How can I enable ABI stability for a SwiftPM package? - swift

Question

Given my bog standard Package Description of
let package = Package(
name: "MyLib",
products: [
.library(name: "MyLib", targets: ["MyLib"]),
],
dependencies: [],
targets: [
.target(
name: "MiniRxSwift",
dependencies: [],
swiftSettings: [
.define("<see below>")
]),
...
I'm trying to get swiftpm to pass the -enable-library-evolution flag through to swiftc, but I've been unsuccessful.
Using swiftSettings of .define("-enable-library-evolution"), I get a compile error which states
"error: conditional compilation flags must be valid Swift identifiers (rather than '-enable-library-evolution')"
I get the same error if I omit the leading hyphen e.g. `.define("enable-library-evolution")
I've tried the Xcode setting of .define("BUILD_LIBRARIES_FOR_DISTRIBUTION") which doesn't result in a compile error, but also doesn't result in the flag getting set, it instead results in -DBUILD_LIBRARIES_FOR_DISTRIBUTION on the command line for swiftc, which doesn't do anything.
After a bit more research, I worked out that .define in swiftSettings is hardwired to produce things with -D - hence it's name.
Instead I needed to use unsafeFlags, which does indeed result in the flag getting passed correctly to the swift compiler:
swiftSettings: [
.unsafeFlags(["-enable-library-evolution"])
]
BUT then when I attempt to consume this package, Xcode fails to load the package, with the error:
The package product 'MyLib' cannot be used as a dependency of this target because it uses unsafe build flags.
If I can't enable library evolution without unsafe build flags, and I can't use unsafe build flags in a library, then what can I do? What is the point of having unsafeFlags if you can't use any libraries which set them?

Answer 1

Why do you need ABI stability if you have access to the package? You should avoid it if possible. That said, you should be able to use unsafeFlags with editable packages (ones you have dragged into the project) but not ones you include the normal way via URL.
If you need to vend someone a swift package as a binary you will need to build your package as an XCFramework and then you can put that in a place where SwiftPM can depend on it. That means it can be hosted at a static place where you give the URL or it can be embedded in the git repo with the Package.swift manifest that you have for vending the package. The only tool I know of to help with this is found here which works for Swift only package when I have used it.