UnsafeMutablePointer to [UInt8] without memory copy

Question

Is it possible to create a [UInt8] from an UnsafeMutablePointer without copying the bytes?

In the NSData world I coul

Answer 1

You can use UnsafeBufferPointer + map to achieve your goal.
KEEP IN MIND: pointer boundary should be maintained by yourself, while arrays will do by itself
let bytesArray = UnsafeBufferPointer(start: bts, count: bytes.count).map{$0}
Swift 5, Xcode 11:

var bytes: [UInt8] = [1, 3, 5, 7]
let count = bytes.count
print(bytes)

// MARK: - [UInt8] -> UnsafePointer

// producing dangling pointer warning bug?
let bts1: UnsafePointer = UnsafePointer(bytes)
print(bts1, bts1[0], bts1[1], bts1[2], bts1[3], bts1[4])

let bts = withUnsafePointer(to: &bytes[0]) {
    $0.withMemoryRebound(to: UInt8.self, capacity: count) { $0 }
}
print(bts, bts[0], bts[1], bts[4])

// MARK: - UnsafePointer -> [UInt8]
let bytesArray = UnsafeBufferPointer(start: bts, count: bytes.count).map{$0}
print(bytesArray, bytesArray[0], bytesArray[1]/*, bytesArray[4]*/)

output:

[1, 3, 5, 7]
0x0000600001946840 1 3 5 7 0
0x0000600001946840 1 3 0
[1, 3, 5, 7] 1 3

bad samples:

/// invalid sample 1
let testA = withUnsafePointer(to: &bytes) {
    $0.withMemoryRebound(to: UInt8.self, capacity: count) { $0 }
}
print(testA, testA[0], testA[1], testA[4])

/// invalid sample 2
let testB = withUnsafePointer(to: bytes[0]) {
    $0.withMemoryRebound(to: UInt8.self, capacity: count) { $0 }
}
print(testB, testB[0], testB[1], testB[4])

output:

0x0000000102b4f520 32 104 0
0x00007ffeed203ac0 192 58 254