strings: simplify code using unsafe.StringData

Updates #54854

Change-Id: I93396dc92bd2decba895f2d059e1aeffcd22312c
Reviewed-on: https://go-review.googlesource.com/c/go/+/428158
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

src/strings/builder.go

@@ -45,7 +45,7 @@ func (b *Builder) copyCheck() {
 
 // String returns the accumulated string.
 func (b *Builder) String() string {
-	return *(*string)(unsafe.Pointer(&b.buf))
+	return unsafe.String(unsafe.SliceData(b.buf), len(b.buf))
 }
 
 // Len returns the number of accumulated bytes; b.Len() == len(b.String()).

src/strings/clone.go

@@ -24,5 +24,5 @@ func Clone(s string) string {
 	}
 	b := make([]byte, len(s))
 	copy(b, s)
-	return *(*string)(unsafe.Pointer(&b))
+	return unsafe.String(&b[0], len(b))
 }

src/strings/clone_test.go

@@ -5,7 +5,6 @@
 package strings_test
 
 import (
-	"reflect"
 	"strings"
 	"testing"
 	"unsafe"
@@ -27,15 +26,12 @@ func TestClone(t *testing.T) {
 			t.Errorf("Clone(%q) = %q; want %q", input, clone, input)
 		}
 
-		inputHeader := (*reflect.StringHeader)(unsafe.Pointer(&input))
-		cloneHeader := (*reflect.StringHeader)(unsafe.Pointer(&clone))
-		if len(input) != 0 && cloneHeader.Data == inputHeader.Data {
+		if len(input) != 0 && unsafe.StringData(clone) == unsafe.StringData(input) {
 			t.Errorf("Clone(%q) return value should not reference inputs backing memory.", input)
 		}
 
-		emptyHeader := (*reflect.StringHeader)(unsafe.Pointer(&emptyString))
-		if len(input) == 0 && cloneHeader.Data != emptyHeader.Data {
-			t.Errorf("Clone(%#v) return value should be equal to empty string.", inputHeader)
+		if len(input) == 0 && unsafe.StringData(clone) != unsafe.StringData(emptyString) {
+			t.Errorf("Clone(%#v) return value should be equal to empty string.", unsafe.StringData(input))
 		}
 	}
 }

src/strings/compare_test.go

@@ -57,7 +57,7 @@ func TestCompareStrings(t *testing.T) {
 	// unsafeString converts a []byte to a string with no allocation.
 	// The caller must not modify b while the result string is in use.
 	unsafeString := func(b []byte) string {
-		return *(*string)(unsafe.Pointer(&b))
+		return unsafe.String(unsafe.SliceData(b), len(b))
 	}
 
 	lengths := make([]int, 0) // lengths to test in ascending order

src/strings/strings_test.go

@@ -660,8 +660,7 @@ func TestMap(t *testing.T) {
 	}
 	orig := "Input string that we expect not to be copied."
 	m = Map(identity, orig)
-	if (*reflect.StringHeader)(unsafe.Pointer(&orig)).Data !=
-		(*reflect.StringHeader)(unsafe.Pointer(&m)).Data {
+	if unsafe.StringData(orig) != unsafe.StringData(m) {
 		t.Error("unexpected copy during identity map")
 	}