std::hint::must_use — Rust function

What `must_use` does

must_use is a free function exposed by the std::hint module of the Rust standard library. It is part of the public, stable API and you can rely on it from any edition of Rust.

must_use is a free function exported by std::hint. Call it directly, or reach for the alternative method-call syntax if it's an inherent method on a related type. The function's parameter and return types declare its contract; the compiler enforces them at every call site. The full canonical path is std::hint::must_use; bring it into scope with use std::hint::must_use; or refer to it by its full path.

When to use it

Call must_use from std::hint when the operation it performs matches the documented behaviour. Many std functions return Result or Option — handle the failure path with `?` or pattern matching.

Annotated examples

Calling must_use from std::hint

use std::hint::must_use;

fn demo() {
    // Look up must_use's signature in the reference to learn what arguments
    // it expects and what type it returns. Many std functions return Result
    // or Option — handle the failure path with `?`, `match`, or `unwrap`
    // (in throwaway code only).
    let _result = must_use();
}

Reach for the function-level reference page first — it shows the exact signature, what each argument means, and what return value to expect. RustDocs Hub mirrors that information for offline reading.

For deeper background, see the canonical Rust patterns reference for the broader context behind this section.

Wrapping must_use in error-handling code

use std::hint::must_use;
use std::error::Error;

fn run() -> Result<(), Box<dyn Error>> {
    // If must_use returns a Result, the ? operator unwraps Ok and propagates
    // Err to the caller. The Box<dyn Error> return type accepts any error
    // implementing std::error::Error, which is the lowest-friction shape
    // for application-level main functions.
    let _result = must_use();
    Ok(())
}

Inside any function that itself returns `Result`, prefer `?` over `.unwrap()` or `match` — it is the canonical Rust short-circuit and integrates with `From` for error conversion.

Common pitfalls

If must_use returns a `Result`, never reach for `.unwrap()` in production code — it panics with no context. Use `?` to propagate, `match` to handle, or `.expect("why this is unreachable")` if you really do know the call cannot fail.

For deeper background, see an in-depth Rust idioms cheat sheet for the broader context behind this section.

Performance & threading notes

Standard-library functions are usually #[inline]-friendly. The optimiser will inline aggressively across crate boundaries when the function is small or marked. Profile before assuming a hot path is fine — `cargo flamegraph` and `perf` are invaluable.

What must_use does