errors, fmt: revert rejected changes for Go 1.13

Reverts the following changes:

  https://go.googlesource.com/go/+/1f90d081391d4f5911960fd28d81d7ea5e554a8f
  https://go.googlesource.com/go/+/8bf18b56a47a98b9dd2fa03beb358312237a8c76
  https://go.googlesource.com/go/+/5402854c3557f87fa2741a52ffc15dfb1ef333cc
  https://go.googlesource.com/go/+/37f84817247d3b8e687a701ccb0d6bc7ffe3cb78
  https://go.googlesource.com/go/+/6be6f114e0d483a233101a67c9644cd72bd3ae7a

Partially reverts the followinng change, removing the errors.Opaque
function and the errors.Wrapper type definition:

  https://go.googlesource.com/go/+/62f5e8156ef56fa61e6af56f4ccc633bde1a9120

Updates documentation referencing the Wrapper type.

Change-Id: Ia622883e39cafb06809853e3fd90b21441124534
Reviewed-on: https://go-review.googlesource.com/c/go/+/176997
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

6 files changed, 5 insertions, 266 deletions

diff --git a/src/errors/errors.go b/src/errors/errors.go
index 51175b13c8..b8a46921be 100644
--- a/src/errors/errors.go
+++ b/src/errors/errors.go
@@ -5,46 +5,16 @@
 // Package errors implements functions to manipulate errors.
 package errors
 
-import (
-	"internal/errinternal"
-	"runtime"
-)
-
 // New returns an error that formats as the given text.
-//
-// The returned error contains a Frame set to the caller's location and
-// implements Formatter to show this information when printed with details.
 func New(text string) error {
-	// Inline call to errors.Callers to improve performance.
-	var s Frame
-	runtime.Callers(2, s.frames[:])
-	return &errorString{text, nil, s}
-}
-
-func init() {
-	errinternal.NewError = func(text string, err error) error {
-		var s Frame
-		runtime.Callers(3, s.frames[:])
-		return &errorString{text, err, s}
-	}
+	return &errorString{text}
 }
 
 // errorString is a trivial implementation of error.
 type errorString struct {
-	s     string
-	err   error
-	frame Frame
+	s string
 }
 
 func (e *errorString) Error() string {
-	if e.err != nil {
-		return e.s + ": " + e.err.Error()
-	}
 	return e.s
 }
-
-func (e *errorString) FormatError(p Printer) (next error) {
-	p.Print(e.s)
-	e.frame.Format(p)
-	return e.err
-}
diff --git a/src/errors/format.go b/src/errors/format.go
deleted file mode 100644
index 12deed3cf7..0000000000
--- a/src/errors/format.go
+++ /dev/null
@@ -1,34 +0,0 @@
-// Copyright 2018 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package errors
-
-// A Formatter formats error messages.
-type Formatter interface {
-	error
-
-	// FormatError prints the receiver's first error and returns the next error in
-	// the error chain, if any.
-	FormatError(p Printer) (next error)
-}
-
-// A Printer formats error messages.
-//
-// The most common implementation of Printer is the one provided by package fmt
-// during Printf. Localization packages such as golang.org/x/text/message
-// typically provide their own implementations.
-type Printer interface {
-	// Print appends args to the message output.
-	Print(args ...interface{})
-
-	// Printf writes a formatted string.
-	Printf(format string, args ...interface{})
-
-	// Detail reports whether error detail is requested.
-	// After the first call to Detail, all text written to the Printer
-	// is formatted as additional detail, or ignored when
-	// detail has not been requested.
-	// If Detail returns false, the caller can avoid printing the detail at all.
-	Detail() bool
-}
diff --git a/src/errors/frame.go b/src/errors/frame.go
deleted file mode 100644
index 487092fa89..0000000000
--- a/src/errors/frame.go
+++ /dev/null
@@ -1,47 +0,0 @@
-// Copyright 2018 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package errors
-
-import (
-	"runtime"
-)
-
-// A Frame contains part of a call stack.
-type Frame struct {
-	frames [1]uintptr
-}
-
-// Caller returns a Frame that describes a frame on the caller's stack.
-// The argument skip is the number of frames to skip over.
-// Caller(0) returns the frame for the caller of Caller.
-func Caller(skip int) Frame {
-	var s Frame
-	runtime.Callers(skip+2, s.frames[:])
-	return s
-}
-
-// location reports the file, line, and function of a frame.
-//
-// The returned function may be "" even if file and line are not.
-func (f Frame) location() (function, file string, line int) {
-	frames := runtime.CallersFrames(f.frames[:])
-	fr, _ := frames.Next()
-	return fr.Function, fr.File, fr.Line
-}
-
-// Format prints the stack as error detail.
-// It should be called from an error's Format implementation,
-// before printing any other error detail.
-func (f Frame) Format(p Printer) {
-	if p.Detail() {
-		function, file, line := f.location()
-		if function != "" {
-			p.Printf("%s\n    ", function)
-		}
-		if file != "" {
-			p.Printf("%s:%d\n", file, line)
-		}
-	}
-}
diff --git a/src/errors/frame_test.go b/src/errors/frame_test.go
deleted file mode 100644
index ba08166966..0000000000
--- a/src/errors/frame_test.go
+++ /dev/null
@@ -1,66 +0,0 @@
-// Copyright 2018 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package errors_test
-
-import (
-	"bytes"
-	"errors"
-	"fmt"
-	"math/big"
-	"regexp"
-	"strings"
-	"testing"
-)
-
-func TestFrame(t *testing.T) {
-
-	// Extra line
-	got := fmt.Sprintf("%+v", errors.New("Test"))
-	got = got[strings.Index(got, "Test"):]
-	const want = "^Test:" +
-		"\n    errors_test.TestFrame" +
-		"\n        .*/errors/frame_test.go:20$"
-	ok, err := regexp.MatchString(want, got)
-	if err != nil {
-		t.Fatal(err)
-	}
-	if !ok {
-		t.Errorf("\n got %v;\nwant %v", got, want)
-	}
-}
-
-type myType struct{}
-
-func (myType) Format(s fmt.State, v rune) {
-	s.Write(bytes.Repeat([]byte("Hi! "), 10))
-}
-
-func BenchmarkNew(b *testing.B) {
-	for i := 0; i < b.N; i++ {
-		_ = errors.New("new error")
-	}
-}
-
-func BenchmarkErrorf(b *testing.B) {
-	err := errors.New("foo")
-	args := func(a ...interface{}) []interface{} { return a }
-	benchCases := []struct {
-		name   string
-		format string
-		args   []interface{}
-	}{
-		{"no_format", "msg: %v", args(err)},
-		{"with_format", "failed %d times: %v", args(5, err)},
-		{"method: mytype", "pi %s %v: %v", args("myfile.go", myType{}, err)},
-		{"method: number", "pi %s %d: %v", args("myfile.go", big.NewInt(5), err)},
-	}
-	for _, bc := range benchCases {
-		b.Run(bc.name, func(b *testing.B) {
-			for i := 0; i < b.N; i++ {
-				_ = fmt.Errorf(bc.format, bc.args...)
-			}
-		})
-	}
-}
diff --git a/src/errors/wrap.go b/src/errors/wrap.go
index b53caa6fc9..62332b1a88 100644
--- a/src/errors/wrap.go
+++ b/src/errors/wrap.go
@@ -8,35 +8,12 @@ import (
 	"internal/reflectlite"
 )
 
-// A Wrapper provides context around another error.
-type Wrapper interface {
-	// Unwrap returns the next error in the error chain.
-	// If there is no next error, Unwrap returns nil.
-	Unwrap() error
-}
-
-// Opaque returns an error with the same error formatting as err
-// but that does not match err and cannot be unwrapped.
-func Opaque(err error) error {
-	return noWrapper{err}
-}
-
-type noWrapper struct {
-	error
-}
-
-func (e noWrapper) FormatError(p Printer) (next error) {
-	if f, ok := e.error.(Formatter); ok {
-		return f.FormatError(p)
-	}
-	p.Print(e.error)
-	return nil
-}
-
 // Unwrap returns the result of calling the Unwrap method on err, if err
 // implements Wrapper. Otherwise, Unwrap returns nil.
 func Unwrap(err error) error {
-	u, ok := err.(Wrapper)
+	u, ok := err.(interface {
+		Unwrap() error
+	})
 	if !ok {
 		return nil
 	}
diff --git a/src/errors/wrap_test.go b/src/errors/wrap_test.go
index f8e907cff7..2055316756 100644
--- a/src/errors/wrap_test.go
+++ b/src/errors/wrap_test.go
@@ -5,7 +5,6 @@
 package errors_test
 
 import (
-	"bytes"
 	"errors"
 	"fmt"
 	"os"
@@ -16,8 +15,6 @@ func TestIs(t *testing.T) {
 	err1 := errors.New("1")
 	erra := wrapped{"wrap 2", err1}
 	errb := wrapped{"wrap 3", erra}
-	erro := errors.Opaque(err1)
-	errco := wrapped{"opaque", erro}
 
 	err3 := errors.New("3")
 
@@ -35,9 +32,6 @@ func TestIs(t *testing.T) {
 		{err1, err1, true},
 		{erra, err1, true},
 		{errb, err1, true},
-		{errco, erro, true},
-		{errco, err1, false},
-		{erro, erro, true},
 		{err1, err3, false},
 		{erra, err3, false},
 		{errb, err3, false},
@@ -45,8 +39,6 @@ func TestIs(t *testing.T) {
 		{poser, err3, true},
 		{poser, erra, false},
 		{poser, errb, false},
-		{poser, erro, false},
-		{poser, errco, false},
 		{errorUncomparable{}, errorUncomparable{}, true},
 		{errorUncomparable{}, &errorUncomparable{}, false},
 		{&errorUncomparable{}, errorUncomparable{}, true},
@@ -108,10 +100,6 @@ func TestAs(t *testing.T) {
 		&errP,
 		true,
 	}, {
-		errors.Opaque(errT),
-		&errT,
-		false,
-	}, {
 		errorT{},
 		&errP,
 		false,
@@ -187,7 +175,6 @@ func TestAsValidation(t *testing.T) {
 func TestUnwrap(t *testing.T) {
 	err1 := errors.New("1")
 	erra := wrapped{"wrap 2", err1}
-	erro := errors.Opaque(err1)
 
 	testCases := []struct {
 		err  error
@@ -198,9 +185,6 @@ func TestUnwrap(t *testing.T) {
 		{err1, nil},
 		{erra, err1},
 		{wrapped{"wrap 3", erra}, erra},
-
-		{erro, nil},
-		{wrapped{"opaque", erro}, erro},
 	}
 	for _, tc := range testCases {
 		if got := errors.Unwrap(tc.err); got != tc.want {
@@ -209,39 +193,6 @@ func TestUnwrap(t *testing.T) {
 	}
 }
 
-func TestOpaque(t *testing.T) {
-	someError := errors.New("some error")
-	testCases := []struct {
-		err  error
-		next error
-	}{
-		{errorT{}, nil},
-		{wrapped{"b", nil}, nil},
-		{wrapped{"c", someError}, someError},
-	}
-	for _, tc := range testCases {
-		t.Run("", func(t *testing.T) {
-			opaque := errors.Opaque(tc.err)
-
-			f, ok := opaque.(errors.Formatter)
-			if !ok {
-				t.Fatal("Opaque error does not implement Formatter")
-			}
-			var p printer
-			next := f.FormatError(&p)
-			if next != tc.next {
-				t.Errorf("next was %v; want %v", next, tc.next)
-			}
-			if got, want := p.buf.String(), tc.err.Error(); got != want {
-				t.Errorf("error was %q; want %q", got, want)
-			}
-			if got := errors.Unwrap(opaque); got != nil {
-				t.Errorf("Unwrap returned non-nil error (%v)", got)
-			}
-		})
-	}
-}
-
 type errorT struct{}
 
 func (errorT) Error() string { return "errorT" }
@@ -255,18 +206,6 @@ func (e wrapped) Error() string { return e.msg }
 
 func (e wrapped) Unwrap() error { return e.err }
 
-func (e wrapped) FormatError(p errors.Printer) error {
-	p.Print(e.msg)
-	return e.err
-}
-
-type printer struct {
-	errors.Printer
-	buf bytes.Buffer
-}
-
-func (p *printer) Print(args ...interface{}) { fmt.Fprint(&p.buf, args...) }
-
 type errorUncomparable struct {
 	f []string
 }