assert_ne!

The assert_ne! macro asserts that two expressions are not equal according to the PartialEq trait (left != right). If the two expressions compare equal, assert_ne! panics and prints the Debug representation of both left and right.

assert_ne!(left, right)
assert_ne!(left, right, "custom message: {}", context)

When to Use assert_ne!

assert_ne! is preferred over assert!(a != b) whenever you want the test failure output to display the offending value that unexpectedly matched:

• Verifying that a state transition, mutation, or encryption step actually modified its input.
• Verifying that unique ID generators, token generators, or port allocators produce distinct values on consecutive calls.
• Checking that a hash or redact function does not return the cleartext secret.

use std::sync::atomic::{AtomicU64, Ordering};

static NEXT_ID: AtomicU64 = AtomicU64::new(1000);

fn generate_request_id() -> u64 {
    NEXT_ID.fetch_add(1, Ordering::Relaxed)
}

fn redact_token(token: &str) -> String {
    if token.len() <= 4 {
        "****".to_string()
    } else {
        format!("{}...****", &token[..4])
    }
}

fn main() {
    let id_first = generate_request_id();
    let id_second = generate_request_id();

    // Verify consecutive IDs are distinct
    assert_ne!(id_first, id_second, "request IDs must be unique");

    // Verify redaction never equals the raw secret
    let raw_secret = "sk_live_987654321";
    let redacted = redact_token(raw_secret);
    assert_ne!(redacted, raw_secret, "redacted output leaked raw secret");

    println!("Generated distinct IDs: {id_first} != {id_second}");
    println!("Redacted token: {redacted}");
}

Generated distinct IDs: 1000 != 1001
Redacted token: sk_l...****

Failure Output of assert_ne!

When left != right evaluates to false, assert_ne! prints both operands along with any custom diagnostic message:

fn main() {
    let primary_host = "db-node-1.internal";
    let replica_host = "db-node-1.internal";

    assert_ne!(
        primary_host, replica_host,
        "failover replica must not point to the primary host"
    );
}

thread 'main' panicked at src/main.rs:5:5:
assertion `left != right` failed: failover replica must not point to the primary host
  left: "db-node-1.internal"
 right: "db-node-1.internal"

Comparing Rust's Standard Assertion Macros