How does error propagation work with the ? operator?

Error propagation with ?: • Shorthand for match on Result • Returns early on Err • Continues on Ok • Only works in functions returning Result or Option • Automatic type conversion with From trait Example: use std::fs::File; use std::io::Read; fn read_file() -> Result { let mut file = File::open("file.txt")?; // returns early if Err let mut contents = String::new(); file.read_to_string(&mut contents)?; Ok(contents) } // Equivalent to: // match File::open("file.txt") { // Ok(file) => file, // Err(e) => return Err(e), // }