Using an unsafe block

rust,editable,ignore

# // Copyright 2026 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
fn main() {
    let numbers = vec![0, 1, 2, 3, 4];
    let i = numbers.len() / 2;

    let x = *numbers.get_unchecked(i);
    assert_eq!(i, x);
}

Walk through the code. Confirm that the audience is familiar with the dereference operator.

Attempt to compile the code, trigger the compiler error.

Add the unsafe block:

rust

# // Copyright 2026 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
# fn main() {
#     let numbers = vec![0, 1, 2, 3, 4];
#     let i = numbers.len() / 2;
#
 let x = unsafe { *numbers.get_unchecked(i) };
#     assert_eq!(i, x);
# }

Prompt audience for a code review. Guide learners towards adding a safety comment.

Add the safety comment:

rust

# // Copyright 2026 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
// SAFETY: `i` must be within 0..numbers.len()

Suggested Solution

rust

# // Copyright 2026 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
fn main() {
    let numbers = vec![0, 1, 2, 3, 4];
    let i = numbers.len() / 2;

    let x = unsafe { *numbers.get_unchecked(i) };
    assert_eq!(i, x);
}

</details>