Who calls the secret `Sequence` methods?

CTMacUser · March 11, 2025, 5:36am

This is about the 3 secret requirements for Sequence: _copyContents(initializing:), _copyToContiguousArray(), and _customContainsEquatableElement(_:). The third method is called by Sequence.contains(_:). But my searching skills on the Standard Library directory couldn't find clients that call either _copyToContiguousArray() or _copyContents(initializing:). Why do they exist if nothing calls them?

Karl · March 11, 2025, 7:42am

The canonical use of _copyToContiguousArray() is by ContiguousArray.init(some Sequence<Element>).

github.com/swiftlang/swift

stdlib/public/core/ContiguousArray.swift

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          public init<S: Sequence>(_ s: S) where S.Element == Element {
            self.init(_buffer: s._copyToContiguousArray()._buffer)
          }

And for _copyContents(initializing:), it is UnsafeMutableBufferPointer.initialize(from: some Sequence<Element>)

github.com/swiftlang/swift

stdlib/public/core/UnsafeBufferPointer.swift.gyb

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          public func initialize<S: Sequence>(
            from source: S
          ) -> (unwritten: S.Iterator, index: Index) where S.Element == Element {
            return unsafe source._copyContents(initializing: self)
          }

These "secret" methods are really just implementation details for public, documented APIs - using dynamic dispatch to ask the parameter <S: Sequence> types in each of these functions how best to copy their contents to a contiguous destination.

You can basically call them directly under either of the above names.