Property delegates as custom attributes

[DougGregor]

This is an implementation of property delegates as custom attributes, under discussion in this pitch thread. The implementation is mostly complete, although there are some annoying known bugs:

• Property delegates cannot be used on local variables; only properties of types work well (lazy is similarly broken and always has been!)

[hartbit]

@DougGregor Would it be possible to have a toolchain created to play around with Property Delegates? I'd like to use them to improve a library Im working on as a way to test them out.

[DougGregor]

@hartbit let me make sure things are back in a working state now, and once they are I'll build a toolchain. I just got to the point where they work end-to-end, so long as you don't touch @autoclosure.

[hartbit]

That's a lot of toolchains 😂

[DougGregor]

@swift-ci please please please please please build toolchain

[shahmishal]

@swift-ci please build toolchain

[DougGregor]

@swift-ci please smoke test

[shahmishal]

@swift-ci please smoke test

[DougGregor]

@swift-ci please smoke test

[DougGregor]

@swift-ci build toolchain

[shahmishal]

@swift-ci build toolchain Linux

[shahmishal]

@swift-ci please smoke test Linux

[DougGregor]

@swift-ci please smoke test

[DougGregor]

@swift-ci please build toolchain

[DougGregor]

@swift-ci please test

[DougGregor]

@swift-ci please build toolchain

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 85ff735eb5c1120af3c47d7c5d682aef71aa5038

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 85ff735eb5c1120af3c47d7c5d682aef71aa5038

[DougGregor]

@swift-ci please smoke test macOS

[hartbit]

With the implementation in the toolchain, the DelayedMutable example from the proposal fails to compile:

@propertyDelegate
struct DelayedMutable<Value> {
    private var _value: Value? = nil

    var value: Value {
        get {
            guard let value = _value else {
                fatalError("property accessed before being initialized")
            }
            return value
        }
        set {
            _value = newValue
        }
    }

    /// "Reset" the delegate so it can be initialized again.
    mutating func reset() {
        _value = nil
    }
}

class Foo {
    @DelayedMutable var x: Int

    init() {
        // We don't know "x" yet, and we don't have to set it
    } // error: Return from initializer without initializing all stored properties

    func getX() -> Int {
        return x // Will crash if 'self.x' wasn't initialized
    }
}

Is this known?

[DougGregor]

@hartbit ah, thank you! that should be @DelayedMutable() var x: Int. I'll fix the proposal

[hartbit]

@DougGregor The () part of the syntax is surprising because the proposal does not talk about it much. When is it required?

[DougGregor]

@hartbit The () is there to call the no-parameter initializer. Without it, the delegate instance hasn't been initialized. There's no special case here

[DougGregor]

And... another rebase is incoming.

[DougGregor]

@swift-ci please smoke test

[DougGregor]

@swift-ci please smoke test

[DougGregor]

@swift-ci please smoke test and merge

[DougGregor]

@swift-ci please smoke test and merge

[compnerd]

@DougGregor - sigh this broke the windows build :-( https://dev.azure.com/compnerd/windows-swift/_build/results?buildId=1697