Add @ArrayBuilder to the standard library

[cal (Cal Stephens)]

Started a discussion in the swift-collections subforum about whether or not folks think an @ArrayBuilder would be a good for the collections package: @ArrayBuilder in swift-collections?

I think there is at least one language improvement that we should make regardless:

@cal

Improving ergonomics of generic @resultBuilders

Today, when using a generic @resultBuilder, you must explicitly specify the generic arguments. Otherwise, you get an error like "reference to generic type 'ArrayBuilder' requires arguments in <...>".

@ArrayBuilder<String>
var arguments2: [String] {
    "format"
    "--in-place"

    if recursive {
        "--recursive"
    }
}

This is redundant since the ArrayBuilder.Result.Element type can be inferred to be the Element type of the attached declaration's return type. Instead, we should allow this to be inferred so that a simpler syntax can be used:

@ArrayBuilder
var arguments2: [String] {
    "format"
    "--in-place"

    if recursive {
        "--recursive"
    }
}

And potentially another, pending investigation on the source compatibility side:

@allevato

But what I really wanted to write, and what I think a lot of people would reach for, would be this:

let someArray = [String] { ... }

which doesn't compile. It looks like the logic that tries to treat a called single-element array literal expression (e.g., [Int](repeating: 0, count: 10)) as an initializer call doesn't work for the case of just a trailing closure, so you have to write this:

let someArray = [String]() { ... }

which is unsatisfying. Might be nice to fix that, if it wouldn't break source somewhere else.

The result builder inferred generics change is new language functionality so probably requires an evolution proposal. Since I have an implementation for that one, I will submit a proposal for it.

I’ll also investigate what it would take to support let someArray = [String] { ... }. It’s possible that’s “just a bug fix” that doesn’t require an evolution proposal. If we decide it does require an evolution proposal, we could bundle them together.