FooのSwiftファイルを変更したときにObjC向けヘッダーを手動で更新しないといけな

// swift-tools-version:5.0 import PackageDescription let package = Package( name: "Foo", products: [ .library(name: "Foo", targets: ["Foo"]), .library(name: "CFoo", targets: ["CFoo"]) ], targets: [ .target(name: "Foo", swiftSettings: [ .unsafeFlags(["-emit-objc-header-path", "Sources/CFoo/include/Foo-swift.h"]) ]), .target(name: "CFoo", dependencies: ["Foo"]) ] )

とか。 (edited)