Resolve unused_must_use warning inside generated code
Resolve unused_must_use warning inside generated code

    warning: unused return value of `Box::<T>::from_raw` that must be used
       --> tests/ffi/lib.rs:337:22
        |
    337 |     impl Box<Shared> {}
        |                      ^^
        |
        = note: `#[warn(unused_must_use)]` on by default
        = note: call `drop(from_raw(ptr))` if you intend to drop the `Box`

    warning: unused return value of `Box::<T>::from_raw` that must be used
      --> tests/ffi/lib.rs:72:27
       |
    72 |         second: Box<Second>,
       |                           ^
       |
       = note: call `drop(from_raw(ptr))` if you intend to drop the `Box`

    warning: unused return value of `Box::<T>::from_raw` that must be used
      --> tests/ffi/lib.rs:99:35
       |
    99 |         fn c_return_box() -> Box<R>;
       |                                   ^
       |
       = note: call `drop(from_raw(ptr))` if you intend to drop the `Box`

dtolnay/cxx,dtolnay/cxx,dtolnay/cxx,dtolnay/cxx