Is there a way to deprecate public APIs for library clients only?

[Karl]

So after working though this for a bit, I realised a couple of things:

1. What I really want is to deprecate this API based on the client's minimum supported Swift language version.

2. As it happens, package declarations include a swiftLanguageVersions property, which gets passed to the compiler. For some reason, this is an array rather than a minimum version, but whatever.

3. I had forgotten to set this, and since I want my library to only use 5.3 language features (unless I opt-in to a newer version via #if swift), I set it to [.version("5.3")].

4. swift build then correctly passes this to the compiler as -swift-version 5.3. Xcode doesn't, and passes -swift-version 5 instead. Because Xcode.

5. Actually it doesn't even matter, because the swift compiler doesn't have a language mode 5.3:

   <unknown>:0: error: invalid value '5.3' in '-swift-version 5.3'
   <unknown>:0: note: valid arguments to '-swift-version' are '4', '4.2', '5'
   

The swift language mode properties are described by:

• SE-0151 Package Manager Swift Language Compatibility Version, and
• SE-0209 Package Manager Swift Language Version API Update

They're very short and, to be honest, not entirely helpful (it's pretty much "support the compiler's -swift-version argument", without a clear description of what that means). But there are hints that it was intended to allow what I'm trying to do:

This is important for packages which want to add support for a newer Swift language version but also want to retain compatibility with an older language and tools version [SE-0209]

@rballard

Our original support for declaring a Swift language version gave package authors the ability to have dual-language-mode packages. This allows you to have a package that can adopt newer Swift features will still being usable by clients on the older version of the tools. In many cases your package may have to conditionalize some code on the Swift language version the code is being compiled with in order to support being used with either language version, but I expect that many package authors are willing to do that to avoid leaving older clients behind.

So ultimately the conclusion I'm coming to is that SwiftPM is able to express what I want. I think the problem is that the compiler only knows about major Swift language versions (v4 vs v5, but not v5.3 vs v5.5). That means I can't deprecate something in a new minor version of the language (5.5) and keep compiling using an older minor version (5.3). All I can tell it is "build in v5 mode".

This is kind of problem. Swift 5.0 was released in March 2019, and that every feature since then has come in a minor language version. And once Swift 6 does come, all we'll be able to say is "build in v5 mode", without saying whether we mean 5.1, 5.3, 5.5, etc.