Fixing the dst type, which is expected to be *mut i8:

245 |                 std::ptr::copy_nonoverlapping(self.as_ptr().cast::<i8>(), dst, self.len());
    |                 -----------------------------                             ^^^ expected `*mut i8`, found `*mut u8`
    |                 |
    |                 arguments to this function are incorrect

gentl/src/ffi/mod.rs

@@ -242,7 +242,7 @@ impl CopyTo for &str {
                 if *dst_size < string_len {
                     return Err(GenTlError::BufferTooSmall);
                 }
-                std::ptr::copy_nonoverlapping(self.as_ptr().cast::<i8>(), dst, self.len());
+                std::ptr::copy_nonoverlapping(self.as_ptr().cast::<i8>(), dst.cast::<i8>(), self.len());
                 dst.add(self.len()).write(0); // Null terminated.
             }
         }