image/unparsed.go
func (i *UnparsedImage) expectedManifestDigest() (digest.Digest, bool) {
	if i.instanceDigest != nil {
		return *i.instanceDigest, true
	}
	ref := i.Reference().DockerReference()
	if ref != nil {
		if canonical, ok := ref.(reference.Canonical); ok {
			return canonical.Digest(), true
		}
	}
	return "", false
}
func (i *UnparsedImage) expectedManifestDigest() (digest.Digest, bool) {
	if i.instanceDigest != nil {
		return *i.instanceDigest, true
	}
	ref := i.Reference().DockerReference()
	if ref != nil {
		if canonical, ok := ref.(reference.Canonical); ok {
			return canonical.Digest(), true
		}
	}
	return "", false
}
// expectedManifestDigest returns a the expected value of the manifest digest, and an indicator whether it is known.
// The bool return value seems redundant with digest != ""; it is used explicitly
// to refuse (unexpected) situations when the digest exists but is "".