just another evidence that @shyouhei is an idiot [ci skip]

(gdb) ptype/o struct RString
/* offset    |  size */  type = struct RString {
/*    0      |    16 */    struct RBasic {
/*    0      |     8 */        VALUE flags;
/*    8      |     8 */        const VALUE klass;

                               /* total size (bytes):   16 */
                           } basic;
/*   16      |    24 */    union {
/*                24 */        struct {
/*   16      |     8 */            long len;
/*   24      |     8 */            char *ptr;
/*   32      |     8 */            union {
/*                 8 */                long capa;
/*                 8 */                VALUE shared;

                                       /* total size (bytes):    8 */
                                   } aux;

                                   /* total size (bytes):   24 */
                               } heap;
/*                24 */        struct {
/*   16      |    24 */            char ary[24];

                                   /* total size (bytes):   24 */
                               } embed;
/* XXX  8-byte padding  */

                               /* total size (bytes):   24 */
                           } as;

                           /* total size (bytes):   40 */
                         }
(gdb)

include/ruby/internal/core/rstring.h

@@ -280,7 +280,7 @@ struct RString {
             /**
             * Embedded contents.  When a string is short enough, it uses this area
             * to store the contents themselves.   This was impractical in the 20th
-            * century, but these days 64 bit  machines can typically hold 48 bytes
+            * century, but these days 64 bit  machines can typically hold 24 bytes
             * here.   Could be  sufficiently large.   In this  case the  length is
             * encoded into the flags.
             */