Function is_val_statically_known

pub const fn is_val_statically_known<T: Copy>(_arg: T) -> bool

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

Returns whether the argument’s value is statically known at compile-time.

This is useful when there is a way of writing the code that will be faster when some variables have known values, but slower in the general case: an if is_val_statically_known(var) can be used to select between these two variants. The if will be optimized away and only the desired branch remains.

Formally speaking, this function non-deterministically returns true or false, and the caller has to ensure sound behavior for both cases. In other words, the following code has Undefined Behavior:

#![feature(is_val_statically_known)]
#![feature(core_intrinsics)]
use std::hint::unreachable_unchecked;
use std::intrinsics::is_val_statically_known;

if !is_val_statically_known(0) { unsafe { unreachable_unchecked(); } }

This also means that the following code’s behavior is unspecified; it may panic, or it may not:

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

assert_eq!(is_val_statically_known(0), is_val_statically_known(0));

Unsafe code may not rely on is_val_statically_known returning any particular value, ever. However, the compiler will generally make it return true only if the value of the argument is actually known.

Stability concerns

While it is safe to call, this intrinsic may behave differently in a const context than otherwise. See the const_eval_select documentation for an explanation of the issues this can cause. Unlike const_eval_select, this intrinsic isn’t guaranteed to behave deterministically even in a const context.

Type Requirements

T must be either a bool, a char, a primitive numeric type (e.g. f32, but not NonZeroISize), or any thin pointer (e.g. *mut String). Any other argument types may cause a compiler error.

Pointers

When the input is a pointer, only the pointer itself is ever considered. The pointee has no effect. Currently, these functions behave identically:

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

fn foo(x: &i32) -> bool {
    is_val_statically_known(x)
}

fn bar(x: &i32) -> bool {
    is_val_statically_known(
        (x as *const i32).addr()
    )
}