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

[Dave Abrahams]

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.
>
> -Andy
>
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

-- 
-Dave