feat(librarian/swift): default output directory (#5024)

Librarian needs to compute the default output directory for an API. For
Swift, I am selecting the full path of the API, as we do in most
languages. The `librarian.yaml` file will include a prefix, such as
`generated/`.

Author: coryan

internal/librarian/generate.go

@@ -269,6 +269,8 @@ func defaultOutput(language string, name, api, defaultOut string) string {
 		return python.DefaultOutput(name, defaultOut)
 	case config.LanguageRust:
 		return rust.DefaultOutput(api, defaultOut)
+	case config.LanguageSwift:
+		return swift.DefaultOutput(api, defaultOut)
 	default:
 		return defaultOut
 	}

internal/librarian/swift/default_output.go

@@ -0,0 +1,23 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package swift
+
+import "path/filepath"
+
+// DefaultOutput derives an output path from an API path and a default
+// output directory.
+func DefaultOutput(api, defaultOutput string) string {
+	return filepath.Join(defaultOutput, api)
+}

internal/librarian/swift/default_output_test.go

@@ -0,0 +1,62 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package swift
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestDefaultOutput(t *testing.T) {
+	for _, tt := range []struct {
+		name   string
+		api    string
+		defOut string
+		want   string
+	}{
+		{
+			name:   "simple",
+			api:    "secretmanager",
+			defOut: "packages",
+			want:   "packages/secretmanager",
+		},
+		{
+			name:   "empty default",
+			api:    "secretmanager",
+			defOut: "",
+			want:   "secretmanager",
+		},
+		{
+			name:   "nested default",
+			api:    "secretmanager",
+			defOut: "a/b/c",
+			want:   "a/b/c/secretmanager",
+		},
+		{
+			name:   "api path",
+			api:    "google/cloud/secretmanager/v1",
+			defOut: "generated",
+			want:   "generated/google/cloud/secretmanager/v1",
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			got := DefaultOutput(tt.api, tt.defOut)
+			if diff := cmp.Diff(tt.want, got); diff != "" {
+				t.Errorf("mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}