hello.rs

use std::io::BufferedReader;
use std::io;

fn main() {
    let mut reader = BufferedReader::new(io::stdin());

    println!("What is your name?");
    let age_input: ~str = reader.read_line().unwrap();
    let age_trimmed: &str = age_input.trim();

    println!("Hello, {}!", age_trimmed);
}

hello_error.rs

use std::io::BufferedReader;
use std::io;

fn main() {
    let mut reader = BufferedReader::new(io::stdin());

    println!("What is your name?");
    let age_input: &str = reader.read_line().unwrap().trim();

    println!("Hello, {}!", age_input);
}

let age_input = reader.read_line().unwrap().trim is roughly equivalent to:

let age_input = {
    let tmp1 = reader.read_line();
    let tmp2 = tmp1.unwrap();

    tmp2.trim()
}

• trim's signature is trim(&self) -> &str. Calling tmp2.trim() thus borrows tmp2.
• the return value of trim is a reference to a substring of tmp2, which still refers the original tmp2.
• at the end of { ... } block, the tmp2 - the original input string - gets destroyed
• so, now that we don't have tmp2, the reference to substring to it is no longer valid and rust compiler detects this