[swift-evolution] [Pitch] `Int.init(ObjectIdentifier)` and `UInt.init(ObjectIdentifier)` should have a `bitPattern:` label

[Arnold Schwaighofer]

Int.init(ObjectIdentifier) and UInt.init(ObjectIdentifier) should have a 'bitPattern:’ label to make it clear at the use site that we interpret the value as a bit pattern.

In Swift we have ObjectIdentifier values which uniquely identify a class instance or metatype. They are implemented as a struct which holds the value of the reference to the instance or metatype as a raw pointer.

  /// A unique identifier for a class instance or metatype.
  public struct ObjectIdentifier : Hashable, Comparable {
    internal let _value: Builtin.RawPointer
    ...
  }


We have constructors for Int and Uint that capture this value. These constructors don’t have an argument label.

  extension UInt {
    /// Create a `UInt` that captures the full value of `objectID`.
    public init(_ objectID: ObjectIdentifier) {
      self.init(Builtin.ptrtoint_Word(objectID._value))
    }
  }

  extension Int {
    /// Create an `Int` that captures the full value of `objectID`.
    public init(_ objectID: ObjectIdentifier) {
      self.init(bitPattern: UInt(objectID))
    }
  }

This proposals suggest adding a label ‘bitPattern’ to the constructor: Int.init(bitPattern: ObjectIdentifier).

  extension UInt {
    /// Create a `UInt` that captures the full value of `objectID`.
    public init(bitPattern objectID: ObjectIdentifier) {
      self.init(Builtin.ptrtoint_Word(objectID._value))
    }
  }

  extension Int {
    /// Create an `Int` that captures the full value of `objectID`.
    public init(bitPattern objectID: ObjectIdentifier) {
      self.init(bitPattern: UInt(objectID))
    }
  }


Motivation
==========

Adding a label ‘bitPattern’ to the constructors makes it clear that we interpret the pointer value as a bit pattern at the use site. It is similar to what we do in other APIs, for example in "UInt(bitPattern: UnsafePointer<Void>(value)))"

Impact on existing code
=======================

Existing code will have to add the bitPattern label.


Alternatives
============

Leave as is.