Bounds Checking

Array access with an out-of-bounds index MUST cause a runtime panic.

On out-of-bounds access, the program MUST terminate with exit code 101 and print an error message.

For constant indices, bounds checking MUST be performed at compile time.

A constant index is an expression that can be fully evaluated at compile time. This includes integer literals, arithmetic operations on constants, comparison operations on constants, and parenthesized constant expressions.

[8.2:5]

For variable indices, bounds checking MUST be performed at runtime before the access.

[8.2:6]

fn main() -> i32 {
    let arr: [i32; 3] = [1, 2, 3];
    let idx: u64 = 10;
    arr[idx]  // Runtime error: index out of bounds
}

[8.2:7]

fn main() -> i32 {
    let arr: [i32; 3] = [1, 2, 3];
    arr[5]  // Compile-time error: index out of bounds
}

[8.2:8]

fn main() -> i32 {
    let arr: [i32; 3] = [1, 2, 3];
    arr[1 + 5]  // Compile-time error: index out of bounds
}