Fix issue where .swift-version file was ignored if not also using config file (#2230)

Sources/SwiftFormat.swift

@@ -353,10 +353,15 @@ private func processDirectory(_ inputURL: URL, with options: inout Options, logg
             logger?("Ignoring swift-version file at \(versionFile.path)")
         } else if Version(rawValue: versionString) != nil {
             logger?("Reading swift-version file at \(versionFile.path) (version \(versionString))")
-            args = args.map {
-                var args = $0
-                args["swift-version"] = versionString
-                return args
+
+            if args.isEmpty {
+                args = [["swift-version": versionString]]
+            } else {
+                args = args.map {
+                    var args = $0
+                    args["swift-version"] = versionString
+                    return args
+                }
             }
         } else {
             // Don't treat as error, per: https://github.com/nicklockwood/SwiftFormat/issues/639

Tests/CommandLineTests.swift

@@ -1423,4 +1423,79 @@ class CommandLineTests: XCTestCase {
             try FileManager.default.removeItem(at: tempFile)
         }
     }
+
+    func testSwiftVersionFileWithNoConfigFile() throws {
+        var errors = [String]()
+
+        CLI.print = { message, type in
+            print(message)
+            if type == .error {
+                errors.append(message)
+            }
+        }
+
+        try withTmpFiles([
+            ".swift-version": """
+            6.1
+            """,
+            "Test.swift": """
+            func foo(
+                foo: Foo,
+                bar: Bar
+            ) {}
+            """,
+        ]) { url in
+            guard url.pathExtension == "swift" else { return }
+            _ = processArguments(["", url.path, "--rules", "trailingCommas"], in: "")
+
+            XCTAssertEqual(try String(contentsOf: url, encoding: .utf8), """
+            func foo(
+                foo: Foo,
+                bar: Bar,
+            ) {}
+            """)
+        }
+
+        XCTAssertEqual(errors, [])
+    }
+
+    func testSwiftVersionFileWithConfigFile() throws {
+        var errors = [String]()
+
+        CLI.print = { message, type in
+            print(message)
+            if type == .error {
+                errors.append(message)
+            }
+        }
+
+        try withTmpFiles([
+            ".swift-version": """
+            6.1
+            """,
+            ".swiftformat": """
+            --rules trailingCommas
+            --rules indent
+            --indent 2
+            """,
+            "Test.swift": """
+            func foo(
+                foo: Foo,
+                bar: Bar
+            ) {}
+            """,
+        ]) { url in
+            guard url.pathExtension == "swift" else { return }
+            _ = processArguments(["", url.path], in: "")
+
+            XCTAssertEqual(try String(contentsOf: url, encoding: .utf8), """
+            func foo(
+              foo: Foo,
+              bar: Bar,
+            ) {}
+            """)
+        }
+
+        XCTAssertEqual(errors, [])
+    }
 }