Use go 1.20 facilities for unsafe string <-> bytes

tools/utils/unsafeutil.go

@@ -1,4 +1,5 @@
 // License: GPLv3 Copyright: 2022, Kovid Goyal, <kovid at kovidgoyal.net>
+//go:build !go1.20
 
 package utils
 
@@ -6,8 +7,6 @@ import (
 	"unsafe"
 )
 
-// TODO: Convert these to use the safer stdlib unsafe functions added in Go 1.20
-
 // stringHeader is the runtime representation of a string.
 // It should be identical to reflect.StringHeader
 type stringHeader struct {

tools/utils/unsafeutil_modern.go

@@ -0,0 +1,26 @@
+// License: GPLv3 Copyright: 2023, Kovid Goyal, <kovid at kovidgoyal.net>
+//go:build go1.20
+
+package utils
+
+import (
+	"fmt"
+	"unsafe"
+)
+
+var _ = fmt.Print
+
+// Unsafely converts s into a byte slice.
+// If you modify b, then s will also be modified. This violates the
+// property that strings are immutable.
+func UnsafeStringToBytes(s string) (b []byte) {
+	return unsafe.Slice(unsafe.StringData(s), len(s))
+}
+
+// Unsafely converts b into a string.
+// If you modify b, then s will also be modified. This violates the
+// property that strings are immutable.
+func UnsafeBytesToString(b []byte) (s string) {
+	unsafe.String(unsafe.SliceData(b), len(b))
+	return s
+}