Identify when buffer is being reused in `read_line` without clearing

[estebank]

Identify when a buffer is being reused without clearing in at least read_line, ideally in a general manner (might be impossible to do so), like in the following case:

use std::io;

fn main() {
    let mut guess = String::new();

    loop {
        println!("Enter a number");

        io::stdin().read_line(&mut guess).unwrap();

        println!("guess: {}", guess);
        let guess: u32 = match guess.trim().parse() {
            Ok(num) => num,
            Err(err) => {
                println!("{:?}", err);
                continue
            },
        };
}

CC rust-lang/rust#56681 (comment)