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>

1 files changed, 3 insertions, 26 deletions

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
 	}