Can we require StringProtocol.UTF8View be a BidirectionalCollection?

Karl · February 21, 2021, 2:25pm

StringProtocol.UTF8View only requires Collection conformance, even though StringProtocol itself, as well as the UTF16View and UnicodeScalarView, are BidirectionalCollections. The actual UTF8Views for String and Substring are of course bidirectional. It's actually commented out from the protocol definition, which makes me think it may have been an oversight not to un-comment it?

github.com

apple/swift/blob/main/stdlib/public/core/StringProtocol.swift#L27


public protocol StringProtocol
  : BidirectionalCollection,
  TextOutputStream, TextOutputStreamable,
  LosslessStringConvertible, ExpressibleByStringInterpolation,
  Hashable, Comparable
  where Iterator.Element == Character,
        Index == String.Index,
        SubSequence: StringProtocol,
        StringInterpolation == DefaultStringInterpolation
{
  associatedtype UTF8View: /*Bidirectional*/Collection
  where UTF8View.Element == UInt8, // Unicode.UTF8.CodeUnit
        UTF8View.Index == Index
  associatedtype UTF16View: BidirectionalCollection
  where UTF16View.Element == UInt16, // Unicode.UTF16.CodeUnit
        UTF16View.Index == Index
  associatedtype UnicodeScalarView: BidirectionalCollection
  where UnicodeScalarView.Element == Unicode.Scalar,
        UnicodeScalarView.Index == Index

This is just a really weird asymmetry which forces me to write a bunch of additional constraints. Given that StringProtocol is kind of "sealed", would it be possible to fix this? Or would it break ABI?