Make all non-user-creatable structures non-comparable (#802)

This change makes all non-user-creatable structures non-comparable. This
makes it easier to add changes later that don't introduce breaking
changes from the go compatibility guarantees perspective.

This, of course, implies that this change _is_ a breaking change, but since
these structures are not intended to be created by users (or de-referenced),
it should be okay.

32 files changed, 62 insertions, 7 deletions

diff --git a/blame.go b/blame.go
index 04e2962..bed29b5 100644
--- a/blame.go
+++ b/blame.go
@@ -87,6 +87,7 @@ func (v *Repository) BlameFile(path string, opts *BlameOptions) (*Blame, error)
 }
 
 type Blame struct {
+	doNotCompare
 	ptr *C.git_blame
 }
 
diff --git a/blob.go b/blob.go
index 091ced1..fa4b1d3 100644
--- a/blob.go
+++ b/blob.go
@@ -15,6 +15,7 @@ import (
 )
 
 type Blob struct {
+	doNotCompare
 	Object
 	cast_ptr *C.git_blob
 }
@@ -96,6 +97,7 @@ func (repo *Repository) CreateFromStream(hintPath string) (*BlobWriteStream, err
 }
 
 type BlobWriteStream struct {
+	doNotCompare
 	ptr  *C.git_writestream
 	repo *Repository
 }
diff --git a/branch.go b/branch.go
index 217b877..9596d0b 100644
--- a/branch.go
+++ b/branch.go
@@ -19,6 +19,7 @@ const (
 )
 
 type Branch struct {
+	doNotCompare
 	*Reference
 }
 
@@ -27,6 +28,7 @@ func (r *Reference) Branch() *Branch {
 }
 
 type BranchIterator struct {
+	doNotCompare
 	ptr  *C.git_branch_iterator
 	repo *Repository
 }
diff --git a/commit.go b/commit.go
index cfc6a63..0869ef2 100644
--- a/commit.go
+++ b/commit.go
@@ -22,6 +22,7 @@ const (
 
 // Commit
 type Commit struct {
+	doNotCompare
 	Object
 	cast_ptr *C.git_commit
 }
diff --git a/config.go b/config.go
index 062df05..bca4d18 100644
--- a/config.go
+++ b/config.go
@@ -52,6 +52,7 @@ func newConfigEntryFromC(centry *C.git_config_entry) *ConfigEntry {
 }
 
 type Config struct {
+	doNotCompare
 	ptr *C.git_config
 }
 
@@ -361,6 +362,7 @@ func OpenOndisk(path string) (*Config, error) {
 }
 
 type ConfigIterator struct {
+	doNotCompare
 	ptr *C.git_config_iterator
 	cfg *Config
 }
diff --git a/credentials.go b/credentials.go
index deb399a..843c6b2 100644
--- a/credentials.go
+++ b/credentials.go
@@ -79,6 +79,7 @@ func (t CredentialType) String() string {
 }
 
 type Credential struct {
+	doNotCompare
 	ptr *C.git_credential
 }
 
diff --git a/deprecated.go b/deprecated.go
index 64f18e0..587fd0e 100644
--- a/deprecated.go
+++ b/deprecated.go
@@ -224,7 +224,7 @@ type SubmoduleCbk = SubmoduleCallback
 
 // Deprecated: SubmoduleVisitor is not used.
 func SubmoduleVisitor(csub unsafe.Pointer, name *C.char, handle unsafe.Pointer) C.int {
-	sub := &Submodule{(*C.git_submodule)(csub), nil}
+	sub := &Submodule{ptr: (*C.git_submodule)(csub)}
 
 	callback, ok := pointerHandles.Get(handle).(SubmoduleCallback)
 	if !ok {
diff --git a/describe.go b/describe.go
index da233d9..f0ebb2c 100644
--- a/describe.go
+++ b/describe.go
@@ -176,6 +176,7 @@ func (repo *Repository) DescribeWorkdir(opts *DescribeOptions) (*DescribeResult,
 //
 // Use Format() to get a string out of it.
 type DescribeResult struct {
+	doNotCompare
 	ptr *C.git_describe_result
 }
 
diff --git a/diff.go b/diff.go
index fab4fce..308859a 100644
--- a/diff.go
+++ b/diff.go
@@ -132,6 +132,7 @@ func diffLineFromC(line *C.git_diff_line) DiffLine {
 }
 
 type Diff struct {
+	doNotCompare
 	ptr          *C.git_diff
 	repo         *Repository
 	runFinalizer bool
@@ -219,6 +220,7 @@ func (diff *Diff) FindSimilar(opts *DiffFindOptions) error {
 }
 
 type DiffStats struct {
+	doNotCompare
 	ptr *C.git_diff_stats
 }
 
diff --git a/git.go b/git.go
index d672d77..3f51048 100644
--- a/git.go
+++ b/git.go
@@ -123,6 +123,10 @@ var (
 	ErrInvalid = errors.New("Invalid state for operation")
 )
 
+// doNotCompare is an idiomatic way of making structs non-comparable to avoid
+// future field additions to make them non-comparable.
+type doNotCompare [0]func()
+
 var pointerHandles *HandleList
 
 func init() {
diff --git a/handles.go b/handles.go
index c0d4b3c..36ca1f0 100644
--- a/handles.go
+++ b/handles.go
@@ -11,6 +11,7 @@ import (
 )
 
 type HandleList struct {
+	doNotCompare
 	sync.RWMutex
 	// stores the Go pointers
 	handles map[unsafe.Pointer]interface{}
diff --git a/index.go b/index.go
index 99dbd7a..48c922c 100644
--- a/index.go
+++ b/index.go
@@ -52,6 +52,7 @@ const (
 )
 
 type Index struct {
+	doNotCompare
 	ptr  *C.git_index
 	repo *Repository
 }
@@ -108,7 +109,7 @@ func freeCIndexEntry(entry *C.git_index_entry) {
 }
 
 func newIndexFromC(ptr *C.git_index, repo *Repository) *Index {
-	idx := &Index{ptr, repo}
+	idx := &Index{ptr: ptr, repo: repo}
 	runtime.SetFinalizer(idx, (*Index).Free)
 	return idx
 }
@@ -616,6 +617,7 @@ func (v *Index) RemoveConflict(path string) error {
 }
 
 type IndexConflictIterator struct {
+	doNotCompare
 	ptr   *C.git_index_conflict_iterator
 	index *Index
 }
diff --git a/indexer.go b/indexer.go
index 601ac9c..53ac420 100644
--- a/indexer.go
+++ b/indexer.go
@@ -19,6 +19,7 @@ import (
 // Indexer can post-process packfiles and create an .idx file for efficient
 // lookup.
 type Indexer struct {
+	doNotCompare
 	ptr        *C.git_indexer
 	stats      C.git_transfer_progress
 	ccallbacks C.git_remote_callbacks
diff --git a/mempack.go b/mempack.go
index bdea224..14898f9 100644
--- a/mempack.go
+++ b/mempack.go
@@ -18,6 +18,7 @@ import (
 
 // Mempack is a custom ODB backend that permits packing object in-memory.
 type Mempack struct {
+	doNotCompare
 	ptr *C.git_odb_backend
 }
 
diff --git a/merge.go b/merge.go
index dc0e7ca..a8e7d6e 100644
--- a/merge.go
+++ b/merge.go
@@ -17,6 +17,7 @@ import (
 )
 
 type AnnotatedCommit struct {
+	doNotCompare
 	ptr *C.git_annotated_commit
 	r   *Repository
 }
@@ -426,11 +427,12 @@ func (r *Repository) MergeBaseOctopus(oids []*Oid) (*Oid, error) {
 }
 
 type MergeFileResult struct {
+	doNotCompare
+	ptr           *C.git_merge_file_result
 	Automergeable bool
 	Path          string
 	Mode          uint
 	Contents      []byte
-	ptr           *C.git_merge_file_result
 }
 
 func newMergeFileResultFromC(c *C.git_merge_file_result) *MergeFileResult {
diff --git a/note.go b/note.go
index 9df1b9d..05eadb8 100644
--- a/note.go
+++ b/note.go
@@ -13,6 +13,7 @@ import (
 // This object represents the possible operations which can be
 // performed on the collection of notes for a repository.
 type NoteCollection struct {
+	doNotCompare
 	repo *Repository
 }
 
@@ -139,6 +140,7 @@ func (c *NoteCollection) DefaultRef() (string, error) {
 
 // Note
 type Note struct {
+	doNotCompare
 	ptr *C.git_note
 	r   *Repository
 }
@@ -189,6 +191,7 @@ func (n *Note) Message() string {
 
 // NoteIterator
 type NoteIterator struct {
+	doNotCompare
 	ptr *C.git_note_iterator
 	r   *Repository
 }
diff --git a/object.go b/object.go
index bf0ca0e..4ea290c 100644
--- a/object.go
+++ b/object.go
@@ -22,6 +22,7 @@ const (
 )
 
 type Object struct {
+	doNotCompare
 	ptr  *C.git_object
 	repo *Repository
 }
diff --git a/odb.go b/odb.go
index cef13bb..5550b91 100644
--- a/odb.go
+++ b/odb.go
@@ -22,10 +22,12 @@ import (
 )
 
 type Odb struct {
+	doNotCompare
 	ptr *C.git_odb
 }
 
 type OdbBackend struct {
+	doNotCompare
 	ptr *C.git_odb_backend
 }
 
@@ -45,7 +47,7 @@ func NewOdb() (odb *Odb, err error) {
 }
 
 func NewOdbBackendFromC(ptr unsafe.Pointer) (backend *OdbBackend) {
-	backend = &OdbBackend{(*C.git_odb_backend)(ptr)}
+	backend = &OdbBackend{ptr: (*C.git_odb_backend)(ptr)}
 	return backend
 }
 
@@ -313,6 +315,7 @@ func (v *OdbBackend) Free() {
 }
 
 type OdbObject struct {
+	doNotCompare
 	ptr *C.git_odb_object
 }
 
@@ -356,6 +359,7 @@ func (object *OdbObject) Data() (data []byte) {
 }
 
 type OdbReadStream struct {
+	doNotCompare
 	ptr  *C.git_odb_stream
 	Size uint64
 	Type ObjectType
@@ -396,6 +400,7 @@ func (stream *OdbReadStream) Free() {
 }
 
 type OdbWriteStream struct {
+	doNotCompare
 	ptr *C.git_odb_stream
 	Id  Oid
 }
@@ -440,6 +445,7 @@ func (stream *OdbWriteStream) Free() {
 
 // OdbWritepack is a stream to write a packfile to the ODB.
 type OdbWritepack struct {
+	doNotCompare
 	ptr        *C.git_odb_writepack
 	stats      C.git_transfer_progress
 	ccallbacks C.git_remote_callbacks
diff --git a/packbuilder.go b/packbuilder.go
index 5d3a933..0ec0fb1 100644
--- a/packbuilder.go
+++ b/packbuilder.go
@@ -16,6 +16,7 @@ import (
 )
 
 type Packbuilder struct {
+	doNotCompare
 	ptr *C.git_packbuilder
 	r   *Repository
 }
diff --git a/patch.go b/patch.go
index b2595f1..9b67687 100644
--- a/patch.go
+++ b/patch.go
@@ -10,6 +10,7 @@ import (
 )
 
 type Patch struct {
+	doNotCompare
 	ptr *C.git_patch
 }
 
diff --git a/rebase.go b/rebase.go
index 119d4f5..98c3e90 100644
--- a/rebase.go
+++ b/rebase.go
@@ -214,6 +214,7 @@ func mapEmptyStringToNull(ref string) *C.char {
 
 // Rebase is the struct representing a Rebase object.
 type Rebase struct {
+	doNotCompare
 	ptr     *C.git_rebase
 	r       *Repository
 	options *C.git_rebase_options
diff --git a/refdb.go b/refdb.go
index 578f43c..a8b1715 100644
--- a/refdb.go
+++ b/refdb.go
@@ -13,11 +13,13 @@ import (
 )
 
 type Refdb struct {
+	doNotCompare
 	ptr *C.git_refdb
 	r   *Repository
 }
 
 type RefdbBackend struct {
+	doNotCompare
 	ptr *C.git_refdb_backend
 }
 
@@ -38,7 +40,7 @@ func (v *Repository) NewRefdb() (refdb *Refdb, err error) {
 }
 
 func NewRefdbBackendFromC(ptr unsafe.Pointer) (backend *RefdbBackend) {
-	backend = &RefdbBackend{(*C.git_refdb_backend)(ptr)}
+	backend = &RefdbBackend{ptr: (*C.git_refdb_backend)(ptr)}
 	return backend
 }
 
diff --git a/reference.go b/reference.go
index e0c7cad..ae49c21 100644
--- a/reference.go
+++ b/reference.go
@@ -17,11 +17,13 @@ const (
 )
 
 type Reference struct {
+	doNotCompare
 	ptr  *C.git_reference
 	repo *Repository
 }
 
 type ReferenceCollection struct {
+	doNotCompare
 	repo *Repository
 }
 
@@ -363,11 +365,13 @@ func (v *Reference) Free() {
 }
 
 type ReferenceIterator struct {
+	doNotCompare
 	ptr  *C.git_reference_iterator
 	repo *Repository
 }
 
 type ReferenceNameIterator struct {
+	doNotCompare
 	*ReferenceIterator
 }
 
@@ -422,7 +426,7 @@ func (repo *Repository) NewReferenceIteratorGlob(glob string) (*ReferenceIterato
 }
 
 func (i *ReferenceIterator) Names() *ReferenceNameIterator {
-	return &ReferenceNameIterator{i}
+	return &ReferenceNameIterator{ReferenceIterator: i}
 }
 
 // NextName retrieves the next reference name. If the iteration is over,
diff --git a/remote.go b/remote.go
index d954766..c8076d5 100644
--- a/remote.go
+++ b/remote.go
@@ -168,6 +168,7 @@ type ProxyOptions struct {
 }
 
 type Remote struct {
+	doNotCompare
 	ptr       *C.git_remote
 	callbacks RemoteCallbacks
 	repo      *Repository
@@ -517,6 +518,7 @@ func (r *Remote) Free() {
 }
 
 type RemoteCollection struct {
+	doNotCompare
 	repo *Repository
 }
 
diff --git a/repository.go b/repository.go
index 9735c63..c7ec9eb 100644
--- a/repository.go
+++ b/repository.go
@@ -14,6 +14,7 @@ import (
 
 // Repository
 type Repository struct {
+	doNotCompare
 	ptr *C.git_repository
 	// Remotes represents the collection of remotes and can be
 	// used to add, remove and configure remotes for this
diff --git a/revparse.go b/revparse.go
index 950932b..34e1fa3 100644
--- a/revparse.go
+++ b/revparse.go
@@ -20,6 +20,7 @@ const (
 )
 
 type Revspec struct {
+	doNotCompare
 	to    *Object
 	from  *Object
 	flags RevparseFlag
diff --git a/stash.go b/stash.go
index 2086e2e..865ea91 100644
--- a/stash.go
+++ b/stash.go
@@ -35,6 +35,7 @@ const (
 // StashCollection represents the possible operations that can be
 // performed on the collection of stashes for a repository.
 type StashCollection struct {
+	doNotCompare
 	repo *Repository
 }
 
diff --git a/status.go b/status.go
index 0b0bc18..74c5814 100644
--- a/status.go
+++ b/status.go
@@ -55,6 +55,7 @@ func statusEntryFromC(statusEntry *C.git_status_entry) StatusEntry {
 }
 
 type StatusList struct {
+	doNotCompare
 	ptr *C.git_status_list
 	r   *Repository
 }
diff --git a/submodule.go b/submodule.go
index 2042125..673cf5f 100644
--- a/submodule.go
+++ b/submodule.go
@@ -21,6 +21,7 @@ type SubmoduleUpdateOptions struct {
 
 // Submodule
 type Submodule struct {
+	doNotCompare
 	ptr *C.git_submodule
 	r   *Repository
 }
@@ -82,6 +83,7 @@ const (
 )
 
 type SubmoduleCollection struct {
+	doNotCompare
 	repo *Repository
 }
 
@@ -117,7 +119,7 @@ type submoduleCallbackData struct {
 
 //export submoduleCallback
 func submoduleCallback(csub unsafe.Pointer, name *C.char, handle unsafe.Pointer) C.int {
-	sub := &Submodule{(*C.git_submodule)(csub), nil}
+	sub := &Submodule{ptr: (*C.git_submodule)(csub)}
 
 	data, ok := pointerHandles.Get(handle).(submoduleCallbackData)
 	if !ok {
diff --git a/tag.go b/tag.go
index dd8477a..9ced680 100644
--- a/tag.go
+++ b/tag.go
@@ -13,6 +13,7 @@ import (
 
 // Tag
 type Tag struct {
+	doNotCompare
 	Object
 	cast_ptr *C.git_tag
 }
@@ -64,6 +65,7 @@ func (t *Tag) TargetType() ObjectType {
 }
 
 type TagsCollection struct {
+	doNotCompare
 	repo *Repository
 }
 
diff --git a/tree.go b/tree.go
index 38c5bdc..14fe7e4 100644
--- a/tree.go
+++ b/tree.go
@@ -24,6 +24,7 @@ const (
 )
 
 type Tree struct {
+	doNotCompare
 	Object
 	cast_ptr *C.git_tree
 }
@@ -167,6 +168,7 @@ func (t *Tree) Walk(callback TreeWalkCallback) error {
 }
 
 type TreeBuilder struct {
+	doNotCompare
 	ptr  *C.git_treebuilder
 	repo *Repository
 }
diff --git a/walk.go b/walk.go
index 6020c97..77ec3a1 100644
--- a/walk.go
+++ b/walk.go
@@ -22,6 +22,7 @@ const (
 )
 
 type RevWalk struct {
+	doNotCompare
 	ptr  *C.git_revwalk
 	repo *Repository
 }