Rust Error Output

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This is a reference to help answer this question: “what will my Rust program print if something goes wrong?”

The examples were compiled with Rust 1.76.0. All output in these examples goes to stderr, nothing goes to stdout.

Setup code for thiserror

#[derive(Debug, thiserror::Error)]
enum CustomError {
    #[error("bad thing: {0}")]
    BadThing(#[from] std::io::Error),
}

fn make_error() -> Result<(), CustomError> {
    Ok(std::fs::remove_file("/this/file/does/not/exist")?)
}

Debug

fn main() {
    eprintln!("{:?}", make_error().unwrap_err())
}

BadThing(Os { code: 2, kind: NotFound, message: "No such file or directory" })

Display

fn main() {
    eprintln!("{}", make_error().unwrap_err())
}

bad thing: No such file or directory (os error 2)

Alternate Display

fn main() {
    eprintln!("{:#}", make_error().unwrap_err())
}

bad thing: No such file or directory (os error 2)

Unwrap

fn main() {
    make_error().unwrap();
}

thread 'main' panicked at src/bin/thiserror_unwrap.rs:12:18:
called `Result::unwrap()` on an `Err` value: BadThing(Os { code: 2, kind: NotFound, message: "No such file or directory" })
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Expect

fn main() {
    make_error().expect("oh no");
}

thread 'main' panicked at src/bin/thiserror_expect.rs:12:18:
oh no: BadThing(Os { code: 2, kind: NotFound, message: "No such file or directory" })
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Return

fn main() -> Result<(), CustomError> {
    make_error()
}

Error: BadThing(Os { code: 2, kind: NotFound, message: "No such file or directory" })