> On Jan 26, 2017, at 7:03 AM, Ray Fix via swift-users <swift-users@swift.org>
> wrote:
>
> To compute a hash, I want to loop through bytes of a trivial type. To get
> the bytes, I use withUnsafeBytes hands me a rawBufferPointer collection type.
> Great!
>
> However this call also requires that the "of arg" be declared as a “var"
> instead of “let” because it is inout. So to do it generically, it forces me
> to make a copy to some var storage.
>
> Since I am using withUnsafeBytes and not withUnsafeMutableBytes, though, it
> doesn’t seem like this copy should be necessary. Is there some way to avoid
> it? Is it something the compiler should be able to reason about and
> therefore elide? Possibly a future evolution (and something for that list)?
>
> On a side note, wouldn’t it be cool if there where a way to enforce that a
> type was trivial?
>
> Then one could write code like:
>
> func consume<T: Trivial>(value: T) {
> var copy = value
> withUnsafeMutableBytes(of: ©) { rawBufferPointer in
> // something interesting
> }
> }
>
> Always appreciate the great insights I get here.
> Thanks as always. Best wishes,
There's no fundamental reason it needs a var; see
https://bugs.swift.org/browse/SR-3679. The formal copy can often be elided by
the LLVM optimizer in most cases, though unfortunately not all
(https://llvm.org/bugs/show_bug.cgi?id=31697 is one such case). There is some
benefit to encouraging people to use the `inout` version when possible in our
current language, though in the fullness of time, the operand would be better
modeled as an "immutable borrow" in the ownership model.
-Joe
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