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 anyhow with context 2x

use anyhow::Context;

fn make_error() -> Result<(), anyhow::Error> {
    Ok(std::fs::remove_file("/this/file/does/not/exist")
        .context("oh no")
        .context("second context")?)
}

Debug

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

second context

Caused by:
    0: oh no
    1: No such file or directory (os error 2)

Display

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

second context

Alternate Display

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

second context: oh no: No such file or directory (os error 2)

Unwrap

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

thread 'main' panicked at src/bin/anyhow_context_2x_unwrap.rs:10:18:
called `Result::unwrap()` on an `Err` value: second context

Caused by:
    0: oh no
    1: No such file or directory (os error 2)
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/anyhow_context_2x_expect.rs:10:18:
oh no: second context

Caused by:
    0: oh no
    1: No such file or directory (os error 2)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Return

fn main() -> Result<(), anyhow::Error> {
    make_error()
}

Error: second context

Caused by:
    0: oh no
    1: No such file or directory (os error 2)