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

[Arnold Schwaighofer]

This is probably uncontroversial.

PR: https://github.com/apple/swift-evolution/pull/434



> On Jul 13, 2016, at 4:39 PM, Arnold Schwaighofer via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 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.
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