[swift-evolution] [Swift 4] Organizing source stability

[Jacob Bandes-Storch]

Here are a few thoughts:

   - -swift=4
   - -swift-version=4
   - -language-version=4
   - a top-of-file "shebang"-style comment indicating the version,
   something like //#swift(4), mirroring the "#if swift" syntax


On Fri, Jul 29, 2016 at 5:27 PM, Chris Lattner <clattner at apple.com> wrote:

>
> On Jul 29, 2016, at 5:20 PM, Jacob Bandes-Storch via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> Chris writes:
>
>> - *Source stability features: *These should be relatively small, but
>> important.  For example, we need a “-std=swift3” sort of compiler flag.  We
>> may also add a way to conditionally enable larger efforts that are under
>> development but not yet stable - in order to make it easier to experiment
>> with them.
>
>
>
> I am curious whether the team has thoughts on how to organize the compiler
> codebase in such a way that new features can be added, and possibly
> source-breaking changes made, while still keeping the old functionality
> around.
>
> Are any obvious areas that will need refactoring to make this feasible?
> (Perhaps they could be turned into StarterBugs.)
>
>
> I think this would be a great thing to do.  We need a few things:
>
> - The actual compiler flag.  It is worth bikeshedding how it is spelled.
> “-std=“ is good inspiration, but clearly the wrong specific name.
>
> - The implementation should be straight forward: the flag should get
> plumbed through to a field in swift::LangOptions.  Code that diverges can
> then check it.
>
> - Handling divergence in the standard library is another big issue.  We
> have some ideas here, but it depends on having the compiler work done
> anyway to hook into.
>
> How many versions back would the compiler be expected to support? Should
> the Swift 5 compiler still support Swift 3 code?
>
>
> To be determined.  Swift 4 should definitely support Swift 3, but Swift 5
> perhaps not.  We can decide that when Swift 4 is winding down.
>
> -Chris
>
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