lib/mmap: convert to using unsafe.Slice to avoid deprecated reflect.SliceHeader

lib/mmap/mmap_windows.go

@@ -7,7 +7,6 @@ package mmap
 
 import (
 	"fmt"
-	"reflect"
 	"unsafe"
 
 	"golang.org/x/sys/windows"
@@ -21,21 +20,10 @@ func Alloc(size int) ([]byte, error) {
 	if err != nil {
 		return nil, fmt.Errorf("mmap: failed to allocate memory for buffer: %w", err)
 	}
-	// SliceHeader is deprecated...
-	var mem []byte
-	sh := (*reflect.SliceHeader)(unsafe.Pointer(&mem)) // nolint:staticcheck
-	sh.Data = p
-	sh.Len = size
-	sh.Cap = size
-	return mem, nil
-	// ...However the correct code gives a go vet warning
-	// "possible misuse of unsafe.Pointer"
-	//
-	// Maybe there is a different way of writing this, but none of
-	// the allowed uses of unsafe.Pointer seemed to cover it other
-	// than using SliceHeader (use 6 of unsafe.Pointer).
-	//
-	// return unsafe.Slice((*byte)(unsafe.Pointer(p)), size), nil
+
+	pp := unsafe.Pointer(&p)
+	up := *(*unsafe.Pointer)(pp)
+	return unsafe.Slice((*byte)(up), size), nil
 }
 
 // Free frees buffers allocated by Alloc.  Note it should be passed