How to overwrite a file in Rust

Recently, I took an interest in the Rust programming language due to its performance and safety.
I chose Rust because of its RAII-based memory model, high-level abstractions which do not compromise perfomance, and the ability to disable its standard library and develop for bare metal. As a bonus, it comes with incredible static checks, dependency management tools, as well as unit test and benchmark frameworks.
The language itself is relatively new and not as popular as other system programming languages, though major enterprises have used it in key products, such as Facebook Diem, AWS Firecracker and Dropbox.

While I was playing around with its capabilities, I attempted to overwrite an existing text file. Naturally, I wrote the following piece of code to do the trick:

use std::io::Write;

fn main() -> std::io::Result<()> {
    let mut f = std::fs::OpenOptions::new().write(true).open("./file")?;
    f.write_all(b"XXX")?;
    f.flush()?;
    Ok(())
}

The original content of the file was AAAAAAAA and after execution of the program, it unexpectedly became XXXAAAAA rather than just XXX. I was quite surprised because in most of the languages that I am familiar with, when a file is opened with the write ("w") flag, the existing content would be automatically truncated and overwritten. Apparently, to achieve the same in Rust, truncate(true) needs to be added:

std::fs::OpenOptions::new().write(true).truncate(true).open("./file")?;

I like the granularity of file settings provided by Rust via low-level flags, such as O_WRONLY and O_TRUNC, but beginners need to be aware of them.

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