Enable swift module interfaces if the package author enables library evolution via unsafe flags

Fixtures/Miscellaneous/LibraryEvolution/Package.swift

@@ -0,0 +1,11 @@
+// swift-tools-version:5.1
+import PackageDescription
+
+let package = Package(
+    name: "LibraryEvolution",
+    products: [
+    ],
+    targets: [
+        .target(name: "A", dependencies: [], swiftSettings: [.unsafeFlags(["-enable-library-evolution"])]),
+        .target(name: "B", dependencies: ["A"], swiftSettings: [.unsafeFlags(["-enable-library-evolution"])]),
+    ])

Sources/Build/BuildPlan.swift

@@ -778,14 +778,14 @@ public final class SwiftTargetBuildDescription {
             args += ["-color-diagnostics"]
         }
 
-        // Add the output for the `.swiftinterface`, if requested.
-        if buildParameters.enableParseableModuleInterfaces {
-            args += ["-emit-parseable-module-interface-path", parseableModuleInterfaceOutputPath.pathString]
-        }
-
         // Add agruments from declared build settings.
         args += self.buildSettingsFlags()
 
+        // Add the output for the `.swiftinterface`, if requested or if library evolution has been enabled some other way.
+        if buildParameters.enableParseableModuleInterfaces || args.contains("-enable-library-evolution") {
+            args += ["-emit-module-interface-path", parseableModuleInterfaceOutputPath.pathString]
+        }
+
         // User arguments (from -Xswiftc) should follow generated arguments to allow user overrides
         args += buildParameters.swiftCompilerFlags
         return args

Sources/XCBuildSupport/PIF.swift

@@ -936,6 +936,7 @@ public enum PIF {
             case WATCHOS_DEPLOYMENT_TARGET
             case MARKETING_VERSION
             case CURRENT_PROJECT_VERSION
+            case SWIFT_EMIT_MODULE_INTERFACE
         }
 
         public enum MultipleValueSetting: String, Codable {

Sources/XCBuildSupport/PIFBuilder.swift

@@ -812,6 +812,21 @@ final class PackagePIFProjectBuilder: PIFProjectBuilder {
 
         return bundleName
     }
+
+    // Add inferred build settings for a particular value for a manifest setting and value.
+    private func addInferredBuildSettings(
+        for setting: PIF.BuildSettings.MultipleValueSetting,
+        value: [String],
+        platform: PIF.BuildSettings.Platform? = nil,
+        configuration: BuildConfiguration,
+        settings: inout PIF.BuildSettings
+    ) {
+        // Automatically set SWIFT_EMIT_MODULE_INTERFACE if the package author uses unsafe flags to enable
+        // library evolution (this is needed until there is a way to specify this in the package manifest).
+        if setting == .OTHER_SWIFT_FLAGS && value.contains("-enable-library-evolution") {
+            settings[.SWIFT_EMIT_MODULE_INTERFACE] = "YES"
+        }
+    }
 
     // Apply target-specific build settings defined in the manifest.
     private func addManifestBuildSettings(
@@ -833,8 +848,10 @@ final class PackagePIFProjectBuilder: PIFProjectBuilder {
                             switch configuration {
                             case .debug:
                                 debugSettings[setting, for: platform, default: ["$(inherited)"]] += value
+                                addInferredBuildSettings(for: setting, value: value, platform: platform, configuration: .debug, settings: &debugSettings)
                             case .release:
                                 releaseSettings[setting, for: platform, default: ["$(inherited)"]] += value
+                                addInferredBuildSettings(for: setting, value: value, platform: platform, configuration: .release, settings: &releaseSettings)
                             }
                         }
 
@@ -847,8 +864,10 @@ final class PackagePIFProjectBuilder: PIFProjectBuilder {
                         switch configuration {
                         case .debug:
                             debugSettings[setting, default: ["$(inherited)"]] += value
+                            addInferredBuildSettings(for: setting, value: value, configuration: .debug, settings: &debugSettings)
                         case .release:
                             releaseSettings[setting, default: ["$(inherited)"]] += value
+                            addInferredBuildSettings(for: setting, value: value, configuration: .release, settings: &releaseSettings)
                         }
                     }
 

Tests/CommandsTests/BuildToolTests.swift

@@ -246,6 +246,18 @@ final class BuildToolTests: XCTestCase {
         }
     }
 
+    func testAutomaticParseableInterfacesWithLibraryEvolution() {
+        fixture(name: "Miscellaneous/LibraryEvolution") { path in
+            do {
+                let result = try build([], packagePath: path)
+                XCTAssert(result.binContents.contains("A.swiftinterface"))
+                XCTAssert(result.binContents.contains("B.swiftinterface"))
+            } catch SwiftPMProductError.executionFailure(_, _, let stderr) {
+                XCTFail(stderr)
+            }
+        }
+    }
+
     func testBuildCompleteMessage() {
         fixture(name: "DependencyResolution/Internal/Simple") { path in
             do {

Tests/XCBuildSupportTests/PIFBuilderTests.swift

@@ -2224,6 +2224,68 @@ class PIFBuilderTests: XCTestCase {
             }
         }
     }
+
+    /// Tests that the inference of XCBuild build settings based on the package manifest's declared unsafe settings
+    /// works as expected.
+    func testUnsafeFlagsBuildSettingInference() throws {
+        let fs = InMemoryFileSystem(emptyFiles:
+            "/MyLib/Sources/MyLib/Foo.swift"
+        )
+
+        let diagnostics = DiagnosticsEngine()
+        let graph = try loadPackageGraph(
+            fs: fs,
+            diagnostics: diagnostics,
+            manifests: [
+                Manifest.createManifest(
+                    name: "MyLib",
+                    path: "/MyLib",
+                    packageKind: .root,
+                    packageLocation: "/MyLib",
+                    v: .v5,
+                    products: [
+                        .init(name: "MyLib", type: .library(.automatic), targets: ["MyLib"]),
+                    ],
+                    targets: [
+                        .init(name: "MyLib", settings: [
+                            .init(
+                                tool: .swift,
+                                name: .unsafeFlags,
+                                value: ["-enable-library-evolution"],
+                                condition: .init(config: "release")),
+                        ]),
+                    ]),
+            ],
+            shouldCreateMultipleTestProducts: true
+        )
+
+        let builder = PIFBuilder(graph: graph, parameters: .mock(), diagnostics: diagnostics)
+        let pif = try builder.construct()
+
+        XCTAssertNoDiagnostics(diagnostics)
+
+        PIFTester(pif) { workspace in
+            workspace.checkProject("PACKAGE:/MyLib") { project in
+                project.checkTarget("PACKAGE-TARGET:MyLib") { target in
+                    target.checkBuildConfiguration("Debug") { configuration in
+                        configuration.checkBuildSettings { settings in
+                            // Check that the `-enable-library-evolution` setting for Release didn't affect Debug.
+                            XCTAssertEqual(settings[.SWIFT_EMIT_MODULE_INTERFACE], nil)
+                            XCTAssertEqual(settings[.OTHER_SWIFT_FLAGS], nil)
+                        }
+                    }
+                    target.checkBuildConfiguration("Release") { configuration in
+                        configuration.checkBuildSettings { settings in
+                            // Check that the `-enable-library-evolution` setting for Release also set SWIFT_EMIT_MODULE_INTERFACE.
+                            XCTAssertEqual(settings[.SWIFT_EMIT_MODULE_INTERFACE], "YES")
+                            XCTAssertEqual(settings[.OTHER_SWIFT_FLAGS], ["$(inherited)", "-enable-library-evolution"])
+                        }
+                    }
+                }
+            }
+        }
+    }
+
   #endif
 }