[swift-evolution] SE-0138 UnsafeBytes

[Andrew Trick]

The proposal is available here:

 <https://github.com/apple/swift-evolution/blob/master/proposals/0138-unsafebytes.md <https://github.com/apple/swift-evolution/blob/master/proposals/0138-unsafebytes.md>>

> On Sep 1, 2016, at 4:59 PM, Drew Crawford <drew at sealedabstract.com> wrote:
> I'm possibly one of the larger users of raw byte stuff in Swift as I maintain an entire client/server network protocol stack in Swift userspace, similar in spirit to one of the examples drawn out a lot longer.  Grepping my code produces over 200 individual uses of unsafe byte accesses.
> 
> I definitely agree that the problem is significant enough to warrant a last-minute change.
> 
> To a first approximation I agree with all the implementation choices.  The naming, the choice of UInt8, length tracking, and debug-bounds checking are all correct IMO.  We have been using something similar for a long time internally [have you been reading my code? :-) ] so I can speak from experience that the basic plan here is sound.
> 
> One thing I would like to see is an (opt-in) release-mode-bounds-check.  Networking is a core use case for this feature, but when you are reading from a socket, production is where you need a guard against out-of-bounds UB the most.  If we can't solve it for Swift 3, affected users can write a wrapper to implement the boundscheck, but I think we should at very least take it up again for Swift 4.
> 
> Drew

In my current implementation:
https://github.com/atrick/swift/blob/unsafebytes/stdlib/public/core/UnsafeBytes.swift.gyb <https://github.com/atrick/swift/blob/unsafebytes/stdlib/public/core/UnsafeBytes.swift.gyb>

The bounds checks in `copyBytes(from:)` are release mode preconditions.

The bounds checks for `subscript`, `load(as:)`, and `storeBytes(of:as:)` are debug only because it’s likely they occur in some loop that could be covered by a single bounds check. By extension, the sequence iterator is only bounds checked in debug mode.

One possibility would be different names for the bounds checked forms of those methods: getByte(atOffset:), setByte(atOffset:), load(fromCheckedOffset:as:), storeBytes(of:toCheckedOffset:as:). Along with some kind of bounds checked Iterator.

I don’t think makes a lot of sense as generic Collection though. Alternatively, we just have an UnsafeBoundsCheckedBytes wrapper.

This would a good thing to experiment with in your project. We may be able to follow-up with a Swift 4 proposal. The important thing now is to determine whether the proposed Swift 3 design will make that wrapper difficult in any way.

-Andy
> On September 1, 2016 at 5:19:02 PM, Andrew Trick via swift-evolution (swift-evolution at swift.org <mailto:swift-evolution at swift.org>) wrote:
> 
>> I’m resending this for Review Manager Dave A. because the announce list is dropping his messages...
>> 
>> Hello Swift community,
>> 
>> The review of "UnsafeBytes" begins now and runs through September
>> 7th. This late addition to Swift 3 is a follow-up to SE-0107:
>> UnsafeRawPointer. It addresses common use cases for UnsafeRawPointer,
>> allowing developers to continue working with collections of UInt8 values,
>> but now doing so via a type safe API. The UnsafeBytes API will not require 
>> direct manipulation of raw pointers or reasoning about binding memory.
>> 
>> The proposal is available here:
>> 
>>  <https://github.com/apple/swift-evolution/blob/master/proposals/0138-unsafebytes.md <https://github.com/apple/swift-evolution/blob/master/proposals/0138-unsafebytes.md>>
>> 
>> Reviews are an important part of the Swift evolution process. All reviews
>> should be sent to the swift-evolution mailing list at
>> 
>>  <https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>>
>> 
>> or, if you would like to keep your feedback private, directly to the
>> review manager. When replying, please try to keep the proposal link at
>> the top of the message:
>> 
>> Proposal link:
>>  <https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>>
>> 
>> What goes into a review?
>> 
>> The goal of the review process is to improve the proposal under review
>> through constructive criticism and, eventually, determine the direction of
>> Swift. When writing your review, here are some questions you might want to
>> answer in your review:
>> 
>>  * What is your evaluation of the proposal?
>>  * Is the problem being addressed significant enough to warrant a
>>    change to Swift?
>>  * Does this proposal fit well with the feel and direction of Swift?
>>  * If you have used other languages or libraries with a similar
>>    feature, how do you feel that this proposal compares to those?
>>  * How much effort did you put into your review? A glance, a quick
>>    reading, or an in-depth study?
>> 
>> More information about the Swift evolution process is available at
>> 
>>  <https://github.com/apple/swift-evolution/blob/master/process.md <https://github.com/apple/swift-evolution/blob/master/process.md>>
>> 
>> Thank you,
>> 
>> -Dave Abrahams
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