assert!

The assert! macro evaluates a boolean expression at runtime and panics if the expression evaluates to false, verifying invariants in both debug and release builds.

Syntactic Forms

assert!(condition);
assert!(condition, "custom format string: {}", arg);

• If condition is true, assert! does nothing and evaluates to ().
• If condition is false, assert! panics, printing either the stringified expression (assertion failed: condition) or your custom formatted message along with the source file and line number.

fn compute_discount_price(price_cents: u64, discount_pct: u8) -> u64 {
    assert!(
        discount_pct <= 100,
        "discount percentage must be in 0..=100, got {discount_pct}%"
    );
    price_cents * u64::from(100 - discount_pct) / 100
}

fn main() {
    let final_price = compute_discount_price(2_000, 25);
    println!("Discounted price: {final_price} cents");
}

Discounted price: 1500 cents

Panicking Output When assert! Fails

When compute_discount_price(2_000, 150) is called with an invalid percentage, assert! halts execution with the custom formatted message:

fn main() {
    let discount_pct = 150_u8;
    assert!(
        discount_pct <= 100,
        "discount percentage must be in 0..=100, got {discount_pct}%"
    );
}

thread 'main' panicked at src/main.rs:3:5:
discount percentage must be in 0..=100, got 150%

Helping the Optimizer Eliminate Bounds Checks with assert!

Because assert!(index < slice.len()) (or assert!(slice.len() >= 4)) guarantees that execution cannot proceed past the assert! if the condition is false, the LLVM optimizer uses that proven fact to eliminate subsequent array bounds checks in the same function:

pub fn sum_first_four(data: &[u32]) -> u32 {
    // Proves to LLVM once that `data` has at least 4 elements;
    // the 4 indexing operations below compile with zero per-element bounds checks!
    assert!(data.len() >= 4);
    data[0] + data[1] + data[2] + data[3]
}

fn main() {
    println!("Sum of first 4: {}", sum_first_four(&[10, 20, 30, 40, 50]));
}

Sum of first 4: 100