repository: add Checker() method to repository to replace unchecked cast

internal/checker/checker.go

@@ -33,10 +33,15 @@ type Checker struct {
 	repo restic.Repository
 }
 
+type checkerRepository interface {
+	restic.Repository
+	Checker() *repository.Checker
+}
+
 // New returns a new checker which runs on repo.
-func New(repo restic.Repository, trackUnused bool) *Checker {
+func New(repo checkerRepository, trackUnused bool) *Checker {
 	c := &Checker{
-		Checker:     repository.NewChecker(repo),
+		Checker:     repo.Checker(),
 		repo:        repo,
 		trackUnused: trackUnused,
 	}

internal/checker/checker_test.go

@@ -435,7 +435,7 @@ func TestCheckerModifiedData(t *testing.T) {
 
 // loadTreesOnceRepository allows each tree to be loaded only once
 type loadTreesOnceRepository struct {
-	restic.Repository
+	*repository.Repository
 	loadedTrees   restic.IDSet
 	mutex         sync.Mutex
 	DuplicateTree bool
@@ -476,7 +476,7 @@ func TestCheckerNoDuplicateTreeDecodes(t *testing.T) {
 
 // delayRepository delays read of a specific handle.
 type delayRepository struct {
-	restic.Repository
+	*repository.Repository
 	DelayTree      restic.ID
 	UnblockChannel chan struct{}
 	Unblocker      sync.Once

internal/checker/testing.go

@@ -3,12 +3,10 @@ package checker
 import (
 	"context"
 	"testing"
-
-	"github.com/restic/restic/internal/restic"
 )
 
 // TestCheckRepo runs the checker on repo.
-func TestCheckRepo(t testing.TB, repo restic.Repository) {
+func TestCheckRepo(t testing.TB, repo checkerRepository) {
 	chkr := New(repo, true)
 
 	hints, errs := chkr.LoadIndex(context.TODO(), nil)