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.

Author: lhchavez

blame.go

@@ -87,6 +87,7 @@ func (v *Repository) BlameFile(path string, opts *BlameOptions) (*Blame, error)
 }
 
 type Blame struct {
+	doNotCompare
 	ptr *C.git_blame
 }
 

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
 }

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
 }

commit.go

@@ -22,6 +22,7 @@ const (
 
 // Commit
 type Commit struct {
+	doNotCompare
 	Object
 	cast_ptr *C.git_commit
 }

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
 }

credentials.go

@@ -79,6 +79,7 @@ func (t CredentialType) String() string {
 }
 
 type Credential struct {
+	doNotCompare
 	ptr *C.git_credential
 }
 

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 {

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.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
 }
 

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() {