black_box

fn black_box<T>(T x) -> T

Returns x unchanged, through a call the optimizer cannot see into.

Use it in benchmarks: at -O2 and above a pure function called with constant arguments is computed while compiling, so fib(30) would time nothing. fib(black_box(30)) runs the call. A function that calls black_box counts as reading host state, so it is never computed at compile time, hoisted out of a loop or run in parallel automatically. It takes a value of one word: not a tuple or a two-word Result.

Parameters#

Returns#

T

Examples#

use tests::assert;
fn sq(int x) -> int {
    return x * x;
}

fn main() {
    assert!(sq(black_box(7)) == 49);
    assert!(black_box("same") == "same");
}

See also#

• Result
• ord: Returns the code of the single character in s as a byte.
• char: Returns the one-character string whose code point is b.