Function core::intrinsics::const_eval_select

pub unsafe fn const_eval_select<ARG, F, G, RET>(
    arg: ARG, 
    _called_in_const: F, 
    called_at_rt: G
) -> RET where
    F: ~constFnOnce<ARG, Output = RET>,
    G: FnOnce<ARG, Output = RET> + ~constDrop, 

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

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.

Type Requirements

The two functions must be both function items. They cannot be function pointers or closures.

arg will be the 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.

Safety

This intrinsic allows breaking referential transparency in const fn and is therefore unsafe.

Code that uses this intrinsic must be extremely careful to ensure that const fns remain referentially-transparent independently of when they are evaluated.

The Rust compiler assumes that it is sound to replace a call to a const fn with the result produced by evaluating it at compile-time. If evaluating the function at run-time were to produce a different result, or have any other observable side-effects, the behavior is undefined.