```(gdb) print raw_slice2
$3 = &mut [u8] [116, 101, 115, ...]
(gdb) print raw_slice2.
data_ptr  length    
(gdb) print raw_slice2.length 
Attempt to extract a component of a value that is not a structure.
```

How do you print the length of a slice in gdb when debugging a rust program?