Make errors.stack comparable (#30)

Fixed #29

errors.New, etc values are not expected to be compared by value but
the change in errors#27 made them incomparable. Assert that various
kinds of errors have a functional equality operator, even if the
result of that equality is always false.

errors.go

@@ -64,17 +64,17 @@ import (
 // stack represents a stack of programm counters.
 type stack []uintptr
 
-func (s stack) Stack() []uintptr { return s }
+func (s *stack) Stack() []uintptr { return *s }
 
-func (s stack) Location() (string, int) {
-	return location(s[0] - 1)
+func (s *stack) Location() (string, int) {
+	return location((*s)[0] - 1)
 }
 
 // New returns an error that formats as the given text.
 func New(text string) error {
 	return struct {
 		error
-		stack
+		*stack
 	}{
 		errors.New(text),
 		callers(),
@@ -95,7 +95,7 @@ func (c cause) Message() string { return c.message }
 func Errorf(format string, args ...interface{}) error {
 	return struct {
 		error
-		stack
+		*stack
 	}{
 		fmt.Errorf(format, args...),
 		callers(),
@@ -120,10 +120,10 @@ func Wrapf(cause error, format string, args ...interface{}) error {
 	return wrap(cause, fmt.Sprintf(format, args...), callers())
 }
 
-func wrap(err error, msg string, st stack) error {
+func wrap(err error, msg string, st *stack) error {
 	return struct {
 		cause
-		stack
+		*stack
 	}{
 		cause{
 			cause:   err,
@@ -198,11 +198,12 @@ func Fprint(w io.Writer, err error) {
 	}
 }
 
-func callers() stack {
+func callers() *stack {
 	const depth = 32
 	var pcs [depth]uintptr
 	n := runtime.Callers(3, pcs[:])
-	return pcs[0:n]
+	var st stack = pcs[0:n]
+	return &st
 }
 
 // location returns the source file and line matching pc.

errors_test.go

@@ -241,3 +241,24 @@ func TestStack(t *testing.T) {
 		}
 	}
 }
+
+// errors.New, etc values are not expected to be compared by value
+// but the change in errors#27 made them incomparable. Assert that
+// various kinds of errors have a functional equality operator, even
+// if the result of that equality is always false.
+func TestErrorEquality(t *testing.T) {
+	tests := []struct {
+		err1, err2 error
+	}{
+		{io.EOF, io.EOF},
+		{io.EOF, nil},
+		{io.EOF, errors.New("EOF")},
+		{io.EOF, New("EOF")},
+		{New("EOF"), New("EOF")},
+		{New("EOF"), Errorf("EOF")},
+		{New("EOF"), Wrap(io.EOF, "EOF")},
+	}
+	for _, tt := range tests {
+		_ = tt.err1 == tt.err2 // mustn't panic
+	}
+}