Remove purego fallbacks (#325)

Having go-cmp panic when using the purego build tag makes it hard to use
go-cmp for testing packages that themselves have purego fallbacks.

Since go-cmp can't implement its functionality without unsafe (the
"fallback" panics) and since environments that prohibit unsafe are much
less common these days anyway, simply remove purego code entirely.

Fixes #283.

cmp/export_unsafe.go → cmp/export.go

@@ -2,18 +2,13 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !purego
-// +build !purego
-
 package cmp
 
 import (
 	"reflect"
 	"unsafe"
 )
 
-const supportExporters = true
-
 // retrieveUnexportedField uses unsafe to forcibly retrieve any field from
 // a struct such that the value has read-write permissions.
 //

cmp/internal/value/pointer_unsafe.go → cmp/internal/value/pointer.go

@@ -2,9 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !purego
-// +build !purego
-
 package value
 
 import (

cmp/options.go

@@ -403,9 +403,6 @@ func (cm comparer) String() string {
 // In other cases, the cmpopts.IgnoreUnexported option can be used to ignore
 // all unexported fields on specified struct types.
 func Exporter(f func(reflect.Type) bool) Option {
-	if !supportExporters {
-		panic("Exporter is not supported on purego builds")
-	}
 	return exporter(f)
 }
 

cmp/report_reflect.go

@@ -199,7 +199,7 @@ func (opts formatOptions) FormatValue(v reflect.Value, parentKind reflect.Kind,
 				break
 			}
 			sf := t.Field(i)
-			if supportExporters && !isExported(sf.Name) {
+			if !isExported(sf.Name) {
 				vv = retrieveUnexportedField(v, sf, true)
 			}
 			s := opts.WithTypeMode(autoType).FormatValue(vv, t.Kind(), ptrs)