Testing Rust Projects

Result Assertion

Check Success Paths

Result-returning helpers are easy to test by asserting that a parse succeeded before unwrapping the value.

Program

Play the program to choose numeric input and verify the success path before printing the parsed count.

text
result_assertion.rs
Replay: real traced execution (multi-file project)
fn main() {
    let text = "4";
    let result = parse_count(text);
    assert!(result.is_ok());
    let count = result.unwrap();
    println!("{count}");
}

fn parse_count(text: &str) -> Result<i32, String> {
    text.parse::<i32>().map_err(|_| String::from("not a number"))
}

fn main() {
    let text = "7";
    let result = parse_count(text);
    assert!(result.is_ok());
    let count = result.unwrap();
    println!("{count}");
}

fn parse_count(text: &str) -> Result<i32, String> {
    text.parse::<i32>().map_err(|_| String::from("not a number"))
}

fn main() {
    let text = "10";
    let result = parse_count(text);
    assert!(result.is_ok());
    let count = result.unwrap();
    println!("{count}");
}

fn parse_count(text: &str) -> Result<i32, String> {
    text.parse::<i32>().map_err(|_| String::from("not a number"))
}

  1. text ← "4"

    1fn main() {2    let tex→ "4"t = "4"; //@text="4", "7", "10"3    let result = parse_count(tex"4"t);4    assert!(result.is_ok());

  2. result ← Ok(4), count ← 4

    2    let text = "4"; //@text="4", "7", "10"3    let resul→ Ok(4)t = parse_count(tex"4"t);4    assert!(result.is_ok());5    let coun→ 4t = result.unwrap();6    println!("{count}");7}
    output4

  1. text ← "7"

    1fn main() {2    let tex→ "7"t = "7";3    let result = parse_count(tex"7"t);4    assert!(result.is_ok());

  2. result ← Ok(7), count ← 7

    2    let text = "7";3    let resul→ Ok(7)t = parse_count(tex"7"t);4    assert!(result.is_ok());5    let coun→ 7t = result.unwrap();6    println!("{count}");7}
    output7

  1. text ← "10"

    1fn main() {2    let tex→ "10"t = "10";3    let result = parse_count(tex"10"t);4    assert!(result.is_ok());

  2. result ← Ok(10), count ← 10

    2    let text = "10";3    let resul→ Ok(10)t = parse_count(tex"10"t);4    assert!(result.is_ok());5    let coun→ 10t = result.unwrap();6    println!("{count}");7}
    output10
Result `Result<T, E>` makes success and failure explicit.
assert! `assert!(result.is_ok())` checks the success condition before unwrapping.
unwrap `unwrap` is safe here because the assertion already checked this controlled input.