Replace duplicate type checking in cache with a function

internal/cache/backend.go

@@ -43,14 +43,13 @@ func (b *Backend) Remove(ctx context.Context, h restic.Handle) error {
 	return b.Cache.remove(h)
 }
 
-var autoCacheTypes = map[restic.FileType]struct{}{
+func autoCached(t restic.FileType) bool {
-	restic.IndexFile:    {},
+	return t == restic.IndexFile || t == restic.SnapshotFile
-	restic.SnapshotFile: {},
 }
 
 // Save stores a new file in the backend and the cache.
 func (b *Backend) Save(ctx context.Context, h restic.Handle, rd restic.RewindReader) error {
-	if _, ok := autoCacheTypes[h.Type]; !ok {
+	if !autoCached(h.Type) {
 		return b.Backend.Save(ctx, h, rd)
 	}
 
@@ -84,11 +83,6 @@ func (b *Backend) Save(ctx context.Context, h restic.Handle, rd restic.RewindRea
 	return nil
 }
 
-var autoCacheFiles = map[restic.FileType]bool{
-	restic.IndexFile:    true,
-	restic.SnapshotFile: true,
-}
-
 func (b *Backend) cacheFile(ctx context.Context, h restic.Handle) error {
 	finish := make(chan struct{})
 
@@ -192,7 +186,7 @@ func (b *Backend) Load(ctx context.Context, h restic.Handle, length int, offset
 	}
 
 	// if we don't automatically cache this file type, fall back to the backend
-	if _, ok := autoCacheFiles[h.Type]; !ok {
+	if !autoCached(h.Type) {
 		debug.Log("Load(%v, %v, %v): delegating to backend", h, length, offset)
 		return b.Backend.Load(ctx, h, length, offset, consumer)
 	}