Initialize shared data once.
Initialize shared data once in a complete Rust program using the standard library.
use std::sync::OnceLock;
fn main() -> Result<(), Box<dyn std::error::Error>> {
let value = OnceLock::new();
println!("{}", value.get_or_init(|| String::from("ready")));
println!("{}", value.get_or_init(|| String::from("unused")));
Ok(())
}Expected output EXAMPLE
ready ready
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.
Tested on Ubuntu 24.04.3 LTS · · 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
cc48cfa559dee9b4ac5edcf2de5cfe20cdb29575ef8c95f78ef05627978ffcbc- 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
ready ready
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.