Update the primitive types explanation in the Rust coding standards

            }

        }

3. Pass only integer types and bytes over the boundary

3. Pass only C-compatible primitive types and bytes over the boundary

   The only non-integer type which may cross the FFI boundary is

   Rust's C-compatible primitive types are integers and floats.

   These types are declared in the [libc crate](https://doc.rust-lang.org/libc/x86_64-unknown-linux-gnu/libc/index.html#types).

   Most Rust objects have different [representations](https://doc.rust-lang.org/libc/x86_64-unknown-linux-gnu/libc/index.html#types)

   in C and Rust, so they can't be passed using FFI.

   Tor currently uses the following Rust primitive types from libc for FFI:

   * defined-size integers: `uint32_t`

   * native-sized integers: `c_int`

   * native-sized floats: `c_double`

   * native-sized raw pointers: `* c_void`, `* c_char`, `** c_char`

   TODO: C smartlist to Stringlist conversion using FFI

   The only non-primitive type which may cross the FFI boundary is

   bytes, e.g. `&[u8]`.  This SHOULD be done on the Rust side by

   passing a pointer (`*mut libc::c_char`) and a length

   (`libc::size_t`).

   passing a pointer (`*mut libc::c_char`). The length can be passed

   explicitly (`libc::size_t`), or the string can be NUL-byte terminated

   C string.

   One might be tempted to do this via doing

   `CString::new("blah").unwrap().into_raw()`. This has several problems: