我正在使用stdlib的raw vec的副本来构建我自己的数据结构。我希望将文件的一个块直接读取到我的数据结构中（没有额外的副本）。RawVec有一个 * const u8作为底层存储，我希望将文件直接读取到它中。

// Goal:
// Takes a file, a pointer to read bytes into, a number of free bytes @ that pointer
// and returns the number of bytes read
fn read_into_ptr(file: &mut File, ptr: *mut u8, free_space: usize) -> usize {
    // read up to free_space bytes into ptr
    todo!()
}

// What I have now requires an extra copy. First I read into my read buffer 
// Then move copy into my destination where I actually want to data.
// How can I remove this extra copy?
fn read_into_ptr(file: &mut File, ptr: *mut u8, read_buf: &mut[u8; 4096]) -> usize {
    let num_bytes = file.read(read_buf).unwrap();
    unsafe { 
      ptr::copy_nonoverlapping(...)
    }
    num_bytes
}
``