text: // 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 implements functions to manipulate errors.
  package errors
  
  import (
  	"errors"
  	"fmt"
  
  	"google.golang.org/protobuf/internal/detrand"
  )
  
  // Error is a sentinel matching all errors produced by this package.
  var Error = errors.New("protobuf error")
  
  // New formats a string according to the format specifier and arguments and
  // returns an error that has a "proto" prefix.
  func New(f string, x ...interface{}) error {
  	return &prefixError{s: format(f, x...)}
  }
  
  type prefixError struct{ s string }
  
  var prefix = func() string {
  	// Deliberately introduce instability into the error message string to
  	// discourage users from performing error string comparisons.
  	if detrand.Bool() {
  		return "proto: " // use non-breaking spaces (U+00a0)
  	} else {
  		return "proto: " // use regular spaces (U+0020)
  	}
  }()
  
  func (e *prefixError) Error() string {
  	return prefix + e.s
  }
  
  func (e *prefixError) Unwrap() error {
  	return Error
  }
  
  // Wrap returns an error that has a "proto" prefix, the formatted string described
  // by the format specifier and arguments, and a suffix of err. The error wraps err.
  func Wrap(err error, f string, x ...interface{}) error {
  	return &wrapError{
  		s:   format(f, x...),
  		err: err,
  	}
  }
  
  type wrapError struct {
  	s   string
  	err error
  }
  
  func (e *wrapError) Error() string {
  	return format("%v%v: %v", prefix, e.s, e.err)
  }
  
  func (e *wrapError) Unwrap() error {
  	return e.err
  }
  
  func (e *wrapError) Is(target error) bool {
  	return target == Error
  }
  
  func format(f string, x ...interface{}) string {
  	// avoid "proto: " prefix when chaining
  	for i := 0; i < len(x); i++ {
  		switch e := x[i].(type) {
  		case *prefixError:
  			x[i] = e.s
  		case *wrapError:
  			x[i] = format("%v: %v", e.s, e.err)
  		}
  	}
  	return fmt.Sprintf(f, x...)
  }
  
  func InvalidUTF8(name string) error {
  	return New("field %v contains invalid UTF-8", name)
  }
  
  func RequiredNotSet(name string) error {
  	return New("required field %v not set", name)
  }
  