Function std::intrinsics::const_eval_select

pub const fn const_eval_select<ARG, F, G, RET>(
    _arg: ARG,
    _called_in_const: F,
    _called_at_rt: G,
) -> RET
where
    ARG: Tuple,
    G: FnOnce<ARG, Output = RET>,
    F: FnOnce<ARG, Output = RET>,

🔬This is a nightly-only experimental API. (core_intrinsics)

Selects which function to call depending on the context.

If this function is evaluated at compile-time, then a call to this intrinsic will be replaced with a call to called_in_const. It gets replaced with a call to called_at_rt otherwise.

This function is safe to call, but note the stability concerns below.

Type Requirements

The two functions must be both function items. They cannot be function pointers or closures. The first function must be a const fn.

arg will be the tupled arguments that will be passed to either one of the two functions, therefore, both functions must accept the same type of arguments. Both functions must return RET.

Stability concerns

Rust has not yet decided that const fn are allowed to tell whether they run at compile-time or at runtime. Therefore, when using this intrinsic anywhere that can be reached from stable, it is crucial that the end-to-end behavior of the stable const fn is the same for both modes of execution. (Here, Undefined Behavior is considered “the same” as any other behavior, so if the function exhibits UB at runtime then it may do whatever it wants at compile-time.)

Here is an example of how this could cause a problem:

#![feature(const_eval_select)]
#![feature(core_intrinsics)]
use std::intrinsics::const_eval_select;

// Standard library
pub const fn inconsistent() -> i32 {
    fn runtime() -> i32 { 1 }
    const fn compiletime() -> i32 { 2 }

    // ⚠ This code violates the required equivalence of `compiletime`
    // and `runtime`.
    const_eval_select((), compiletime, runtime)
}

// User Crate
const X: i32 = inconsistent();
let x = inconsistent();
assert_eq!(x, X);

Currently such an assertion would always succeed; until Rust decides otherwise, that principle should not be violated.