feat(internal/rust): clean output directory before generation (#3102)

Remove generated files before regenerating, preserving only the files in
Library.Keep. This ensures stale files from previous generations don't
persist.

Author: julieqiu

internal/librarian/generate.go

@@ -18,6 +18,9 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
 
 	"github.com/googleapis/librarian/internal/config"
 	"github.com/googleapis/librarian/internal/fetch"
@@ -111,6 +114,10 @@ func generate(ctx context.Context, language string, library *config.Library, sou
 	case "testhelper":
 		err = testGenerate(library)
 	case "rust":
+		keep := append(library.Keep, "Cargo.toml")
+		if err := cleanOutput(library.Output, keep); err != nil {
+			return err
+		}
 		err = rust.Generate(ctx, library, sources)
 	default:
 		err = fmt.Errorf("generate not implemented for %q", language)
@@ -132,3 +139,33 @@ func fetchGoogleapisDir(ctx context.Context, sources *config.Sources) (string, e
 	}
 	return fetch.RepoDir(ctx, googleapisRepo, sources.Googleapis.Commit, sources.Googleapis.SHA256)
 }
+
+// cleanOutput removes all files in dir except those in keep. The keep list
+// should contain paths relative to dir. It returns an error if any file in
+// keep does not exist.
+func cleanOutput(dir string, keep []string) error {
+	keepSet := make(map[string]bool)
+	for _, k := range keep {
+		path := filepath.Join(dir, k)
+		if _, err := os.Stat(path); errors.Is(err, fs.ErrNotExist) {
+			return fmt.Errorf("%s: file %q in keep list does not exist", dir, k)
+		}
+		keepSet[k] = true
+	}
+	return filepath.WalkDir(dir, func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if d.IsDir() {
+			return nil
+		}
+		rel, err := filepath.Rel(dir, path)
+		if err != nil {
+			return err
+		}
+		if keepSet[rel] {
+			return nil
+		}
+		return os.Remove(path)
+	})
+}

internal/librarian/generate_test.go

@@ -19,6 +19,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"slices"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -137,3 +138,90 @@ libraries:
 		})
 	}
 }
+
+func TestCleanOutput(t *testing.T) {
+	for _, test := range []struct {
+		name    string
+		files   []string
+		keep    []string
+		want    []string
+		wantErr bool
+	}{
+		{
+			name:  "removes all except keep list",
+			files: []string{"Cargo.toml", "README.md", "src/lib.rs"},
+			keep:  []string{"Cargo.toml"},
+			want:  []string{"Cargo.toml"},
+		},
+		{
+			name:    "empty directory with keep list",
+			files:   []string{},
+			keep:    []string{"Cargo.toml"},
+			wantErr: true,
+		},
+		{
+			name:  "only kept file",
+			files: []string{"Cargo.toml"},
+			keep:  []string{"Cargo.toml"},
+			want:  []string{"Cargo.toml"},
+		},
+		{
+			name:    "keep file not found",
+			files:   []string{"README.md", "src/lib.rs"},
+			keep:    []string{"Cargo.toml"},
+			wantErr: true,
+		},
+		{
+			name:  "keep multiple files",
+			files: []string{"Cargo.toml", "README.md", "src/lib.rs"},
+			keep:  []string{"Cargo.toml", "README.md"},
+			want:  []string{"Cargo.toml", "README.md"},
+		},
+		{
+			name:  "empty keep list",
+			files: []string{"Cargo.toml", "README.md"},
+			keep:  []string{},
+			want:  []string{},
+		},
+		{
+			name:  "keep nested files",
+			files: []string{"Cargo.toml", "README.md", "src/lib.rs", "src/operation.rs", "src/endpoint.rs"},
+			keep:  []string{"src/operation.rs", "src/endpoint.rs"},
+			want:  []string{"src/endpoint.rs", "src/operation.rs"},
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			dir := t.TempDir()
+			for _, f := range test.files {
+				path := filepath.Join(dir, f)
+				if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
+					t.Fatal(err)
+				}
+				if err := os.WriteFile(path, []byte("test"), 0644); err != nil {
+					t.Fatal(err)
+				}
+			}
+			err := cleanOutput(dir, test.keep)
+			if test.wantErr {
+				if err == nil {
+					t.Fatal("expected error, got nil")
+				}
+				return
+			}
+			if err != nil {
+				t.Fatal(err)
+			}
+			var got []string
+			for _, f := range test.files {
+				if _, err := os.Stat(filepath.Join(dir, f)); err == nil {
+					got = append(got, f)
+				}
+			}
+			slices.Sort(got)
+			slices.Sort(test.want)
+			if !slices.Equal(got, test.want) {
+				t.Errorf("got %v, want %v", got, test.want)
+			}
+		})
+	}
+}