Array.description uses `debugDescription` instead of `description` of its elements

[dnadoba]

Given the following programs:

struct MyType {}

extension MyType: CustomStringConvertible {
    var description: String {
        "description of MyType"
    }
}

extension MyType: CustomDebugStringConvertible {
    var debugDescription: String {
        "debugDescription of MyType"
    }
}

@main
struct Main {
    static func main() async throws {
        print([MyType()])
        debugPrint([MyType()])
    }
}

this prints

[debugDescription of MyType]
[debugDescription of MyType]

but I would expect that the first print

[description of MyType]

This seems to be on purpose from the code comments in _makeCollectionDescription: https://github.com/apple/swift/blob/86265881c3a8c0480f94ebeedd67aef607d4a1f1/stdlib/public/core/ArrayShared.swift#L119
However, this is very unexpected and also probably quite slow as it looks like debugPrint cannot be specialised.

This also affects ArraySlice, ContiguousArray, Dictionary and Set.

[dnadoba]

rdar://112911414

[Azoy]

I think the main reason for this is that String.description prints the contents of the string unquoted and this would be very awkward in an array: [hello, world] instead of using its debugDescription: ["hello", "world"]. We could branch and say if the element type is X, Y, or Z print debug description otherwise description, but at least we have something that's uniform for all types.

[dnadoba]

Yeah, it seems that this is how it is used, but the naming and documentation of CustomDebugStringConvertible does not mentioned it at all. I think we should at least update the documentation to reflect that. Ideally actually change the name of it as well but this will likely need to go through swift evolution, if it is even possible ABI wise.