# [−][src]Function std::hint::black_box

    pub fn black_box<T>(dummy: T) -> T

🔬 This is a nightly-only experimental API. (`test` #50297)

An identity function that hints to the compiler to be maximally pessimistic about what `black_box` could do.

Unlike `std::convert::identity`, a Rust compiler is encouraged to assume that `black_box` can use `x` in any possible valid way that Rust code is allowed to without introducing undefined behavior in the calling code. This property makes `black_box` useful for writing code in which certain optimizations are not desired, such as benchmarks.

Note however, that `black_box` is only (and can only be) provided on a "best-effort" basis. The extent to which it can block optimisations may vary depending upon the platform and code-gen backend used. Programs cannot rely on `black_box` for correctness in any way.