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(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(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.

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(core_intrinsics)]
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()
    )
}