Replace a value and keep the old value.

Replace a value and keep the old value in a complete Rust program using the standard library.

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut state = String::from("pending");
    let previous = std::mem::replace(&mut state, String::from("done"));
    println!("{previous} -> {state}");
    Ok(())
}

Expected output EXAMPLE

pending -> done

Details

Save as main.rs, compile with rustc --edition=2024 main.rs -o snippet, then run ./snippet. Uses only the Rust standard library. Work in a scratch directory for file examples.

✓ Verified

Tested on Ubuntu 24.04.3 LTS · 2026-11-02 18:54:12 UTC · rustc 1.99.1 (58cf8f9c3 2026-10-13) · Rust standard library

Test inputs, results and environment

The exact published code ran in a dedicated Linux test container with prepared inputs. Dedicated test container, external networking disabled, read-only root, temporary per-example working directory. Browser network requests are blocked. Fixtures and output records are isolated from the application database. Testing covers this example and its stated checks, not every input, operating system, or runtime version. The Linux container shares its host's kernel; this is not a native Windows test.

Architecture / host kernel: x86_64 / 6.18.54.1-microsoft-standard-WSL2
Snippet identity / locale: uid=65534, gid=65534 (unprivileged) / C.UTF-8
Interpreter invocation: rustc --edition=2024 -D warnings main.rs -o snippet && ./snippet
Container image ID: sha256:c9bce90110f636fe04907469a37e9c8ac888171fd569cbf87101743ec7cdf099
Code SHA-256: 726b0b97eb5e1f62458f4478ab792a2ec98b0215561b879dfd63bbdabbaa6014
Dependencies: Rust standard library

Test input setup

These are the prepared inputs used by the test harness. See Details above for the prerequisites and how to run this example.

# No input files required.

Observed standard output

pending -> done

Exit code: 0. Standard error: empty. Validation: Exact stdout comparison. System-dependent values above are one recorded run.

Additional assertions that passed

Checks recorded by the test harness. Where Python assertions are shown, browser cases use Playwright and simulator cases may inspect pin transitions: result.stdout is captured output, work is the temporary directory, and Path is pathlib.Path. The harness also imports json and csv.

Exact stdout comparison; process returned zero without stderr.