[swift_package] .defaultIsolation in swiftSettings is not passed to swiftc

Hi! I’m trying out Once on a Swift 6 project and ran into an issue with actor isolation settings.

When a Swift package target defines .defaultIsolation(MainActor.self) in its swiftSettings, the setting doesn’t seem to be forwarded to swiftc (the -default-isolation MainActor flag is missing). As a result, the build fails under Swift 6 with concurrency errors on global or static variables that rely on default isolation.

Environment: Xcode 27, Swift 6.4, Once 0.56.0.

Reproduction

Package.swift:

// swift-tools-version: 6.2
import PackageDescription

let package = Package(
  name: "DefaultIsolationRepro",
  platforms: [.macOS(.v14)],
  targets: [
    .target(
      name: "DefaultIsolation",
      swiftSettings: [
        .defaultIsolation(MainActor.self)
      ]
    )
  ]
)

Sources/DefaultIsolation/DefaultIsolation.swift:

public final class MutableBox {
  public var value: Int

  public init(value: Int = 0) {
    self.value = value
  }
}

public enum GlobalState {
  public static let shared = MutableBox()
}

Steps

Native SwiftPM builds fine:

swift build

Building the same target with Once:

once query targets
once build SwiftPackage_DefaultIsolationRepro_DefaultIsolation

Fails with:

Sources/DefaultIsolation/DefaultIsolation.swift:10:21: error: static property 'shared' is not concurrency-safe because non-'Sendable' type 'MutableBox' may have shared mutable state [#MutableGlobalVariable]
public static let shared = MutableBox()
                  ^
swift_module_compile_DefaultIsolation (2) failed for SwiftPackage_DefaultIsolationRepro_DefaultIsolation with exit code 1

When checking the swiftc command:

• Native swift build passes -default-isolation MainActor
• once build passes -swift-version 6, but -default-isolation MainActor is missing

No GitHub comments yet.