Consuming, borrowing and type inference

Development Compiler
1

The following fails to compile:

struct Ref<T>: ~Copyable {
    var value: T
    init(_ value: T) { self.value = value }
}

func consumingMap<A>(
    _ f: @escaping (inout A) -> Void
) -> (consuming Ref<A>) -> Ref<A> {
    { ref in
        var newRef = ref
        f(&newRef.value)
        return newRef
    }
}

with the error:

'ref' is borrowed and cannot be consumed.

Shouldn't consuming be inferred here, or am I missing something fundamental? (I feel like I've missed a huge amount of fundamental stuff on move-only types of late).

2 Likes
2

ok, fooling around with it more, this does compile:

func consumingMap<A>(
    _ f: @escaping (inout A) -> Void
) -> (consuming Ref<A>) -> Ref<A> {
    { (ref: consuming Ref<A>) -> Ref<A> in
        var newRef = ref
        f(&newRef.value)
        return newRef
    }
}

Seems like I should not have to spell out the full type-annotation here..

1 Like
3

I think ‘consuming’ ought to be inferred here from the return type (similarly if it were bound to a variable with that explicit type annotation).

I’m not sure if that inference was part of the corresponding SE proposal, but it should be a source-compatible addition if today it’s defaulting to ‘borrowing’, the weakest ownership kind.

4

that was my thinking too, but this stuff is all so new to me that I'm never sure if I have properly understood the concept. Another quick question, should ref be mutable there? i.e. should I be able to say:

func consumingMap<A>(
    _ f: @escaping (inout A) -> Void
) -> (consuming Ref<A>) -> Ref<A> {
    { (ref: consuming Ref<A>) -> Ref<A> in
        f(&ref.value)
        return ref
    }
}

Intuitively I would think I could or at least that it would be desirable. At present trying it that way yields:

error: couldn't IRGen expression. Please enable the expression log by running "log enable lldb expr", then run the failing expression again, and file a bug report with the log output.

I've reported that as a compiler bug, but my I can't tell if (when the compiler gets fixed) that should work or not.

5

Yes, a function should always be able to mutate its own parameter if that parameter is consuming.

1 Like