[Pitch] Noncopyable (or "move-only") structs and enums

Evolution Pitches
41

I don’t think the problem here is the negative copyable constraint ?: Copyable, but rather implicit requirements. I think a better name for your attribute is @implicitConstraint(MyProtocol), which —as you say— could generalize to other feature like reflection.

This syntax generalizes a lot better. Swift already synthesizes some implicit conformances (e.g. Sendable), so a general syntax would be a great way to opt out. The syntax would also naturally extend to opting out of the aforementioned implicit constraints. But more importantly, Copyable should be a protocol. IIRC, the compiler generates witnesses for copyable types that describe how they should be copied. Even if Copyable’s requirement is an underscore-prefixed __copy() method, implementing and explaining copies in Swift would be more straightforward. So it only makes sense that a move-only type would shed its implicit Copyable conformance, like any other conformance (?: Sendable), with a ?: Copyable syntax.

42

I think the key distinction to draw is that I’m suggesting @noImplicitCopy be automatically applied to all code that isn’t in an ImplicitlyCopyable scope – or, to put it another way, that code that isn’t in an ImplicitlyCopyable scope doesn’t automatically synthesise conformances to ImplicitlyCopyable for Copyable types that are used within said scope (wherever they may be defined). That’s in addition to suppressing automatic conformance of Copyable for all types defined outside of an ImplicitlyCopyable scope.

Rephrasing without negative constraints: code within an ImplicityCopyable scope automatically generates conformances to ImplicitlyCopyable for all Copyable types regardless of where they’re defined, and additionally synthesises conformances to Copyable for all types defined within that scope. All current Swift code is assumed to be implicitly within an ImplicityCopyable scope.

1 Like
43

Yes, I also think this is very important for a large number of use cases for move-only types. I think I would want to make it possible to mark deinit as public, private, fileprivate, or internal, because the type itself might just be a move-only token used as an API currency type.

44

This is very nicely subsetted out from the full feature set. Obviously I look forward to generics-compatible non-copyable types, but I agree that this has use cases on its own, and it is definitely easier to review these proposals in chunks rather than all at once, even as we treat them as part of a whole. Kudos!

People already seem to be discussing a number of things I'm interested in, especially "should the attribute affect the type's generic parameters" because that does affect future proposals. There's only one thing that's jumped out at me that no one's mentioned:

For a local var or let binding, or consume function parameter, that is not itself consumed, deinit runs after the last non-consuming use.

What does "after" mean here? The example shows a non-consuming use in a function call, followed by deinitialization on the next line. What if there are nested function calls? Thus far, Swift does not share C++'s concept of a "full-expression", so would it happen between the inner and outer calls, like inout ending/cleanup? If the function is inlined, could the deinit happen before it's even complete? And if I want my binding to live longer, I can certainly use an explicit consume x, but the cost of forgetting that could result in a bug.

Between the discussion about class instances being released unexpectedly early (which I'm having trouble finding at the moment), and the precedent set by Rust, I would appreciate more discussion on why the default behavior isn't to deinit at end of scope, with an early consume allowing for more control when needed.

EDIT: I can think of one reason why this rule isn’t sufficient: directly passing an owned return value to a borrow parameter, or discarding one. It would make sense to me if those behave like inout while named bindings behave like defer, just as struct members and enum payloads have their own ordering.

5 Likes
45

Yes please. A major reason to adopt move semantics is to end reliance on optimizer magic. It seems counterproductive for the default mode to defer to the optimizer’s lifetime analysis.

2 Likes
46

Sorry for not being clear. The intent is to specify that the value is destroyed immediately after its last borrowing use ends. So that's stricter than end of scope, but should still be a well-defined location, not subject to optimizer whims, since borrows of noncopyable values begin and end at well-defined places. Looking toward values with lifetime dependencies, which may be lifetime-bound to borrows or directly contain borrows of other values, I expect that shrinkwrapping the lifetimes would get us closer to Rust's "non-lexical lifetimes" model, so code doesn't need to manually shorten lifetimes to avoid interfering borrows when values linger. I thought that was how Rust worked in general—is there a different rule for Drop types?

On that note, another area of design here has to do with library evolution and deinits—do we want to allow for public types to add deinits without affecting API or ABI? Lifetime aside, the presence of a deinit also puts some restrictions on how code outside of the type can consume it—there needs to be a whole value for the deinit to consume, so you can't partially destructure a value with a deinit by consuming some of a struct's fields or doing a consuming switch on an enum. Library evolution would also be a wrinkle in allowing for different lifetime semantics for types with or without deinits.

1 Like
47

Yeah, non-lexical lifetimes only apply to references; other types still use the classic “end of scope” model. See 2094-nll - The Rust RFC Book

3 Likes
48

Just a naming thing: I find myself thinking about what @John_McCall said about naming conventions, and my own vague intuition about there being an lvalue/rvalue-like distinction here that naming should respect.

Looking at the code examples in context in this proposal, my gut feeling is that the keyword should be a gerund (consuming / borrowing) in declarations:

  func write(_ data: [UInt8], to file: borrowing FileDescriptor) {
                                       ^^^^^^^^^

  func close(file: consuming FileDescriptor) {
                   ^^^^^^^^^

That reads better to my eye. John was skeptical of the gerund, but darn it, in context that just flows off the mental tongue, as it were. The write function writes to a file by borrowing a FileDescriptor. It’s right there in the code.

I do also see the case for a past participle:

  func write(_ data: [UInt8], to file: borrowed FileDescriptor) {
                                       ^^^^^^^^

The colon usually reads as “which is a”, and this naming fits: “Write data, which is a UInt8 array, to file, which is a borrowed FileDescriptor.” Both those options read better to me than the imperative verb borrow.

To be clear, the keyword should still be an imperative verb (consume / borrow) when applied to an expression that supplies a value:

  munge(borrow thinger)
        ^^^^^^

  funge(consume thinger)
        ^^^^^^^

Trying to articulate my intuition here: one describes what will happen elsewhere whenever the thing is used; the other describes what does happen right there in the usage site. That's post hoc explanation, to be clear; I'm just reacting to the fluency of the code itself.

5 Likes
49

@Paul_Cantrell Did you mean to post this over in the other review thread?

51

Perhaps? It's the code examples from this proposal I was referring to, though you're right that it's more on-topic for the other proposal. Feel free to move it (or let me know if I should).

52

I just think perhaps they'd like to hear about your opinions over there in that thread :)

53

munge(lend thinger) in that case, or?! :-)

1 Like
54

I had a similar thought. lend / loan seemed like a bridge too far at first blush…but you do have a point!

1 Like
55

You could argue for lend(ing)/borrow(ing) too…

56

That set of alternatives was already covered during the review for parameter ownership modifiers, which is also still ongoing here. Thoughts on naming the modifiers might be better discussed in the active review thread; we will use whatever modifiers get selected there in the noncopyable types proposal when it is ready for review.

4 Likes
57

Thanks Joe, apologies for the noise - I must say it is very hard to keep up with all threads and the preceding discussions in general, utmost respect for those of you who manage.

3 Likes
58

No worries, I'm not sure the review managers for those other proposals are keeping on top of this thread, so I want to make sure they see all of your naming feedback relevant to the proposals too.

1 Like
59

I would be interested to see whether the Rust community has a sense of how often lexical-scoped lifetimes prevent issues that would've been caused by using non-lexical time-of-use lifetimes for everything instead (@Gankra maybe you've blogged about this in the past?).

60

Mutex guards are the classic C++ example…that immediately becomes irrelevant once you put the data inside the mutex like Rust does.

5 Likes
61

Yeah, my current thinking is very much along those lines. Semantically relying on destructor ordering is inherently subtle in a way that makes it dangerous even without compiler action. It promotes bugs, and good APIs don't do that.