rust - "cannot move out of variable because it is borrowed" when rotating variables

Question

I am writing a program that writes to a file and rotates the file it's writing to every now and then. When I check to rotate the file, I can't seem to change the file since it is borrowed by my struct. Even if I drop the instance of the struct, I can't seem to regain ownership of the file to rename it. Here is my example:

use std::fs::File;
use std::io::{Write};
use std::mem::{drop};

pub struct FileStruct<W: Write> {
    pub writer: Option<W>,
}

impl <W: Write> FileStruct<W> {
    pub fn new(writer: W) -> FileStruct<W> {
        FileStruct {
            writer: Some(writer),
        }
    }
}

fn main() {
    let mut file = File::create("tmp.txt").unwrap();
    let mut tmp = FileStruct::new(&mut file);
    loop {
        if true { //will be time based if check
            drop(tmp);
            drop(file);
            file = File::create("tmp2.txt").unwrap();
            tmp = FileStruct::new(&mut file);
        }
        // write to file
    }
}

I know I can get this to work by moving the file creation into the new function call of FileStruct instead of having an intermediate variable, file, but I would like to know why this method where I forcibly drop all the variables where all the variables references should be returned doesn't work.

Answer 1

As the std::mem::drop documentation says,

While this does call the argument's implementation of Drop, it will not release any borrows, as borrows are based on lexical scope.

So even if you call drop, file will remain borrowed nonetheless.