SE-0076: Add overrides taking an UnsafePointer source to non-destructive copying methods on UnsafeMutablePointer

Introduction

UnsafeMutablePointer includes several methods to non-destructively copy elements from memory pointed to by another UnsafeMutablePointer instance. I propose adding overloads of these methods to UnsafeMutablePointer that allow an UnsafePointer source.

Swift-evolution thread: [\[Pitch\] Add overrides with UnsafePointer sources to non-destructive copying methods on UnsafeMutablePointer](https://forums.swift.org/t/pitch-add-overrides-with-unsafepointer-sources-to-non-destructive-copying-methods-on-unsafemutablepointer/1294)

Motivation

To copy values from memory pointed to by an UnsafePointer it is currently necessary to perform a cast to UnsafeMutablePointer beforehand:

let source: UnsafePointer<Int> = ...
let destination: UnsafeMutablePointer<Int> = ...

// Today:
destination.assignFrom(UnsafeMutablePointer(source), count: count)

These casts are unnecessary visual noise as non-destructively copying from an UnsafePointer source is perfectly safe. Furthermore, these casts are a source of confusion and increased cognitive load on a reader since any such cast is likely to throw up a red flag at first.

Proposed solution

In addition to these existing methods on UnsafeMutablePointer:

func assignBackwardFrom(source: UnsafeMutablePointer<Pointee>, count: Int)
func assignFrom(source: UnsafeMutablePointer<Pointee>, count: Int)
func initializeFrom(source: UnsafeMutablePointer<Pointee>, count: Int)

I propose adding the following overloads:

func assignBackwardFrom(source: UnsafePointer<Pointee>, count: Int)
func assignFrom(source: UnsafePointer<Pointee>, count: Int)
func initializeFrom(source: UnsafePointer<Pointee>, count: Int)

This would transform the given example as follows:

let source: UnsafePointer<Int> = ...
let destination: UnsafeMutablePointer<Int> = ...

// Today:
destination.assignFrom(UnsafeMutablePointer(source), count: count)

// This proposal:
destination.assignFrom(source, count: count)

Detailed design

The following methods are added to UnsafeMutablePointer:

/// Assign from `count` values beginning at source into initialized
/// memory, proceeding from the first element to the last.
public func assignFrom(source: UnsafePointer<Pointee>, count: Int)
  
  
/// Assign from `count` values beginning at `source` into
/// initialized memory, proceeding from the last value to the first.
/// Use this for assigning ranges into later memory that may overlap
/// with the source range.
///
/// - Requires: Either `source` precedes `self` or follows `self + count`.
public func assignBackwardFrom(source: UnsafePointer<Pointee>, count: Int)
  
  
/// Copy `count` values beginning at source into raw memory.
///
/// - Precondition: The memory is not initialized.
///
/// - Requires: `self` and `source` may not overlap.
public func initializeFrom(source: UnsafePointer<Pointee>, count: Int)

Impact on existing code

This proposal is additive and does not impact existing code.

Alternatives considered

• Keep the status quo: I'd argue that they provide enough benefit to justify their existence while only minimally increasing the stdlib surface area, especially by merit of being overloads.

• Introduce a PointerProtocol protocol: A common protocol could be used to avoid the need for overloads in this case. However, without additional use cases this seems like severe over-engineering for this simple issue. This would also require a lot more design work, performance considerations, etc...

• Leverage implicit conversions: The implicit conversions from UnsafeMutablePointer to UnsafePointer could be leveraged to work around the need for overloads by dropping the existing methods taking UnsafeMutablePointer source arguments. Adding explicit overloads seems a better solution than depending on compiler magic and is clearer from a documentation and auto-completion perspective.