"struct field is never used", but needed by RAII

Question

I'm trying to implement a memory mapped file by combining os::MemoryMap and fs::File in some kind of RAII style. Consider the following example:

#![feature(fs, os, io, path, std_misc, core)]

use std::{io, os, mem, raw};
use std::io::{Seek};
use std::fs::{File};
use std::path::{Path};
use std::os::{MemoryMap};
use std::borrow::{Cow};
use std::error::{FromError};
use std::os::unix::{AsRawFd};

struct Mmapped {
    file: File,
    map: MemoryMap,
    map_len: usize,
}

#[derive(Debug)]
enum Error {
    IoError(io::Error),
    MmapError(os::MapError),
}

impl FromError<io::Error> for Error { 
    fn from_error(err: io::Error) -> Error { Error::IoError(err) }
}

impl FromError<os::MapError> for Error { 
    fn from_error(err: os::MapError) -> Error { Error::MmapError(err) }
}

impl Mmapped {
    fn new(filename: &str) -> Result<Mmapped, Error> {
        let mut file = try!(File::open(Path::new(filename)));
        let map_len = try!(file.seek(io::SeekFrom::End(0))) as usize;
        let map = try!(MemoryMap::new(map_len, &[os::MapOption::MapReadable, os::MapOption::MapFd(file.as_raw_fd())]));
        Ok(Mmapped { file: file, map: map, map_len: map_len })
    }

    unsafe fn as_string<'a>(&self) -> Cow<'a, String, str> {
        String::from_utf8_lossy(mem::transmute(raw::Slice { data: self.map.data() as *const u8, 
                                                            len: self.map_len }))
    }
}

fn main() {
    let m = Mmapped::new("test.txt").unwrap();
    println!("File contents: {:?}", unsafe { m.as_string() });
}

playpen

It works, but compiler treats the file field in Mmapped object as dead code:

<anon>:13:5: 13:15 warning: struct field is never used: `file`, #[warn(dead_code)] on by default
<anon>:13     file: File,
              ^~~~~~~~~~

Could I be sure that it will not optimize it out, and file will be closed in new method? And is there any standard way to mark my field "not dead" code?

Answer 1

You can annotate the specific field to ignore the dead-code warning:

struct Mmapped {
    #[allow(dead_code)]
    file: File,
    map: MemoryMap,
    map_len: usize,
}

I don't believe that the optimizer will completely drop the field from the struct, and a quick look at the LLVM IR for a struct that has an unused String field shows that there is still space for that field and even calls the drop glue for that String.