[Xcodeproj] Fix logic for generating SWIFT_VERSION setting

Sources/Xcodeproj/pbxproj().swift

@@ -78,7 +78,7 @@ func xcodeProject(
             interpreterFlags[3] = "$(TOOLCHAIN_DIR)/usr/lib/swift/pm/" + String(package.manifest.manifestVersion.rawValue)
         }
         pdTarget.buildSettings.common.OTHER_SWIFT_FLAGS += interpreterFlags
-        pdTarget.buildSettings.common.SWIFT_VERSION = "\(package.manifest.manifestVersion.rawValue).0"
+        pdTarget.buildSettings.common.SWIFT_VERSION = package.manifest.manifestVersion.swiftLanguageVersion.xcodeBuildSettingValue
         pdTarget.buildSettings.common.LD = "/usr/bin/true"
     }
 
@@ -459,7 +459,7 @@ func xcodeProject(
 
         // Set the correct SWIFT_VERSION for the Swift targets.
         if case let swiftTarget as SwiftTarget = target.underlyingTarget {
-            targetSettings.common.SWIFT_VERSION = "\(swiftTarget.swiftVersion).0"
+            targetSettings.common.SWIFT_VERSION = swiftTarget.swiftVersion.xcodeBuildSettingValue
         }
 
         // Add header search paths for any C target on which we depend.
@@ -716,3 +716,16 @@ private extension ResolvedTarget {
         }
     }
 }
+
+private extension SwiftLanguageVersion {
+    /// Returns the build setting value for the given Swift language version.
+    var xcodeBuildSettingValue: String {
+        // Swift version setting are represented differently in Xcode:
+        // 3 -> 4.0, 4 -> 4.0, 4.2 -> 4.2
+        var swiftVersion = "\(rawValue)"
+        if !rawValue.contains(".") {
+            swiftVersion += ".0"
+        }
+        return swiftVersion
+    }
+}

Tests/XcodeprojTests/PackageGraphTests.swift

@@ -311,13 +311,44 @@ class PackageGraphTests: XCTestCase {
         XCTAssertEqual(schemes["Foo-Package"]?.testTargets.map({ $0.name }).sorted(), ["aTests", "bcTests", "dTests", "libdTests"])
         XCTAssertEqual(schemes["Foo-Package"]?.regularTargets.map({ $0.name }).sorted(), ["a", "b", "c", "d", "libd"])
     }
+
+    func testSwiftVersion() throws {
+        // FIXME: Unfortunately, we can't test 4.2 right now.
+        for swiftVersion in [3, 4] {
+            let fs = InMemoryFileSystem(emptyFiles:
+                "/Foo/Sources/a/main.swift",
+                "/end"
+            )
+
+            let diagnostics = DiagnosticsEngine()
+            let graph = loadMockPackageGraph4([
+                "/Foo": .init(
+                    name: "Foo",
+                    targets: [
+                        .target(name: "a"),
+                    ],
+                    swiftLanguageVersions: [swiftVersion]
+                 ),
+            ], root: "/Foo", diagnostics: diagnostics, in: fs)
+
+            let project = try xcodeProject(xcodeprojPath: AbsolutePath("/Foo").appending(component: "xcodeproj"), graph: graph, extraDirs: [], options: XcodeprojOptions(), fileSystem: fs, diagnostics: diagnostics)
+            XCTAssertNoDiagnostics(diagnostics)
+
+            XcodeProjectTester(project) { result in
+                result.check(target: "a") { targetResult in
+                    XCTAssertEqual(targetResult.target.buildSettings.common.SWIFT_VERSION, "\(swiftVersion).0")
+                }
+            }
+        }
+    }
 
     static var allTests = [
         ("testAggregateTarget", testAggregateTarget),
         ("testBasics", testBasics),
         ("testModuleLinkage", testModuleLinkage),
         ("testModulemap", testModulemap),
         ("testSchemes", testSchemes),
+        ("testSwiftVersion", testSwiftVersion),
     ]
 }