[Build] Add test for '@_implementationOnly' import of 'Foundation.Bundle'.

Author: xwu

Fixtures/Resources/FoundationlessClient/AppPkg/Package.swift

@@ -0,0 +1,17 @@
+// swift-tools-version:5.7
+import PackageDescription
+
+let package = Package(
+    name: "AppPkg",
+    dependencies: [
+        .package(path: "../UtilsPkg"),
+    ],
+    targets: [
+        .executableTarget(
+            name: "App",
+            dependencies: [
+                .product(name: "Utils", package: "UtilsPkg")
+            ],
+            path: "./Sources/App"),
+    ]
+)

Fixtures/Resources/FoundationlessClient/AppPkg/Sources/App/main.swift

@@ -0,0 +1,4 @@
+import Utils
+// Note the lack of 'import Foundation'
+
+print(FooUtils.foo.trimmingCharacters(in: .whitespaces))

Fixtures/Resources/FoundationlessClient/UtilsPkg/Package.swift

@@ -0,0 +1,14 @@
+// swift-tools-version:5.7
+import PackageDescription
+
+let package = Package(
+    name: "UtilsPkg",
+    products: [
+        .library(name: "Utils", targets: ["Utils"]),
+    ],
+    targets: [
+        .target(name: "Utils", dependencies: [], resources: [
+            .copy("foo.txt"),
+        ]),
+    ]
+)

Fixtures/Resources/FoundationlessClient/UtilsPkg/Sources/Utils/FooUtils.swift

@@ -0,0 +1,6 @@
+@frozen
+public enum FooUtils { }
+
+extension FooUtils {
+  public static let foo: String = "Hello, World!"
+}

Fixtures/Resources/FoundationlessClient/UtilsPkg/Sources/Utils/foo.txt

@@ -0,0 +1 @@
+foo

Sources/Build/BuildPlan.swift

@@ -791,7 +791,7 @@ public final class SwiftTargetBuildDescription {
 
         let stream = BufferedOutputByteStream()
         stream <<< """
-        \(toolsVersion > .v5_7 ? "@_implementationOnly import" : "import") class Foundation.Bundle
+        \(toolsVersion >= .v5_7 ? "@_implementationOnly import" : "import") class Foundation.Bundle
 
         extension Foundation.Bundle {
             static var module: Bundle = {

Tests/FunctionalTests/ResourcesTests.swift

@@ -88,4 +88,18 @@ class ResourcesTests: XCTestCase {
             }
         }
     }
+
+    func testFoundationlessClient() throws {
+        try fixture(name: "Resources/FoundationlessClient") { fixturePath in
+            #if os(Linux) && swift(>=5.7)
+            let pkgPath = fixturePath.appending(components: "AppPkg")
+            guard let failure = XCTAssertBuildFails(pkgPath) else {
+              XCTFail("missing expected command execution error")
+              return
+            }
+            // Check that the following code expectedly doesn't compile for lack of 'import Foundation'
+            XCTAssertMatch(failure.stdout, .contains("print(FooUtils.foo.trimmingCharacters(in: .whitespaces))"))
+            #endif
+        }
+    }
 }