investigate ByteBufferView supporting fast String(decoding:as:) as a workaround for SR-12125

[weissi]

The program below allocates for ByteBufferView:

              libsystem_malloc.dylib`malloc
              libswiftCore.dylib`swift_slowAlloc+0x19
              libswiftCore.dylib`_swift_allocObject_(swift::TargetHeapMetadata<swift::InProcess> const*, unsigned long, unsigned long)+0x14
              TestApp`specialized _ContiguousArrayBuffer.init(_uninitializedCount:minimumCapacity:)+0x53
              TestApp`specialized _copyCollectionToContiguousArray<A>(_:)+0x2d
              TestApp`specialized makeString<A>(_:)+0x1dc
              TestApp`doByteBufferView()+0xf0
              TestApp`main+0x13
              libdyld.dylib`start+0x1
              TestApp`0x1
            10000

the problem is that String(decoding:count:) does not seem to leverage withContiguousStorageIfAvaiable which is SR-12125. CC @milseman

We should investigate if we can find a workaround.

===

dev/make-single-file-spm test.swift mallocs

with dev/make-single-file-spm from the NIO repo.

===

// DEPENDENCY: https://github.com/apple/swift-nio 2.13.0 NIO
import NIO

func makeString<Bytes: Collection>(_ bytes: Bytes) -> String where Bytes.Element == UInt8 {
    return String(decoding: bytes, as: Unicode.UTF8.self)
}

func testArray(_ array: [UInt8]) -> String {
    return makeString(array)
}

func testArraySlice(_ array: ArraySlice<UInt8>) -> String {
    return makeString(array)
}

func testByteBufferView(_ array: ByteBufferView) -> String {
    return makeString(array)
}

@inline(never)
func doArray() {
    let array: [UInt8] = Array(repeating: UInt8(ascii: "X"), count: 15)

    for _ in 0..<10000 {
        precondition(testArray(array) == "XXXXXXXXXXXXXXX")
    }
}

@inline(never)
func doArraySlice() {
    let array: ArraySlice<UInt8> = Array(repeating: UInt8(ascii: "X"), count: 15)[...]

    for _ in 0..<10000 {
        precondition(testArraySlice(array) == "XXXXXXXXXXXXXXX")
    }
}

@inline(never)
func doByteBufferView() {
    var buffer = ByteBufferAllocator().buffer(capacity: 16)
    buffer.writeString(String(repeating: "X", count: 15))
    let view = buffer.readableBytesView

    for _ in 0..<10000 {
        precondition(testByteBufferView(view) == "XXXXXXXXXXXXXXX")
    }
}

doArray()
doArraySlice()
doByteBufferView()

[weissi]

This is fixed. In both Swift and NIO.