     @inlinable // This is @inlinable as generic and reasonably small.
     public mutating func replaceSubrange<ByteCollection : Collection>(_ subrange: Range<Index>, with newElements: ByteCollection) where ByteCollection.Iterator.Element == Data.Iterator.Element {
         let totalCount = Int(newElements.count)
-        _withStackOrHeapBuffer(totalCount) { conditionalBuffer in
+        __withStackOrHeapBuffer(totalCount) { conditionalBuffer in
             let buffer = UnsafeMutableBufferPointer(start: conditionalBuffer.pointee.memory.assumingMemoryBound(to: UInt8.self), count: totalCount)
             var (iterator, index) = newElements._copyContents(initializing: buffer)
             while let byte = iterator.next() {

        }
        
    }
    
    // slightly faster paths for common sequences
    @inlinable // This is @inlinable as an important generic funnel point, despite being a non-trivial initializer.
    public init<S: Sequence>(_ elements: S) where S.Element == UInt8 {


            // Copy as much as we can in one shot from the sequence.
            let underestimatedCount = Swift.max(elements.underestimatedCount, 1)
            __withStackOrHeapBuffer(underestimatedCount) { (buffer) in
                // In order to copy from the sequence, we have to bind the buffer to UInt8.
                // This is safe since we'll copy out of this buffer as raw memory later.
                let capacity = buffer.pointee.capacity
