2 files changed, 137 insertions, 0 deletions
diff --git a/src/internal/unsafeheader/unsafeheader.go b/src/internal/unsafeheader/unsafeheader.go
new file mode 100644
index 0000000..6d092c6
--- /dev/null
+++ b/src/internal/unsafeheader/unsafeheader.go
@@ -0,0 +1,37 @@
+// Copyright 2020 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 unsafeheader contains header declarations for the Go runtime's slice
+// and string implementations.
+//
+// This package allows packages that cannot import "reflect" to use types that
+// are tested to be equivalent to reflect.SliceHeader and reflect.StringHeader.
+package unsafeheader
+
+import (
+	"unsafe"
+)
+
+// Slice is the runtime representation of a slice.
+// It cannot be used safely or portably and its representation may
+// change in a later release.
+//
+// Unlike reflect.SliceHeader, its Data field is sufficient to guarantee the
+// data it references will not be garbage collected.
+type Slice struct {
+	Data unsafe.Pointer
+	Len  int
+	Cap  int
+}
+
+// String is the runtime representation of a string.
+// It cannot be used safely or portably and its representation may
+// change in a later release.
+//
+// Unlike reflect.StringHeader, its Data field is sufficient to guarantee the
+// data it references will not be garbage collected.
+type String struct {
+	Data unsafe.Pointer
+	Len  int
+}
diff --git a/src/internal/unsafeheader/unsafeheader_test.go b/src/internal/unsafeheader/unsafeheader_test.go
new file mode 100644
index 0000000..6fb7cca
--- /dev/null
+++ b/src/internal/unsafeheader/unsafeheader_test.go
@@ -0,0 +1,100 @@
+// Copyright 2020 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 unsafeheader_test
+
+import (
+	"bytes"
+	"internal/unsafeheader"
+	"reflect"
+	"testing"
+	"unsafe"
+)
+
+// TestTypeMatchesReflectType ensures that the name and layout of the
+// unsafeheader types matches the corresponding Header types in the reflect
+// package.
+func TestTypeMatchesReflectType(t *testing.T) {
+	t.Run("Slice", func(t *testing.T) {
+		testHeaderMatchesReflect(t, unsafeheader.Slice{}, reflect.SliceHeader{})
+	})
+
+	t.Run("String", func(t *testing.T) {
+		testHeaderMatchesReflect(t, unsafeheader.String{}, reflect.StringHeader{})
+	})
+}
+
+func testHeaderMatchesReflect(t *testing.T, header, reflectHeader interface{}) {
+	h := reflect.TypeOf(header)
+	rh := reflect.TypeOf(reflectHeader)
+
+	for i := 0; i < h.NumField(); i++ {
+		f := h.Field(i)
+		rf, ok := rh.FieldByName(f.Name)
+		if !ok {
+			t.Errorf("Field %d of %v is named %s, but no such field exists in %v", i, h, f.Name, rh)
+			continue
+		}
+		if !typeCompatible(f.Type, rf.Type) {
+			t.Errorf("%v.%s has type %v, but %v.%s has type %v", h, f.Name, f.Type, rh, rf.Name, rf.Type)
+		}
+		if f.Offset != rf.Offset {
+			t.Errorf("%v.%s has offset %d, but %v.%s has offset %d", h, f.Name, f.Offset, rh, rf.Name, rf.Offset)
+		}
+	}
+
+	if h.NumField() != rh.NumField() {
+		t.Errorf("%v has %d fields, but %v has %d", h, h.NumField(), rh, rh.NumField())
+	}
+	if h.Align() != rh.Align() {
+		t.Errorf("%v has alignment %d, but %v has alignment %d", h, h.Align(), rh, rh.Align())
+	}
+}
+
+var (
+	unsafePointerType = reflect.TypeOf(unsafe.Pointer(nil))
+	uintptrType       = reflect.TypeOf(uintptr(0))
+)
+
+func typeCompatible(t, rt reflect.Type) bool {
+	return t == rt || (t == unsafePointerType && rt == uintptrType)
+}
+
+// TestWriteThroughHeader ensures that the headers in the unsafeheader package
+// can successfully mutate variables of the corresponding built-in types.
+//
+// This test is expected to fail under -race (which implicitly enables
+// -d=checkptr) if the runtime views the header types as incompatible with the
+// underlying built-in types.
+func TestWriteThroughHeader(t *testing.T) {
+	t.Run("Slice", func(t *testing.T) {
+		s := []byte("Hello, checkptr!")[:5]
+
+		var alias []byte
+		hdr := (*unsafeheader.Slice)(unsafe.Pointer(&alias))
+		hdr.Data = unsafe.Pointer(&s[0])
+		hdr.Cap = cap(s)
+		hdr.Len = len(s)
+
+		if !bytes.Equal(alias, s) {
+			t.Errorf("alias of %T(%q) constructed via Slice = %T(%q)", s, s, alias, alias)
+		}
+		if cap(alias) != cap(s) {
+			t.Errorf("alias of %T with cap %d has cap %d", s, cap(s), cap(alias))
+		}
+	})
+
+	t.Run("String", func(t *testing.T) {
+		s := "Hello, checkptr!"
+
+		var alias string
+		hdr := (*unsafeheader.String)(unsafe.Pointer(&alias))
+		hdr.Data = (*unsafeheader.String)(unsafe.Pointer(&s)).Data
+		hdr.Len = len(s)
+
+		if alias != s {
+			t.Errorf("alias of %q constructed via String = %q", s, alias)
+		}
+	})
+}