assert_eq!

The assert_eq! macro asserts that two expressions are equal (left == right via PartialEq), and if they are not equal, panics and prints the Debug ({:?}) values of both operands side by side.

Why assert_eq!(a, b) Beats assert!(a == b)

When assert!(a == b) fails, the panic message only prints the literal expression text assertion failed: a == b without showing what a and b actually evaluated to! By contrast, assert_eq!(a, b) binds &left and &right once, compares *left_val == *right_val, and formats both values with {:?} on failure:

Single-Evaluation Guarantee and Custom Messages

Like a well-designed hygienic macro, assert_eq!(left, right) expands to match (&left, &right) { (left_val, right_val) => ... }, guaranteeing that left and right are each evaluated exactly once even when an assertion fails. You can also pass an optional format string and arguments as the third parameter:

#[derive(Debug, PartialEq, Eq)]
struct Point {
    x: i32,
    y: i32,
}

fn translate(p: Point, dx: i32, dy: i32) -> Point {
    Point {
        x: p.x + dx,
        y: p.y + dy,
    }
}

fn main() {
    let start = Point { x: 2, y: 3 };
    let moved = translate(start, 10, -1);

    assert_eq!(
        moved,
        Point { x: 12, y: 2 },
        "translation by (10, -1) produced wrong coordinates"
    );
    println!("Verified point: {moved:?}");
}

Verified point: Point { x: 12, y: 2 }

Diagnostic Output When assert_eq! Fails

fn main() {
    let actual = vec![10, 20, 99];
    let expected = vec![10, 20, 30];
    assert_eq!(actual, expected, "vector mismatch at final element");
}

thread 'main' panicked at src/main.rs:4:5:
assertion `left == right` failed: vector mismatch at final element
  left: [10, 20, 99]
 right: [10, 20, 30]