[swift-dev] What can you change in a fixed-contents struct?

Greg Parker gparker at apple.com
Tue Sep 5 17:52:49 CDT 2017

> On Sep 5, 2017, at 11:59 AM, Jordan Rose via swift-dev <swift-dev at swift.org> wrote:
> Hey, all. In preparation for the several proposals we have to come this year, I cleaned up docs/LibraryEvolution.rst <https://github.com/apple/swift/pull/11742> a little bit based on what's changed in Swift 4. This is mostly just mentioning things about generic subscripts, but there is one issue that I remember John bringing up some time in this past year: once you've made a struct fixed-contents, what can you change about its stored properties?
>> To opt out of this flexibility, a struct may be marked '@fixedContents'. This promises that no stored properties will be added to or removed from the struct, even non-public ones.
> Interestingly, this means that you can have non-public members of a fixed-contents struct. This is actually pretty sensible: it's the equivalent of a C++ class with non-public fields but a defaulted, inlinable copy constructor. Any inlinable members can access these properties directly as well; it's just outsiders that can't see them. But if inlinable code can reference these things, and if we really want them to be fast, that means they have to have a known offset at compile-time.
> Now, we don't plan to stick to C's layout for structs, even fixed-contents structs. We'd really like users to not worry about manually packing things into trailing alignment space. But we still need a way to lay out fields consistently; if you have two stored properties with the same type, one of them has to go first. There are two ways to do this: sort by name, and sort by declaration order. That means we can either allow reordering or allow renaming, but not both. Which do people think is more important?
> At the moment I'm inclined to go with "allow renaming" just because that's what C does. It's not great because you're allowed to reorder nearly everything else in the language, but there's a "least surprise" principle here that I think is still useful. It'd be surprising for the name of a non-public property to affect your library's ABI.

In a somewhat similar semantic situation, Objective-C non-fragile ivars chose to allow reordering and disallow renaming. One reason is that we wanted to prevent accidental ABI breaks as best we could. The assumption was that a name change is probably deliberate, but an order change is likely to be an accidental effect of source code rearrangement. In addition, erroneous name changes are more likely to be caught downstream (for example when clients fail to build because they didn't change their use of the name), but erroneous order changes are less likely to be caught later.

The goal of preventing accidental ABI breaks becomes less important if Swift gets good ABI-diffing tools that can help catch errors after they are introduced. ObjC had no effective way to verify that your current ABI matched your previously-published ABI.

Greg Parker     gparker at apple.com <mailto:gparker at apple.com>     Runtime Wrangler

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