xerrors: add Format method to errorString

Fixes #30036.

Change-Id: I8155e94f3a4bbda77170ebbecb24c27d9e26860b
Reviewed-on: https://go-review.googlesource.com/c/160658
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: neild

errors.go

@@ -4,6 +4,8 @@
 
 package xerrors
 
+import "fmt"
+
 // errorString is a trivial implementation of error.
 type errorString struct {
 	s     string
@@ -22,6 +24,8 @@ func (e *errorString) Error() string {
 	return e.s
 }
 
+func (e *errorString) Format(s fmt.State, v rune) { FormatError(e, s, v) }
+
 func (e *errorString) FormatError(p Printer) (next error) {
 	p.Print(e.s)
 	e.frame.Format(p)

errors_test.go

@@ -6,6 +6,7 @@ package xerrors_test
 
 import (
 	"fmt"
+	"regexp"
 	"testing"
 
 	"golang.org/x/xerrors"
@@ -34,6 +35,18 @@ func TestErrorMethod(t *testing.T) {
 	}
 }
 
+func TestNewDetail(t *testing.T) {
+	got := fmt.Sprintf("%+v", xerrors.New("error"))
+	want := `(?s)error:.+errors_test.go:\d+`
+	ok, err := regexp.MatchString(want, got)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !ok {
+		t.Errorf(`fmt.Sprintf("%%+v", New("error")) = %q, want %q"`, got, want)
+	}
+}
+
 func ExampleNew() {
 	err := xerrors.New("emit macho dwarf: elf header corrupted")
 	if err != nil {