[swift-evolution] [swift-dev] Rebinding UnsafePointer makes it mutable

[Andrew Trick]

> On Sep 19, 2016, at 11:44 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
> on Mon Sep 19 2016, Andrew Trick <swift-evolution at swift.org> wrote:
> 
>>> On Sep 19, 2016, at 1:24 AM, Martin R via swift-dev <swift-dev at swift.org> wrote:
>>> 
>>> I noticed that both UnsafePointer and UnsafeMutablePointer have the identical method
>>> 
>>>   public func withMemoryRebound<T, Result>(to: T.Type, capacity count: Int, _ body: (UnsafeMutablePointer<T>) throws -> Result) rethrows -> Result
>> 
>>> 
>>> so that rebinding an immutable pointer gives you a _mutable_ pointer. That is different from what
>>> 
>>>   Unsafe[Mutable]Pointer<Pointee> {
>>>     func withMemoryRebound<T>(to: T.Type, capacity count: Int,
>>>       _ body: (Unsafe[Mutable]Pointer<T>) throws -> ()) rethrows
>>>   }
>>> 
>>> in https://github.com/apple/swift-evolution/blob/master/proposals/0107-unsaferawpointer.md indicates. Perhaps I am misunderstanding something. Shouldn't rebinding an UnsafePointer result in an UnsafePointer again?
>>> 
>>> Martin
>> 
>> I think you’re right about that. I didn’t notice the discrepancy until
>> source breaking changes were frozen and was concerned that fixing it
>> would be more restrictive.
>> 
>> Some users may migrate their code to:
>> 
>> constPtr.withMemoryRebound(to: T.self, capacity: 1) {
>> takesMutablePointer($0)
>> }
>> 
>> We probably want them to be more explicit:
>> 
>> constPtr.withMemoryRebound(to: T.self, capacity: 1) {
>> takesMutablePointer(UnsafeMutablePointer(mutating: $0))
>> }
>> 
>> We could possibly justify correcting this in Swift 3 though on these grounds:
>> 
>> - It’s effectively a bug considering that the proposal and
>> implementation are inconsistent.
> 
> It's definitely a bug, IMO.
> 
>> - There is a correct way write the code that will continue to work
>> before and after fixing the bug.  
>> - A simple fixit will tell them to add the “mutating” label.
>> 
>> If not, it’s something I was already planning to roll into Swift 4.

If there’s no objection, I’ll just go ahead with a fix on the 3.0 branch since it was already covered by SE-0107.
-Andy