compile-time effect, that of ending the lifetime of the original name, but I agree it’s better not to allow that if it doesn’t come with the run-time effect of releasing the value.)

[hborla (Holly Borla)]

@jrose

Would it be better to make this a warning rather than an error?

I'll provide a complete summary of the review outcome with the acceptance, but since this is right at the top: the Language Workgroup discussed this and we decided that we'd like more real-world experience with take/consume before considering downgrading this restriction. It's easy to remove restrictions later. It's not easy to add back a restriction later if we find that only producing a warning is harmful or confusing in practice.

[hborla (Holly Borla)]

Thanks everyone for your feedback so far! I've kicked off another re-review of SE-0366 here:

Combined SE-0366 (third review) and SE-0377 (second review): rename `take`/`taking` to `consume`/`consuming`

Hello, Swift community! The combined third review of SE-0366: consume operator to end the lifetime of a variable binding and second review of SE-0377: borrow and consume parameter ownership modifiers begins now and run through December 14th, 2022. Throughout the reviews for SE-0366 and SE-0377, reviewers surfaced the concern that ‘take’ is too general a name to describe ownership transfer; ‘take’ is used colloquially to describe general passing of arguments to parameters, e.g. a function takes…

Please provide further feedback on the new review thread. Thank you!

Holly Borla
Review Manager