text: // Copyright (c) 2015-2016 Dave Collins <dave@davec.name>
  //
  // Permission to use, copy, modify, and distribute this software for any
  // purpose with or without fee is hereby granted, provided that the above
  // copyright notice and this permission notice appear in all copies.
  //
  // THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
  // WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
  // MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
  // ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
  // WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
  // ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
  // OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  
  // NOTE: Due to the following build constraints, this file will only be compiled
  // when the code is not running on Google App Engine, compiled by GopherJS, and
  // "-tags safe" is not added to the go build command line.  The "disableunsafe"
  // tag is deprecated and thus should not be used.
  // Go versions prior to 1.4 are disabled because they use a different layout
  // for interfaces which make the implementation of unsafeReflectValue more complex.
  // +build !js,!appengine,!safe,!disableunsafe,go1.4
  
  package spew
  
  import (
  	"reflect"
  	"unsafe"
  )
  
  const (
  	// UnsafeDisabled is a build-time constant which specifies whether or
  	// not access to the unsafe package is available.
  	UnsafeDisabled = false
  
  	// ptrSize is the size of a pointer on the current arch.
  	ptrSize = unsafe.Sizeof((*byte)(nil))
  )
  
  type flag uintptr
  
  var (
  	// flagRO indicates whether the value field of a reflect.Value
  	// is read-only.
  	flagRO flag
  
  	// flagAddr indicates whether the address of the reflect.Value's
  	// value may be taken.
  	flagAddr flag
  )
  
  // flagKindMask holds the bits that make up the kind
  // part of the flags field. In all the supported versions,
  // it is in the lower 5 bits.
  const flagKindMask = flag(0x1f)
  
  // Different versions of Go have used different
  // bit layouts for the flags type. This table
  // records the known combinations.
  var okFlags = []struct {
  	ro, addr flag
  }{{
  	// From Go 1.4 to 1.5
  	ro:   1 << 5,
  	addr: 1 << 7,
  }, {
  	// Up to Go tip.
  	ro:   1<<5 | 1<<6,
  	addr: 1 << 8,
  }}
  
  var flagValOffset = func() uintptr {
  	field, ok := reflect.TypeOf(reflect.Value{}).FieldByName("flag")
  	if !ok {
  		panic("reflect.Value has no flag field")
  	}
  	return field.Offset
  }()
  
  // flagField returns a pointer to the flag field of a reflect.Value.
  func flagField(v *reflect.Value) *flag {
  	return (*flag)(unsafe.Pointer(uintptr(unsafe.Pointer(v)) + flagValOffset))
  }
  
  // unsafeReflectValue converts the passed reflect.Value into a one that bypasses
  // the typical safety restrictions preventing access to unaddressable and
  // unexported data.  It works by digging the raw pointer to the underlying
  // value out of the protected value and generating a new unprotected (unsafe)
  // reflect.Value to it.
  //
  // This allows us to check for implementations of the Stringer and error
  // interfaces to be used for pretty printing ordinarily unaddressable and
  // inaccessible values such as unexported struct fields.
  func unsafeReflectValue(v reflect.Value) reflect.Value {
  	if !v.IsValid() || (v.CanInterface() && v.CanAddr()) {
  		return v
  	}
  	flagFieldPtr := flagField(&v)
  	*flagFieldPtr &^= flagRO
  	*flagFieldPtr |= flagAddr
  	return v
  }
  
  // Sanity checks against future reflect package changes
  // to the type or semantics of the Value.flag field.
  func init() {
  	field, ok := reflect.TypeOf(reflect.Value{}).FieldByName("flag")
  	if !ok {
  		panic("reflect.Value has no flag field")
  	}
  	if field.Type.Kind() != reflect.TypeOf(flag(0)).Kind() {
  		panic("reflect.Value flag field has changed kind")
  	}
  	type t0 int
  	var t struct {
  		A t0
  		// t0 will have flagEmbedRO set.
  		t0
  		// a will have flagStickyRO set
  		a t0
  	}
  	vA := reflect.ValueOf(t).FieldByName("A")
  	va := reflect.ValueOf(t).FieldByName("a")
  	vt0 := reflect.ValueOf(t).FieldByName("t0")
  
  	// Infer flagRO from the difference between the flags
  	// for the (otherwise identical) fields in t.
  	flagPublic := *flagField(&vA)
  	flagWithRO := *flagField(&va) | *flagField(&vt0)
  	flagRO = flagPublic ^ flagWithRO
  
  	// Infer flagAddr from the difference between a value
  	// taken from a pointer and not.
  	vPtrA := reflect.ValueOf(&t).Elem().FieldByName("A")
  	flagNoPtr := *flagField(&vA)
  	flagPtr := *flagField(&vPtrA)
  	flagAddr = flagNoPtr ^ flagPtr
  
  	// Check that the inferred flags tally with one of the known versions.
  	for _, f := range okFlags {
  		if flagRO == f.ro && flagAddr == f.addr {
  			return
  		}
  	}
  	panic("reflect.Value read-only flag has changed semantics")
  }
  