runtime: simplify code using unsafe.{Slice,String}

Updates #54854

Change-Id: Ie18665e93e477b6f220acf4c6c070b2af4343064
Reviewed-on: https://go-review.googlesource.com/c/go/+/428157
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

src/runtime/debuglog.go

@@ -657,11 +657,7 @@ func (r *debugLogReader) printVal() bool {
 	case debugLogConstString:
 		len, ptr := int(r.uvarint()), uintptr(r.uvarint())
 		ptr += firstmoduledata.etext
-		str := stringStruct{
-			str: unsafe.Pointer(ptr),
-			len: len,
-		}
-		s := *(*string)(unsafe.Pointer(&str))
+		s := unsafe.String((*byte)(unsafe.Pointer(ptr)), len)
 		print(s)
 
 	case debugLogStringOverflow:

src/runtime/memmove_linux_amd64_test.go

@@ -6,7 +6,6 @@ package runtime_test
 
 import (
 	"os"
-	"reflect"
 	"syscall"
 	"testing"
 	"unsafe"
@@ -45,11 +44,7 @@ func TestMemmoveOverflow(t *testing.T) {
 		defer syscall.Syscall(syscall.SYS_MUNMAP, base+off, 65536, 0)
 	}
 
-	var s []byte
-	sp := (*reflect.SliceHeader)(unsafe.Pointer(&s))
-	sp.Data = base
-	sp.Len, sp.Cap = 3<<30, 3<<30
-
+	s := unsafe.Slice((*byte)(unsafe.Pointer(base)), 3<<30)
 	n := copy(s[1:], s)
 	if n != 3<<30-1 {
 		t.Fatalf("copied %d bytes, expected %d", n, 3<<30-1)

src/runtime/proc.go

@@ -620,7 +620,7 @@ func cpuinit() {
 
 		for i := int32(0); i < n; i++ {
 			p := argv_index(argv, argc+1+i)
-			s := *(*string)(unsafe.Pointer(&stringStruct{unsafe.Pointer(p), findnull(p)}))
+			s := unsafe.String(p, findnull(p))
 
 			if hasPrefix(s, prefix) {
 				env = gostring(p)[len(prefix):]