Hints to the compiler that a branch condition is likely to be true. Returns the value passed to it.

It can be used with if or boolean match expressions.

When used outside of a branch condition, it may still influence a nearby branch, but probably will not have any effect.

It can also be applied to parts of expressions, such as likely(a) && unlikely(b), or to compound expressions, such as likely(a && b). When applied to compound expressions, it has the following effect:

    likely(!a) => !unlikely(a)
    likely(a && b) => likely(a) && likely(b)
    likely(a || b) => a || likely(b)

See also the function [cold_path()] which may be more appropriate for idiomatic Rust code.

Examples

#![feature(likely_unlikely)]
use core::hint::likely;

fn foo(x: i32) {
    if likely(x > 0) {
        println!("this branch is likely to be taken");
    } else {
        println!("this branch is unlikely to be taken");
    }

    match likely(x > 0) {
        true => println!("this branch is likely to be taken"),
        false => println!("this branch is unlikely to be taken"),
    }

    // Use outside of a branch condition may still influence a nearby branch
    let cond = likely(x != 0);
    if cond {
        println!("this branch is likely to be taken");
    }
}